A

absolute zero

The coldest possible temperature. It is the lowest temperature on the Kelvin temperature scale, equivalent to -459.67 F (–273.15 C). At this temperature, molecules have reached their minimum energy and cannot transfer heat, but they are never completely motionless because they retain energy to vibrate. As of 2021, the lowest temperature achieved in experiments was 38 picokelvin (3.8 x 10^-11 K), or 38 trillionths of a degree above absolute zero.

absorption

The process in which atoms or molecules retain the energy delivered by electromagnetic radiation that strikes them. When an atom absorbs light, its electrons are excited into higher energy states.

accretion

Growth by the gradual accumulation of additional matter. In astronomy, the term describes growth by the gravitational attraction of matter in the form of gas, dust, rocky or icy debris, and other objects. Young stars, planets, and black holes grow in this way.

accretion disk

A relatively flat, rapidly rotating cloud of gas or dust surrounding any massive object growing in size by attracting matter through gravity (accretion), such as a newborn star or a black hole.

active galactic nucleus (AGN), active galaxy

A class of galaxies with central regions that appear much brighter than ordinary galaxies and emit unusually large amounts of X-ray, ultraviolet, and other radiation. The central region is called the galactic nucleus (plural, nuclei). Astronomers think supermassive black holes at the centers of AGN produce the additional energy as matter stored in vast accretion disks falls into them. Seyfert galaxies, radio galaxies, quasars, and blazars are all AGN. The term is sometimes shortened to active galaxy.

afterglow

Lower-energy electromagnetic radiation, including X-rays, visible light, and radio waves, that often follow the initial burst of gamma rays in a gamma-ray burst. Afterglows can persist for days to weeks, gradually dissipating with time. The BeppoSAX satellite discovered the afterglow phenomenon in 1997.

ammonia

A molecule composed of one atom of nitrogen and three of hydrogen (NH³). It can be found as either a gas (in planetary atmospheres and accretion disks) or a solid (as ice on the surfaces of dust grains, comets, and planets). Ammonia was the first multiatomic molecule found in interstellar space (1968).

amplitude

The size of a wave from the top of a wave crest to its midpoint.

angstrom (Å)

A metric unit of length equal to 0.1 nanometer (10^-10 meters), used to specify wavelengths of light, named for Swedish physicist Anders Jonas Ångström (1814-1874).

angular momentum

A property of a mass or system of masses turning about some fixed point, for example, a planet in orbit around a star. It is defined as the body's mass multiplied by its velocity and its distance from the center of motion. In a closed system, it is a conserved quantity in physics, which means it remains constant. For example, a planet in an eccentric orbit constantly changes the distance from its star. Since the planet's mass is constant, its velocity must change too — faster at perihelion, slower at aphelion — so that its angular momentum remains the same at every point in the orbit. The conservation of angular momentum explains Kepler's second law of planetary motion.

annihilation

The mutual conversion of a particle and its antiparticle into electromagnetic radiation, converting their mass (m) into energy (E) according to Einstein's famous formula, E = mc², where c is the speed of light. An electron and a positron annihilate to produce two gamma-ray photons, each with an energy of 511 keV.

anticoincidence detector

A system on a gamma-ray observatory that triggers when it detects a charged particle entering the telescope so a detector does not mistake the incoming particle for a gamma ray.

antimatter

Counterparts of the subatomic particles making up normal matter that have opposite properties, typically electrical charge (exceptions: the antineutron has reversed magnetic polarity, and antineutrinos have reversed spin). When a particle and its antiparticle collide, they annihilate. The positron is the antiparticle of the electron. By manipulating positrons into orbit around antiprotons, scientists at the European Organization for Nuclear Research (CERN) created the first antimatter atoms — antihydrogen — in 1995.

apoapsis [ap-oh-AP-sis]

The farthest point in the path of an orbiting body; its opposite is periapsis. The concept has special names in commonly used systems:

• apastron, for a body orbiting a star
• apogee, for a body orbiting Earth
• apojove, for a body orbiting Jupiter
• aphelion, for a body orbiting the Sun
• apolune, for a body orbiting the Moon

arcminute

An angular measurement equal to 1/60th of a degree. It is approximately the smallest angle discernible to the unaided human eye. As viewed from Earth, both the Sun and the Moon appear about 30 arcminutes across.

arcsecond

An angular measurement equal to 1/3,600th of a degree or 1/60th of an arcminute. This is about the apparent thickness of a U.S. dime coin viewed from a distance of about 913 feet (278 meters).

asteroid

A rocky object larger than 3.3 feet (1 meter) and smaller than a planet that orbits a star; also called minor planets. Most asteroids in our solar system orbit the Sun between Mars and Jupiter in a region called the main asteroid belt. The largest asteroid, Ceres, has a diameter of 966 kilometers (600 miles) and is additionally classified as a dwarf planet.

astronomical unit (au or AU)

A unit of length approximating the average distance from Earth to the Sun and formally defined as 149,597,870.7 km (92,955,807.3 miles). It is commonly used to describe distances within our own and other planetary systems.

astronomy

The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.

astrophysics

The branch of astronomy concerned with the physical nature of stars, galaxies, interstellar clouds and other objects, including their luminosities, temperatures, densities and chemical composition.

atmosphere

The envelope of gases surrounding a planet, moon, or star.

Atmospheric Cherenkov Telescope (ACT)

A ground-based telescope that observes high-energy gamma rays and cosmic rays through the faint flashes of blue light they produce on interacting with Earth's atmosphere. When they strike an air molecule, the collision shatters its atoms into their constituent particles — protons, neutrons, electrons — and produces others, such as muons and pions. These particles then strike other atoms, producing more fast-moving debris that breaks apart more atoms, and so on. This cascade of particles (air shower) moves faster through the air than light is able to, which results in a broad blue flash called Cherenkov light, named for the Russian physicist Pavel Cherenkov (1904–1990).

atom

The smallest unit of an element that retains the element's chemical characteristics. An atom consists of a central nucleus containing one or more positively charged protons and uncharged neutrons (except for hydrogen, which may have no neutrons) orbited by one or more negatively charged electrons. Atoms of each element possess a unique number of protons (for example, one for hydrogen, eight for oxygen).

atomic nucleus

The tiny, dense center of an atom, consisting of positively charged protons and (except for hydrogen) uncharged neutrons. A nucleus (plural, nuclei) accounts for more than 99.9% of an atom’s mass but represents only 0.01% of its volume. Most are shaped like spheres or ellipsoids, and some are unstable and undergo radioactive decay.

aurora

A natural light display predominantly seen in Earth's northern and southern high-latitude regions, appearing as dynamic streamers, curtains, and arches of shimmering light; also called the northern and southern lights. The phenomenon occurs around two vast ovals in the upper atmosphere. When the paths of electrons and protons near Earth become disturbed by solar activity, they can follow magnetic field lines into the atmosphere, exciting atoms and molecules that respond by emitting light of characteristic colors. Similar processes create auroras detected on Mars, Jupiter, Jupiter's moon Io, Saturn, Uranus, Neptune, comet 67P/Churyumov–Gerasimenko, and — beyond the solar system — the brown dwarf LSR J1835+3259.

B

Balmer series

A set of visible emission or absorption lines in the spectrum of hydrogen that arise from electron transitions between the atom's first excited state and those at higher energies, discovered by the Swiss physicist Johann Balmer (1825–1898). The first four lines are identified using the chemical symbol for hydrogen (H) and the lowercase Greek letters alpha (α), beta (β), gamma (γ) and delta (δ). Because hydrogen is the universe's most abundant element, the Balmer lines play a major role in how we see the cosmos. For example, the light of the Hα line provides the prominent reddish color to images of emission nebulae taken in natural light. Hydrogen also produces additional line sets beyond visible light, including the Lyman series (ultraviolet, important for cosmology), the Paschen series (infrared) and the Brackett series (far infrared).

bar

A metric unit of pressure. At sea level, Earth's atmosphere exerts a pressure just over 1 bar. By contrast, the very thin atmosphere of Mars has a surface pressure less the 1% of Earth's, while the thick atmosphere of Venus produces an average pressure 92 times greater than Earth's.

baryon [BARRY-on]

A class of hadrons built from three quarks that includes protons and neutrons. The other class, called mesons, are unstable particles built from only two quarks. Baryons produce both electromagnetic and gravitational signals that reveal their presence throughout the universe. In astronomy, the term "baryonic matter" refers more loosely to the normal atomic matter (including electrons) that makes up people, planets, stars and galaxies. It represents just 4.9% of the mass-energy in the universe.

baryonic acoustic oscillations (BAO)

Sound waves from the early universe whose imprint is reflected in the large-scale distribution of matter. When the universe was a hot, dense plasma, regions with greater concentrations of light and matter were at higher pressure and sound waves rippled outward from them. These waves carried along ordinary (baryonic) matter until the universe cooled enough that light could travel freely, which is when the cosmic microwave background formed. The movement of matter from sound waves up to this time resulted in spherical shells of excess density throughout the cosmos, and they grew in size as the universe expanded. BAOs appear in the temperature variations of the cosmic microwave background, and today they leave a faint imprint in the clustering of galaxies — detectable as a small excess in the number of galaxy pairs separated by about 500 million light-years. These features serve as "standard rulers" that allow astronomers to probe the expansion rate of the universe and learn more about dark energy.

basalt

A volcanic rock, dark grey to black in color, made of iron- and magnesium-rich silicate minerals. More than 90% of all volcanic rock on Earth is basalt, and it is also common on the Moon, Mercury, Venus, and Mars.

Be [bee-ee] star

A hot, often variable star of spectral class B that shows emission lines (the "e" in the name) in its spectrum. Be stars rotate so rapidly that they eject gas, which collects into a surrounding disk thought to be many times the star's size. Hydrogen emission lines arise because the star's ultraviolet light interacts with the gaseous disk. Examples include Gamma Cassiopeiae A — the first one identified, in 1866 — and the bright southern star Achernar (Alpha Eridani A).

big bang

A widely accepted model of the universe that explains its observed properties by postulating that it originated as a tiny, superdense, rapidly expanding fireball about 13.8 billion years ago. The universe expanded, cooled, and condensed into subatomic particles that later accumulated into the matter that formed stars and galaxies. The model naturally explains the cosmic microwave background, the expansion of space, and the ratio of hydrogen, helium, and other light elements observed throughout the universe.

binary star

Two stars that are gravitationally bound together and orbit their common center of mass.

biosignature

An indicator that provides scientific evidence for the presence of past or present life. In exoplanet research, scientists have suggested that certain gases — such as oxygen, ozone, methane, nitrous oxide, methyl bromide, methyl chloride, hydrogen sulfide, carbonyl sulfide, phosphine, sulfur dioxide and others — could accumulate in the atmosphere of an exoplanet as byproducts of life. Detecting spectral lines from such gases could, in principle, allow astronomers to determine the presence of life on these worlds.

blackbody

An idealized opaque, non-reflective object that both emits and absorbs all forms of light perfectly. This results in a specific continuous spectrum (a blackbody spectrum)that depends only on the blackbody's temperature. Stars and planets are often described by their blackbody temperature, that is, the temperature of a perfect radiator that would emit the same total amount of light observed. The body's actual surface temperature may be quite different depending on additional properties, such as the presence of a heat-retaining atmosphere around a planet. An extreme example is Venus, which has a blackbody temperature of -42 F (-41 C) but a globally averaged surface temperature of 867 F (464 C).

black dwarf

A hypothetical stellar remnant formed once a white dwarf has radiated away all of its energy and no longer emits heat or light. The time required to reach this state is expected to be hundreds of billions of years. Because this is far longer than the age of our universe, they do not yet exist.

black hole

An object whose gravity is so strong that not even light can escape it. Black holes are completely characterized by only three qualities: mass, rotation, and electrical charge. Abundant observational evidence exists for stellar-mass black holes, with masses from four to dozens of times the mass of our Sun formed through the collapse of massive stars at the ends of their lives, with the most massive objects formed through mergers with other black holes. Supermassive black holes weighing hundreds of thousands to billions of solar masses are observed at the centers of most large galaxies and power AGNs. Although expected to exist, there is less certain evidence for intermediate-mass black holes of hundreds to 100,000 solar masses. Primordial black holes are hypothetical objects that may have formed in the early universe; they represent a possible explanation for dark matter and could have masses ranging from asteroid-like to about as much as stellar-mass black holes.

blazar [BLEY-zar]

A type of active galactic nucleus often appearing as a bright, highly variable point-like source that does not display emission lines. Blazars vary dramatically in all forms of light, so studying them requires simultaneous multiwavelength observations. Astronomers think an accreting supermassive black hole at the center of a blazar drives jets of matter that travel outward in opposite directions at nearly the speed of light. Blazars appear so bright and variable because we are looking almost directly down the barrel of one jet.

blue supergiant star

A massive blue star of extreme luminosity, surface temperature, and size. Blue supergiants are among the brightest stars in the universe. Several such stars can be seen in the constellation Orion, the brightest being Rigel, which has about 20 times the mass of our Sun, is at least 70 times larger, has a surface more than twice as hot as the Sun's, and emits 120,000 times the Sun's luminosity. The sizes and temperatures of blue supergiants allow them to drive strong outflows (stellar winds) into their surroundings.

blueshift

An apparent shift toward shorter (bluer) wavelengths of an object's spectral lines or emitted light caused by its motion toward an observer.

bolometric luminosity

The total energy radiated by an object at all wavelengths, usually given in watts. The Sun's bolometric luminosity is 3.83 x 10²⁶ watts.

Boltzmann constant (k)

A number that relates the temperature of an ideal gas to the average kinetic energy of molecules in the gas. It is defined as 1.380649 x 10^–23 joule per kelvin. Named for Austrian physicist Ludwig Boltzmann (1844–1906).

bremsstrahlung [BREM-shtrah-luhng]

Light emitted when a charged particle, such as an electron, decelerates when deflected by the electric and magnetic fields of another particle or atomic nucleus. Very fast charged particles primarily lose energy by this process when traveling through matter. The term is a German word that literally means "braking radiation."

brown dwarf

A substellar object lacking the mass needed to sustain the hydrogen fusion processes found in the cores of full-fledged stars. Brown dwarfs have masses between 1% and 8% of our Sun's, or between 13 and 80 times the mass of Jupiter, spectral types of M, L, T, and Y, and surface temperatures below about 5,800 F (3,200 C).

C

calibration

A process for translating the signals produced by a measuring instrument (such as a detector attached to a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.

calorimeter

An instrument for measuring the energy of a particle or photon through the amount of heat deposited in a material.

Canadian Space Agency (CSA)

The Canadian Space Agency is an independent federal agency responsible for managing all of Canada's civil space-related activities.

carbon dioxide

A molecule composed of one atom of carbon and two of oxygen (CO2). In its frozen form, it's known as dry ice. Carbon dioxide is a common molecule and can be found as a gas in planetary atmospheres, as solid grains in molecular clouds, and in icy features on the surfaces of comets and planets. The south polar ice cap of Mars is topped by a permanent layer of frozen carbon dioxide about 26 feet (8 meters) thick.

cataclysmic variable

A close binary star system where a white dwarf accretes gas from a low-mass normal star. The gas collects into an accretion disk around the white dwarf and then flows onto its surface, producing X-rays. In some systems, this gas accumulates and eventually begins thermonuclear fusion reactions, producing a nova outburst that suddenly increases the system's apparent brightness by up to 100,000 times.

centaur

A small solar system body orbiting the Sun between Jupiter and Neptune that crosses the orbit of at least one giant planet. Centaurs are widely thought to have been ejected from the Kuiper Belt, but some may have been produced through the breakup of comets or by impact events on icy moons.

Cepheid [SEE-fee-id]

A type of star that undergoes repeating cycles of brightness changes as it regularly expands and contracts. The length of this cycle is directly related to the star's luminosity, and astronomers can determine its distance by comparing this to the star's observed brightness. This discovery, made in 1912 by Henrietta Leavitt (1868-1921), makes cepheid variables a powerful astronomical tool for measuring cosmic distances. The class is named for its prototype, Delta Cephei, and includes Polaris, the closest cepheid to the Sun.

Chandrasekhar limit

The theoretical maximum mass of a white dwarf, equal to about 1.4 solar masses. Beyond it, a white dwarf must collapse into a neutron star. Named for Subrahmanyan Chandrasekhar (1910–1995), who first proposed the idea in 1931.

chemical evolution

Chemical changes that transformed simple molecules into the more complex substances needed for the origin of life under conditions very different from those on present-day Earth. Amino acids found in asteroids and meteorites could have arrived on Earth via impacts, for example, but the steps by which they were assembled into self-replicating molecules like RNA and DNA, which carry the genetic information of living organisms, remain unknown.

circumstellar disk/protoplanetary disk

A flattened, circular, rotating cloud of gas, dust, rocky or icy fragments, and larger objects orbiting a star. Around the youngest stars, such disks serve as cocoons where planets may form.

climate

The long-term pattern of oceanic and atmospheric conditions at a location, described by measurements of average and extreme temperature, precipitation, and other variables, and by the intensity, frequency, and duration of weather events.

cluster of galaxies

A system of galaxies containing anywhere between a few and a few thousand member galaxies that are gravitationally bound together. Also called a galaxy cluster.

collecting area

The area of a telescope that is capable of collecting light. The more light a telescope can collect (that is, the larger its collecting area), the better it can detect faint objects.

comet

A small ice-rich body, typically less than about 20 km (12 miles) across, that travels around the Sun, often in highly eccentric orbits. The solid body is also called the comet's nucleus. It is composed of frozen gases — including water ice, carbon dioxide, carbon monoxide, methane, and ammonia — mixed with dust and organic molecules. When a comet nears the Sun, it warms up, and ice on the surface converts to gas, forming jets that carry ice and dust into space. This material surrounds the comet with a vast atmosphere, called a coma, typically up to 50,000 miles (80,000 kilometers) across. Ultraviolet sunlight breaks up water molecules released by the comet, and this forms a vast, thin neutral hydrogen cloud visible only in ultraviolet light that may span 62 million miles (100 million kilometers). Radiation pressure from sunlight pushes dust grains averaging a micrometer across — about the size of particles in smoke — out of the coma. They sweep back into a gently curved dust tail that glows by reflecting sunlight and may extend tens of millions of kilometers; it is the most visually striking phenomenon associated with comets. Additionally, the solar wind acts on charged ions broken off of molecules by ultraviolet sunlight, sweeping them into a straight, narrow, bluish, and visually faint gas tail (also called an ion or plasma tail) that shines by fluorescence. Comets also emit X-rays as they interact with the solar wind, a phenomenon first detected in 1996 by the German-led ROSAT mission.

compact object

An extremely dense stellar object, such as a black hole, neutron star, or white dwarf. We observe such an object as a point source, too small to resolve.

composition

The materials that make up an object or group of objects.

Compton scattering

A transfer of energy between a photon and a particle, such as an electron, that lowers the photon's energy and increases the velocity of the particle. Inverse Compton scattering occurs when a charged particle transfers some of its energy to a photon, boosting it to higher energy, and plays an important role in astrophysics. When light from microwave to ultraviolet wavelengths collides with a high-energy particle, the interaction can boost the photon to gamma-ray energies.

constellation

One of 88 regions dividing the celestial sphere, formalized in 1930. They enclose recognizable star patterns recorded by various cultures, particularly those from ancient Greece. The patterns are imaginatively described as representing, animals (Leo, Taurus), mythological figures (Orion, Andromeda, Perseus), or recognizable objects (Telescopium, Triangulum). Not all familiar star patterns are constellations. For example, the Big Dipper is a well-known arrangement (called an asterism) within the constellation Ursa Major.

corona (plural: coronae)

The faint, tenuous, uppermost level of a star's atmosphere. The Sun's corona extends many thousands of kilometers above its visible surface, gradually transforming into the solar wind that flows outward through our solar system. The solar corona is characterized by very low densities and enormously high temperatures (over 2 million F, 1 million C), although scientists aren’t sure exactly why the corona is so hot.

coronagraph

An instrument designed to block direct light from the Sun or another star so that objects otherwise lost in its glare may be observed.

cosmic dust

Tiny, irregularly-shaped solid particles — measuring between a few molecules and 0.004 inches (0.1 mm) across — found in space. Much of the dust in our solar system originates from comets, asteroids, and planets and their satellites, but material from interstellar space, formed in the atmospheres of aging stars, stellar outbursts, and supernova explosions, also passes through. Such particles are the largest source of extraterrestrial material arriving on present-day Earth, with an estimated 30 or more tons entering the atmosphere each day; most of these particles were shed by Jupiter-family comets. Particles originating from interstellar space, called presolar grains because they formed before the Sun did, are found both in in meteorites and cosmic dust. Dust is important because stars and their planets form inside vast, cool, dusty interstellar clouds. These clouds absorb visible light, looking like large dark features in our own and other galaxies. In broad terms, there are three types of dust: icy grains, found in regions like the outer solar system; silicate particles, which resemble grains of sand; and polycyclic aromatic hydrocarbons (PAHs), which are large molecules of carbon and hydrogen that resemble soot.

cosmic microwave background (CMB)

A faint signal present everywhere in the sky and the oldest light we can detect, discovered in 1965 by American astronomers Arno Penzias and Robert Wilson. It is the afterglow of the hot, young, expanding universe at a time when it was only about 380,000 years old, a fraction of its current age of 13.8 billion years. Before this time, the universe was a hot, dense plasma filled with electrons, protons, nuclei, neutrinos, other particles, and light. All light was tied up with interactions between particles, especially electrons, so it couldn't travel far before striking one of them, making the universe an opaque fog. As the universe expanded and cooled below about 4,900 F (2,700 C), electrons, protons, and nuclei could combine into atoms. This greatly reduced the number of free electrons that light could crash into, and for the first time the universe became transparent — light could travel unimpeded through the cosmos. The continued expansion of space has redshifted the wavelength of this glow by about 1,100 times, so we detect the CMB in the microwave part of the electromagnetic spectrum, brightest at a wavelength of 1.9 mm (corresponding to near-ultraviolet light when it last bounced off an electron). The CMB exhibits a perfect blackbody spectrum with an average temperature of about -454.765 F (-270.425 C or 2.725 K) and shows minute temperature differences (measured in microkelvins) that were first detected by NASA's COBE satellite and mapped in greater detail by the WMAP and Planck missions. These variations reflect differences in the density of the universe that were already present before the CMB formed and that ultimately led to the formation of the first stars and galaxies.

cosmic rays

Atomic nuclei — about 90% of them protons — and elementary particles, such as electrons, that travel through interstellar space at speeds approaching that of light. When the term was coined in 1925, it was not yet known that they were particles, not rays, but the name stuck. Trillions upon trillions of cosmic rays hit Earth every day, most from the Sun, and nearly all blocked by our planet's atmosphere and magnetic field. Sometimes, though, a cosmic ray will strike molecules in the atmosphere and create a shower of secondary particles that reach the ground. They are important cosmic "messengers," but because they carry electric charge, they are easily diverted by our galaxy's magnetic field and do not travel to us straight from their sources (unlike light and neutrinos).

cosmochemistry

The study of the chemical compositions of matter in the universe and the processes that produced them, primarily focused on objects in our own solar system. For example, meteorites contain features that provide insight into the circumstellar disk from which the planets grew. Some, called presolar grains, are samples of matter ejected by dying stars before the solar system formed.

cosmological constant (Λ)

A mathematical fix to the general theory of relativity, symbolized by the capital Greek letter lambda (Λ), which Einstein proposed in the mistaken belief that the universe was static, neither expanding nor contracting. By 1929 observations made clear that the universe is expanding, and so this fix was unnecessary. If Einstein had believed what his equations were telling him, he could have claimed the expansion of the universe as perhaps the greatest and most convincing prediction of general relativity. He called his introduction of the term the "biggest blunder of my life."

cosmological distance

A distance far beyond the boundaries of our galaxy. When viewing objects at cosmological distances, the curved nature of space-time introduces effects such as time dilation and cosmological redshift.

cosmological principle

This principle states that the distribution of matter across very large distances is the same everywhere in the universe and that the universe looks the same in all directions. According to this principle, our view of the universe is like the view from a boat on an ocean, which is essentially the same for any other person on any other boat on any other ocean. Measurements of matter and energy in the universe on the largest observable scales support the cosmological principle.

cosmological redshift

An effect where light emitted from a distant source appears redshifted because of the expansion of space-time.

cosmology

The branch of astrophysics concerned with the origin, development, and structure of the universe.

D

dark energy

A theoretical pressure that affects the universe on the largest scales and appears to drive the accelerating expansion of space. While cosmologists have determined that it makes up 68% of the total energy in the present-day universe, the nature of dark energy is unknown.

dark matter

Hypothetical non-baryonic matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. Observations of the motions of stars and gas in galaxies, the properties of hot gas in galaxy clusters and the radial velocities of their member galaxies, and gravitational lensing of distant background galaxies by foreground galaxy clusters indicate the presence of large amounts of dark matter. It is now thought to comprise 27% of the universe's total energy. Candidate particles include WIMPs, axions, and even primordial black holes.

de Broglie wavelength

The quantum mechanical wavelength associated with a particle as part of the dual wave-particle nature of matter, named after Louis de Broglie (1892–1987). In 1924 he proposed the concept that matter behaves like a wave. In quantum mechanics, all massive particles have wave characteristics, with the wavelength of a particle equal to the Planck constant divided by the particle's momentum.

debris disk

A flattened, circular, rotating cloud of dust, rocky or icy fragments, and larger objects orbiting a star. Unlike circumstellar and protoplanetary disks, debris disks contain little gas and resemble the Kuiper Belt in our solar system.

declination

A celestial coordinate which, along with right ascension, locates any position in the sky. Analogous to latitude on Earth, it extends from +90 degrees to –90 degrees.

degree

An angular measure devised so there are 360° in a circle. From Earth, the Sun and Moon both appear to be about 0.5° across. One degree equals 60 arcminutes. 2. A unit in the Celsius and Fahrenheit temperature scales.

density

The ratio of an object's mass to its volume. For example, water has a density of one gram of mass for every milliliter (cubic centimeter) of volume. The density of lead is 11 times greater.

dewar [doo-ur]

A double-walled container of metal or silvered glass with a vacuum between the walls, used to hold low-temperature liquids. In astronomy, dewars are often used for liquid nitrogen (-321 F or -196 C), but can also be used for liquid helium (-452 F or -269 C) or solid neon (-429 F or -256 C). These materials are used because astronomical detectors work better at such cold temperatures. Named for the inventor Sir James Dewar (1842–1923), a Scottish chemist and physicist.

diffraction grating

A device that splits light into its component colors, used for obtaining a spectrum. A diffraction grating often consists of a mirror etched with thousands of closely spaced parallel lines, which spread out the light into parallel bands of color or distinct lines or bars.

diffuse galactic emission

Widespread gamma-ray emission along the plane of our Milky Way galaxy. It arises mostly from cosmic rays interacting with interstellar gas, dust, and starlight.

digital image

A visible image that is recorded by an electronic detector and subdivided into small picture elements (pixels). Each element is assigned a number that corresponds to the brightness recorded at its physical location on the detector. Computer software converts the numerical information into a visual image. The Hubble Space Telescope records digital images.

disk

1. The apparent circular shape of the Sun, a planet, or the Moon when seen in the sky or through a telescope.
2. A flattened, rotating, circular region of gas, dust, and/or stars. It may refer to material surrounding a star (circumstellar disk, protoplanetary disk, debris disk), material falling onto a black hole or neutron star (accretion disk), a type of flattened galaxy that includes spiral galaxies, or the component of such a galaxy that is rich in gas, dust, young stars and includes its spiral arms (galactic disk).

dispersion

The separation of light, as by a prism, into a series of colors. The speed of light passing through a transparent medium depends on its wavelength, so a beam of light passing through a prism fans out into its component colors. Shorter wavelengths, like purple and blue light, bend the most. Longer wavelengths, like red and orange light, bend the least.

Doppler effect

The apparent change in wavelength of sound or light caused by the motion of the source, observer, or both. In astronomy, the Doppler effect seen in the shift of spectral lines can reveal companion objects (stars or planets) orbiting stars, stellar motion through the galaxy, the motions of galaxies themselves, and the expansion of the universe. Waves emitted by a moving object as received by an observer will be blueshifted (compressed, with shorter wavelengths) if approaching, redshifted (elongated, with longer wavelengths) if receding. This change occurs both in sound and light — and indeed for any wave when there is relative motion between the observer and the source. An everyday example is the sound of a siren, which slides from a higher to lower pitch as it passes an observer. How much the frequency changes depends on how fast the object is moving toward or away from the receiver. It is named for Austrian physicist and mathematician Christian Andreas Doppler (1803-1853), who described the phenomenon in 1842.

dwarf galaxy

The most common galaxy type, often found as companions to larger galaxies. They typically have low masses (less than a billion Suns), small sizes (33,000 light-years or less), and low luminosities that make them difficult to detect. The Large and Small Magellanic Clouds, visible in the Southern Hemisphere, are two of many satellite galaxies that orbit our Milky Way; they are classified as dwarf irregular galaxies. Two other forms, dwarf ellipticals and dwarf spheroidals, also exist.

dwarf planet

A celestial body within the solar system that is like a planet in that it orbits the Sun directly (it is not a moon) and has enough mass to have a spherical or nearly spherical shape. Unlike a planet, however, a dwarf planet is not massive enough to clear its orbital path of similar-sized objects. Dwarf planets include Ceres, Pluto, Eris, Makemake, and Haumea.

dwarf star

With no other qualification, the term refers to a main sequence star. Dwarfs have average or low luminosity, mass, and size, range in color from blue to red, with corresponding surface temperatures from above 17,000 F (9,400 C) to 4,000 F (2,200 C). Red dwarfs, the least massive and luminous stars, are also the most numerous, accounting for 75% of the stars in our galaxy; they have masses from 8% to 60% of the Sun's and spectral types K or M. Orange dwarfs have masses between 60% and 90% of the Sun's, have surface temperatures between 6,500 F (3,600 C) and 9,000 F (5,000 C) and spectral type K. Yellow dwarfs, which includes the Sun, have a mass range from 84% to 115% of the Sun's, surface temperatures of 5,300 to 6,000 K, and spectral type G. For rare, hot, blue O- and B-type stars, the term dwarf is generally not used since these stars have from 2 to more than 90 times the Sun's mass and surface temperatures that reach over 50,000 F (30,000 C). Other exceptions are white dwarfs — stellar remnants that no longer produce energy in their centers — and brown dwarfs, which are substellar objects incapable of producing energy via the hydrogen fusion processes occurring in main-sequence stars.

E

Earth

The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometers (93 million miles) away from the Sun. Earth has one satellite, "the Moon."

eccentricity (e)

The amount by which an elliptical orbit deviates from a perfect circle, which has an eccentricity of 0. Elliptical orbits have values between 0 and 1, parabolic orbits have a value of exactly 1, and hyperbolic orbits have values exceeding 1. Eccentricity is the ratio of the distance between the focus points and the major axis of the ellipse. Equivalently, the eccentricity is (r_a– r_p) / (r_a+r_p), where r_a is the apoapsis distance and r_p is the periapsis distance.

eclipse

The total or partial obscuring of one celestial body by another, as when the Moon blocks the Sun during a solar eclipse, or the passage of one celestial body into the shadow of another, as when the Moon passes through Earth's shadow during a lunar eclipse. Orbiting objects such as binary stars or stars with exoplanets will be seen to eclipse their companions once every orbital period if the plane of their orbits lies edge-on toward Earth.

eclipse depth

The measured decrease in brightness of a system that occurs when one object in the system moves in front or behind the other. For example, the decrease in brightness of a star-planet system that occurs when the planet moves in front of the star (transit or eclipse) or when the planet moves behind the star (secondary eclipse). Eclipse depth is commonly presented in terms of the portion of light that is blocked in percent or parts per million.

ecliptic

A great circle on the celestial sphere representing the Sun's apparent path during a year (so called because eclipses of the Sun or Moon can occur only when the Moon is close to this path). It is also the plane of Earth's orbit around the Sun.

Eddington limit

The theoretical maximum luminosity a body, such as a star, can attain when the outward force of its radiation balances the inward force of its gravitational pull. A body exceeding this luminosity will emit an intense outflow (called a stellar wind) from its outer layers. Supernovae, novae, gamma-ray bursts, and accreting black holes in active galaxies can briefly exceed their Eddington luminosities, producing short, intense outflows. Named for English astronomer and physicist Sir Arthur Stanley Eddington (1882-1944).

ejecta

Material that is material that is forced or thrown out, especially as a result of a volcanic eruption, impact on the surface of a planet or Moon, or stellar explosion.

electromagnetic radiation (waves)

Light (electromagnetic radiation) in all of its forms, including radio, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Electromagnetic radiation moves at the speed of light and is a traveling periodic fluctuation of an electric and magnetic field. These fields oscillate at right angles to each other and to the wave's travel direction.

electromagnetic spectrum

The full range of frequencies, from radio waves to gamma rays, that characterizes electromagnetic radiation (light).

electron

A negatively charged particle commonly found in the outer layers of atoms. The electron has only 0.0005 the mass of the proton.

electron volt (eV)

A unit of energy equal to the work required to accelerate an electron through a potential difference of one volt. In high-energy astrophysics, it is more common to refer to the energy of photons than their corresponding wavelengths. Visible light has energies ranging from 2-3 eV. X-rays have energies thousands of times greater, measured in kiloelectron volts (keV), while gamma rays can be measured in millions (MeV), billions (GeV) and trillions (TeV) of electron volts. These units are also used when referring to the energy of accelerated particles.

element

A substance made from the same kind of atoms that cannot be chemically interconverted or broken down into simpler substances. Elements are primary constituents of matter, with 118 known. Atoms with the same number of protons (atomic number) in the nucleus have identical chemical properties. Hydrogen and helium are the simplest and most abundant elements in the universe, making up about 74% and 24% of all matter, respectively. Heavier elements — often collectively called "metals" in astronomy — comprise only about 2% of the Milky Way's disk, but their presence can greatly influence astronomical phenomena.

elementary particle

Particles with no measurable internal structure and thought to be the basic building blocks of the universe. There are elementary fermions (six leptons — including electrons and neutrinos — and six quarks, plus all their antimatter partners) and elementary bosons, which carry forces. Other particles, such as the protons and neutrons in atomic nuclei, are built from two or more quarks. The force carriers are the photon (electromagnetism), the W and Z bosons (the weak force, responsible for radioactive decay), the gluon (the strong force, which binds atomic nuclei together), and the still hypothetical graviton (gravity). Additionally, the Higgs boson, discovered in 2012, gives rise to the masses of elementary particles.

ellipse

A curve on a plane surrounding two focal points, such that the sum of the distances from both of these points is constant for all points on the curve. A circle is a special type of ellipse where both focal points are located in the same spot. The orbits of the planets are non-circular ellipses, a fact first recognized by German astronomer Johannes Kepler (1571-1630) through analysis of careful observations of Mars collected by the Danish astronomer Tycho Brahe (1546-1601).

elliptical galaxy

A galaxy with a spherical or ellipsoidal shape characterized by little structure, little gas and dust, and populated by older, redder stars than those in spiral galaxies. Astronomers suspect that most elliptical galaxies formed from the collisions and subsequent mergers of spiral galaxies, which are more common than ellipticals.

emission

The process in which atoms or molecules lose the energy delivered by electromagnetic radiation that strikes them. When an atom is excited to a higher-energy state, it gives off light in order to return to its previous state.

emission line spectrum

A spectrum consisting of bright lines at certain wavelengths separated by dark regions in which there is no light.

emitted light

Light that is given off by an object or material. Emitted light is light that originates in the object or material itself. The light that we observe from stars is emitted light — light that is generated by the star itself. Light bulbs, fires, and fireflies emit visible light.

equatorial coordinates

A celestial coordinate system for locating any position in the sky that is tied to Earth's position in space commonly used to locate the positions of astronomical objects. Declination, analogous to latitude on Earth, has values from +90° degrees to -90°, corresponding to the north and south celestial poles, with 0° at the celestial equator. Right ascension, the astronomical equivalent of longitude on Earth, is measured eastward along the celestial equator from the vernal equinox, customarily given in hours, minutes, and seconds, with 24 hours being equivalent to the full circle. Because of the slow wobble of Earth's rotation axis called precession, the equatorial coordinates of a fixed object drift with time, so each position is also tied to a specific year, or epoch.

era of reionization (epoch of reionization, EOR)

More than 13 billion years ago, the gas between galaxies was largely opaque to energetic light, making it difficult to observe young galaxies. It's not yet known what allowed the universe to become completely ionized, or transparent, which eventually led to the "clear" conditions detected in much of the universe today.

erg/sec

A form of the metric unit for power. It is equal to 10^–10 kilowatts. See also: scientific notation.

European Space Agency (ESA)

An intergovernmental organization of 22 European states dedicated to the exploration of space, established in 1975, and headquartered in Paris. ESA maintains and operates the Guiana Space Centre, a major spaceport near Kourou, French Guiana.

event horizon

The distance from a black hole where the velocity required to escape it equals or exceeds the speed of light, the fastest possible speed known, and from within which nothing can escape. The event horizon is a black hole's "point of no return."

excited state

A higher-energy state of any molecule, atom, or nucleus relative to its previous lower-energy state or its minimum-energy (ground) state. Excitation is usually brief and quickly followed by the emission of energy (such as photon) and a return to the previous lower-energy state.

exoplanet

A planet beyond our solar system, typically one orbiting a star. Some exoplanets move freely through space, unbound to a star, and are often called starless, free-floating, or rogue planets.

exposure

The process of allowing electromagnetic radiation to fall on light-sensitive materials such as photographic films or plates. An exposure is also the image created by the process. A long exposure time is needed in order to obtain an image of dim and distant celestial objects.

extragalactic

Beyond our own galaxy, the Milky Way.

extraterrestrial

Beyond Earth.

F

far-infrared light

The region of the infrared spectrum that is closest to microwave or radio light. Far-infrared light has the longest wavelengths and lowest frequencies and energies of any infrared range. Webb's science instruments cannot detect far-infrared light.

Fermi acceleration

A mechanism to accelerate charged particles to the high energies observed in cosmic rays, proposed in 1949 by Italian-born physicist Enrico Fermi (1901-1954). In a magnetic field, a charged particle moves in a trajectory that wraps around a magnetic field line. Areas of weaker and stronger fields can behave like mirrors, reversing the particle's direction. If the line itself is moving, as in the turbulent shock wave of a supernova, the particle gets an energy boost, like a rubber ball bouncing off of a wall moving toward it. Repeated interactions keep accelerating the particle until it has enough energy to completely escape the magnetic field and freely travel through space.

field of view (FOV)

The viewing area of a telescope, expressed in degrees, arcminutes, or arcseconds. A telescope able to show the entire disk of the full moon has a field of view at least 30 arcminutes across. Often abbreviated FOV.

filter

A device that selectively absorbs some colors of light while allowing others to pass through. Astronomers use filters to observe how celestial objects appear in certain colors of light or to reduce the light of exceptionally bright objects.

fission

A nuclear process that releases energy when heavyweight atomic nuclei break down into lighter nuclei. Fission is the basis of the atomic bomb.

fluorescence

The emission of light by a substance that has absorbed light or other electromagnetic radiation. Typically, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Ions of carbon monoxide (CO) in the gas tails of comets absorb ultraviolet sunlight, then emit bluish light at a wavelength of 420 nm.

flux

A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area. Because the energy is measured per time and area, flux measurements make it easy for astronomers to compare the relative energy output of objects with very different sizes or ages.

frequency

A property of a wave that describes how many wave patterns or cycles pass by a fixed point in a given period of time. Frequency is often measured in hertz (Hz), where a wave with a frequency of 1 hertz completes 1 cycle each second.

fusion

The process in which atomic nuclei collide so fast that they stick together and emit a large amount of energy. In the centers of most stars, hydrogen fuses into helium. The energy emitted by fusion supports the star's enormous mass from collapsing in on itself and causes the star to glow.

G

galactic center

The center of a galaxy. The center of our Milky Way galaxy lies 28,000 light-years away in the direction of the constellation Sagittarius and contains a supermassive black hole.

galactic coordinates

A celestial coordinate system, analogous to longitude and latitude on Earth, with the Sun as its center, its central longitude aligned with the approximate center of the Milky Way, and its equator parallel to an approximation of the galactic plane but offset to its north (since the Sun lies about 57 light-years north of the actual midplane). It is often used to specify the positions of objects in our galaxy to provide their context relative to other galactic structures. Galactic latitude ranges from -90° to +90° (below and above the galactic plane respectively), and galactic longitude ranges from 0° (the galactic center) to 360° measured eastward from the galactic center along the galactic equator. The galactic equator is inclined by 63 degrees to Earth's equator.

galactic halo

An extended, roughly spherical galaxy component that extends its main visible features. It includes a low-luminosity stellar halo of typically old stars and globular clusters; hot, ionized gas, a feature called the galactic corona; and dark matter. Stellar halos surround most disk galaxies and some elliptical galaxies.

galaxy

A collection of stars, gas, dust, and dark matter bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions. The Milky Way galaxy contains our solar system. Galaxies are classified or grouped by their shape. Round or oval galaxies are elliptical galaxies and those showing a pinwheel structure are spiral galaxies. All others are called irregular because they do not resemble elliptical or spiral galaxies.

galaxy cluster

Groupings of hundreds to thousands of galaxies and the largest objects in the universe held together by their own gravity. The largest galaxies in the universe, which also host supermassive black holes, are found in the centers of galaxy clusters. The galaxies of a cluster make up only about 5% of its total mass. Hot plasma accounts for another 15% of its mass and makes them bright X-ray sources. The remaining mass is in the form of invisible dark matter. A galaxy cluster’s enormous mass warps the space-time around it, bending the path of more distant light passing through it (called gravitational lensing). This makes galaxy clusters giant telescopes that magnify background galaxies too faint for astronomers to see otherwise.

galaxy evolution

The study of the formation of the first galaxies, the way galaxies change over time, and the processes that have given rise the variety of structures observed in nearby galaxies.

galaxy group

The smallest collection of galaxies held together by their own gravity, typically containing a few to a few dozen members. Our Milky Way galaxy is part of the Local Group, which includes two other large spirals (the Andromeda and Triangulum galaxies), their satellites, and numerous other dwarf galaxies.

galaxy supercluster

A vast collection of smaller galaxy clusters or galaxy groups that can span hundreds of millions of light-years, making them among the largest-known structures in the universe. The Milky Way is part of the Local Group, which in turn is part of the Virgo Supercluster, which is part of the Laniakea Supercluster, which is part of the Pisces-Cetus Supercluster Complex.

gamma rays

The highest-energy, shortest-wavelength light, usually thought of as any light having energies greater than about 500 keV (hundreds of times the energy of visible light). They can penetrate materials opaque to X-rays. Gamma rays are emitted by some radioactive elements (such as cesium-137 and americium-141) and are produced by various astronomical processes, including cosmic ray interactions, neutron star mergers, and the formation of black holes in collapsing stars.

gamma-ray burst (GRB)

The most powerful class of explosion in the universe, GRBs are brief, powerful blasts of gamma rays originating in distant galaxies. They occur randomly and can appear anywhere in the sky. In general, longer GRBs occur when a massive star collapses, shorter bursts occur when neutron stars and/or black holes collide. Such events produce high-speed jets of matter that emit gamma rays, which can be detected by satellites when one jet points in Earth's direction.

gas

One of the three states of matter, in which atoms, molecules, ions, and electrons are far apart and move freely, allowing the substance to expand and fill both the shape and volume of any container.

gas giant (Jovian planet)

A large planet composed mostly of hydrogen and/or helium. These planets, like Jupiter and Saturn in our solar system, don't have hard surfaces and instead have swirling gases above a solid core. Gas giants can be much larger than Jupiter, and much closer to their stars than anything found in our solar system.

general relativity

Theories developed by German-born physicist Albert Einstein (1879-1955) that are central to modern science. The special theory of relativity (1905) is based on the principles that all constant, straight-line motion is relative, that light has the same speed in a vacuum for all observers, and that space-time is a four-dimensional continuum. From it emerged the equivalence of matter (m) and energy (E) as described by the famous equation E=mc², where c is the speed of light. The general theory of relativity (1916) extends the special theory to systems that are accelerating with respect to one another, covering gravitation and the curvature of space-time. It predicted the existence of black holes, gravitational waves, and gravitational lensing.

giant molecular cloud (GMC)

A cold, dense cloud primarily composed of hydrogen molecules that blocks all visible light behind it. Most star formation occurs within these clouds, which can sport temperatures below -280 F (-180 C) and span several hundred light-years.

giant star

An evolved star. Giant stars are brighter and have a larger radius than main sequence stars of the same temperature. They have exhausted their hydrogen fuel to fuse in their cores, which puffs up their outer layers.

globular cluster

A large collection of old stars closely packed in a roughly spherical shape. They can contain hundreds of thousands to millions of stars, including some of the oldest yet observed, in a ball typically less than 300 light-years across. The Milky Way hosts at least 150 globular clusters.

granite

A light-colored igneous rock made of silica-rich minerals like quartz and feldspar. Earth's continents are granitic in composition, with a mineral and chemical composition similar to granite.

gravitational clustering

The process by which a large-scale structure grows as its gravity attracts smaller building blocks. Astronomers believe that all the large-scale structures (such as galaxies, galaxy clusters, and galaxy superclusters) that we see in the universe today formed through gravitational clustering.

gravitational collapse

When a massive body collapses under its own weight. For example, interstellar clouds collapse to become stars until the onset of nuclear fusion stops the collapse.

gravitational lensing

A phenomenon where a concentration of matter, such as a star or a galaxy cluster, distorts and magnifies the light of more distant objects passing through it. Einstein's general theory of relativity describes how masses distort the space-time around them. Light from more distant objects passing through this region must travel through the distorted space-time. This allows astronomers to detect planets around stars that would otherwise be undetectable (microlensing), and on larger scales allows researchers to study the details of galaxies that would otherwise be too far away to be seen with current telescopes.

gravitational waves

Ripples in space-time caused by the acceleration of massive objects. The first directly detected sources include binary systems where orbiting black holes or neutron stars merged together. Gravitational waves are also thought to emanate from the motion of matter during the big bang.

gravitationally bound

Objects held in orbit about each other by their gravitational attraction. For example, satellites in orbit around Earth are gravitationally bound to it because they can't escape Earth's gravity. By contrast, the Voyager spacecraft were launched with enough energy to escape the Sun's gravity and hence are not gravitationally bound.

gravity

The universal force of attraction between all bodies of matter. The more massive the object, the stronger its gravitational attraction. The gravitational force between objects diminishes with the inverse square of the distance between them, so if their distance is doubled, the force weakens to one-fourth its original strength. Gravity is the weakest of the four fundamental forces in nature.

H

habitable zone

The range of orbits around a star where a planetary surface can support liquid water given sufficient atmospheric temperature and pressure; sometimes referred to as the Goldilocks zone (the extent that's "just right" for the presence of water). Since liquid water is a key ingredient for life as we know it, exoplanets in other stars' habitable zones become compelling places to search for life. Scientists estimate that the present-day habitable zone of the solar system extends from about 10% closer to the Sun than Earth's orbit to 50% beyond it (0.9 to 1.5 AU); it may have been larger in the past, when the young Sun was more luminous. Mars is on the outer edge of the present-day habitable zone, making it a compelling planet in the search for evidence of past life. Since the habitable zone definition only applies to a planet's surface, liquid water below the surface may exist well beyond a star's formal habitable zone, such as the internal ocean under the crust of Jupiter's moon Europa.

hadron

A subatomic particle that experiences the strong interaction and is built from two or more quarks, including baryons (such as protons and neutrons) and mesons. All observed subatomic particles are hadrons except for the force-carriers of the fundamental interactions (gauge bosons) and leptons (which includes electrons and neutrinos).

Hawking radiation

A theory first proposed in 1974 by British physicist Stephen Hawking (1942-2018) holding that, due to a combination of properties of quantum mechanics and general relativity, black holes emit radiation in the form of light or other particles from near their event horizons. Hawking showed that the amount of energy released by a black hole in this way is inversely proportional to its mass, which means that the higher the black hole’s mass, the smaller its energy release and temperature (Hawking temperature). A black hole of the Sun's mass might have a temperature of 100-millionth of a kelvin (10^-8 K), with more massive black holes being even colder. If matter or energy does not enter to offset the emission of Hawking radiation, a black hole's mass and the diameter of its event horizon gradually shrinks until it vanishes in a final flash of radiation. For a black hole the mass of the Sun, the time for evaporation is enormous — 10⁶⁴ years, which is far greater than the current age of the universe. The cosmic microwave background, which has a higher temperature than such black holes, provides photons to them that will offset losses by Hawking radiation until cosmic expansion cools the universe below the black holes' Hawking temperature.

helium

The second-lightest element, symbol He, and second-most abundant element in the observable universe, after hydrogen. Helium was formed in the early universe and is responsible for about 24% of the total mass of elements. Today, helium forms as a result of the hydrogen fusion process occurring in stars. A neutral helium atom has a nucleus containing two protons and, usually, two neutrons, together with two electrons orbiting the nucleus. Some elements (uranium, thorium, radium, etc.) emit helium nuclei (called alpha particles) as part of their radioactive decay.

hertz (Hz)

A unit of frequency, defined as a frequency of 1 cycle per second, named for Heinrich Hertz (1857-1894), who first demonstrated the existence of electromagnetic waves. For higher frequencies, the unit is commonly expressed in multiples: a thousand (kilohertz, kHz), a million, (megahertz, MHz), a billion (gigahertz, GHz), and a trillion (terahertz, THz).

horizontal coordinate system

A celestial coordinate system that uses the observer's local horizon as a reference plane to define the position of a point on the celestial sphere at a given time by its angular distance above the horizon (altitude) and the angular distance measured east from north and parallel to the horizon (azimuth); also called the alt/az system. Altitude ranges from 0° at the horizon to 90° at the zenith. Azimuth ranges from 0° (due north), to 90° (due east), to 180° (due south), and 270° (due west). Because the system is fixed to the rotating Earth and not the stars, the coordinates of an object change with time.

hot Jupiter

A class of gas giant exoplanets thought to have properties similar to Jupiter but that orbit their stars in less than 10 days. Such close distances to their host stars result in high surface temperatures, hence the name. The first confirmed exoplanet orbiting a main-sequence star, discovered in 1995, is the hot Jupiter 51 Pegasi b, now officially named Dimidium, which orbits its star every 4.2 days.

Hubble constant (H_o)

A number that expresses the rate at which the universe expands with time. Its value is measured to be between 60 and 75 kilometers per second per megaparsec. It is named for American astronomer Edwin Hubble (1889-1953).

Hubble's law

A relationship between a galaxy's distance from us and its velocity through space, which is interpreted as evidence of the expanding universe. The farther away a galaxy lies from us, the faster it's receding away from us. It is named for American astronomer Edwin Hubble (1889-1953).

hydrogen

The lightest smallest atom, symbol H. Hydrogen formed in the early universe and is its most abundant element, making up about 75% of its normal matter. A hydrogen atom consists of just one proton and one electron.

I

ice giant

A giant planet that typically has an atmosphere dominated by hydrogen and helium, and a core made up of rock and heavier metals. The ice giants in our solar system are Uranus and Neptune, which are primarily made of oxygen, carbon, nitrogen, and sulfur.

implosion

A sudden, violent collapse. When the core of a massive star runs out of fuel, there is no longer any force countering its own gravity, so the core collapses under its own weight. This implosion may create a neutron star or black hole and generate an outward-moving shock wave that forms a supernova, heating and ejecting the overlying gas layers.

inclination

1. The angle between the orbital plane of a planet, comet, or other body, and the ecliptic.
2. The angle between the orbital plane of a moon or satellite and the equatorial plane of its primary; the Moon's orbit is inclined 5.16° to Earth's equator.
3. For orbiting systems beyond the solar system, such as those hosting exoplanets or multiple stars, the angle of the orbital plane relative to the plane of the sky, that is, a plane perpendicular to the line of sight from Earth, with 0° being face on and 90° being edge on.

infrared

Invisible light that we sense as radiant heat, with longer wavelengths (0.78 microns to 1 mm) and lower frequencies and energies than visible light. In astronomy, it is subdivided into three regions: the near infrared (nearest to visible light) ranges from 0.78 to about 2.5 microns; the mid-infrared spans 2.5 to 50 microns; and the far infrared includes wavelengths from 50 to 1,000 microns. (Although some astronomers place the cutoff at 300 microns.) Water vapor absorbs a significant amount of infrared and the atmosphere itself glows at these wavelengths, but some regions can be observed by telescopes atop mountains or aboard high-altitude aircraft, balloons, or rockets where they are above most of the atmosphere's water vapor. The entire infrared range can be detected by observatories in space.

infrared telescope

An instrument that collects the infrared radiation emitted by celestial objects. There are several Earth- and space-based infrared observatories.

interacting galaxies (colliding galaxies)

Two or more galaxies that pass close enough to gravitationally disrupt each other's shapes. The collision rips streamers of stars from the galaxies, fuels an explosion of star formation, and can ultimately result in both galaxies merging into one.

intermediate-mass black holes

This black hole is estimated to have a mass between 100 and 1,000 solar masses. No single star could ever form such a heavy black hole. Instead, researchers suggest these black holes form either by devouring lots of material or individual black holes merge.

interstellar dust

Small particles of solid matter, similar to smoke, in the space between stars. Interstellar dust reflects and reddens visible starlight, and in large quantities absorbs ultraviolet light. Longer wavelengths of infrared light can penetrate interstellar dust.

interstellar medium

A collective term for the gas (in ionized, atomic, and molecular form), dust particles, light, and cosmic rays located between the stars of a galaxy.

invisible radiation

Forms of light (electromagnetic radiation) that human eyes cannot detect, including gamma rays, X-rays, ultraviolet light, infrared light, microwaves, and radio waves.

ion

An atom with one or more electrons removed (or added), giving the atom a net positive (or negative) charge. Gases including large numbers of such atoms are said to be ionized.

ionization

The process by which ions are produced, typically by collisions with other atoms or electrons, or by absorption of electromagnetic radiation.

ionosphere

A region of Earth's upper atmosphere containing abundant ions and free electrons mainly formed by ultraviolet light and X-rays from the Sun. The term is also used to describe corresponding regions above the surfaces of other planets. The ionosphere is important because it reflects and modifies radio waves used for communication and navigation. More precisely, the ionosphere consists of three distinct regions, known as the D, E, and F layers, embedded within standard atmospheric layers called the mesosphere and the thermosphere. The D layer ranges from about 60 km (37 miles) to 90 km (56 miles) altitude, the E region extends from around 90 km (56 miles) up to 150 km (93 miles) high, and the F layer ranges from altitudes of about 150 km to up to 600 km (373 miles). The height, fraction of ionized particles, and even the existence of these regions varies over time, with significant differences between day and night, as well as over the course of the Sun's 11-year sunspot cycle.

irregular galaxy

A galaxy that appears disorganized and disordered, without a distinct spiral or elliptical shape. Irregular galaxies are usually rich in interstellar matter, such as dust and gas. The Large and Small Magellanic Clouds are examples of nearby irregular galaxies.

isolated planetary-mass object

An object that has the mass of a planet, but does not orbit a star. Some isolated planetary-mass objects are probably brown dwarfs, while others could be free-floating planets. The term is typically used when researchers are not certain how to classify the object.

J

Japan Aerospace Exploration Agency (JAXA)

The Japanese national aerospace and space agency, formed in 2003 through the merger of three previously independent organizations. JAXA operates the Tanegashima Space Center, the nation's largest spaceport.

jets

Narrow, high-energy streams of gas and other particles generally ejected in two opposite directions from some central source. Jets appear to originate in the vicinity of an extremely dense object, such as a black hole, pulsar, or protostar, with a surrounding accretion disk. The jets are thought to emerge perpendicular to the plane of the accretion disk. In active galaxies and gamma-ray bursts, the jets contain beams of particles moving near the speed of light and emit gamma rays.

K

Kelvin temperature scale (K)

A kelvin is the fundamental measure temperature. Water freezes at 273 kelvins (not degrees), which equals 32 degrees Fahrenheit or 0 degrees Celsius. The scale is frequently used in science and is named for the Scottish physicist Sir William Thompson (1824-1907, also known as Lord Kelvin), who devised it in 1848. A change of 1 kelvin (1 K) is the same magnitude as a temperature change of 1 C, but the Kelvin scale measures from absolute zero, the coldest possible temperature. The boiling point of water is 373 K (212 F or 100 CF), and the Sun's effective temperature is 5,772 K (9,930 F or 5,499 C).

Kepler's laws

A set of three scientific laws describing the motion of planets around the Sun published by Johannes Kepler (1571–1630), a German astronomer and mathematician. They define the shape of planetary orbits, explain how a planet's velocity changes during an orbit, and relate a planet's orbital period to its distance from the Sun.

1. All planets move along elliptical orbits with the Sun located at one focus. 
2. During each orbit, a line connecting a planet with the Sun sweeps out equal areas in equal time intervals.
3. The square of the planet's orbital period (P) is proportional to the cube of its semimajor axis (a, or half the longest width of its elliptical orbit): P² ∝ a³.

kilogram (kg)

The fundamental unit of mass in the sciences. One kilogram is equivalent to 1,000 grams or about 2.2 pounds; the mass of a liter of water.

kilonova

A transient astronomical event following the merger of two compact objects, either two neutron stars or a neutron star and a black hole. The expanding debris generates ultraviolet, visible, and infrared light from heating supplied by the decay of radioactive elements heavier than iron that formed during the merger. Kilonovae are about 1,000 times brighter than novae and about 1,000 times less bright than supernovae. These elements form via rapid neutron capture (called the r-process), which is responsible for the creation (nucleosynthesis) of approximately half of the atomic nuclei heavier than iron, including platinum and gold. Current models suggest that a single neutron star merger event may produce several Earth masses of gold alone. On Aug. 17, 2017, the neutron star merger GW170817 was detected via both gravitational waves and, within two seconds, a gamma-ray burst — the first detection of light from any gravitational wave source and a confirmation of the link between such mergers and short gamma-ray bursts. The associated kilonova was extensively studied and represents the first direct observation of r-process nucleosynthesis in the cosmos.

kinematics

Often referred to as the "geometry of motion," the term refers to the calculation or description of the underlying mechanics of motion of an astronomical object. For example, in radioastronomy, spectral line graphs are used to determine the kinematics or relative motions of material at the center of a galaxy or surrounding a star as it is born.

Kirchhoff's spectroscopy laws

Rules that describe the kinds of spectra seen based on the state, density, and temperature of a substance. Named for the German physicist Gustav Robert Kirchhoff (1824-1887), who summarized these relationships in 1859.

1. A hot object or dense gas will emit radiation and produce a continuous spectrum.
2. A hot, low-density gas in front of a cooler background will emit radiation at a discrete set of isolated wavelengths (an emission-line spectrum).
3. A continuous spectrum viewed through a cool, low-density gas will absorb light from the background source at discrete wavelengths (an absorption-line spectrum).

Kuiper Belt (object)

A region in our outer solar system where many "short-period" comets originate. The orbits of short-period comets are less than 200 years. This region begins near Neptune's orbit at 30 astronomical units (AU) and extends to about 50 AU away from the Sun. An astronomical unit is the average distance between Earth and the Sun. The Kuiper Belt may have as many as 100 million comets.

L

Lagrange points

One of five locations (denoted L1 through L5) in the vicinity of two massive bodies (such as Earth and the Moon) where a third body, such as an asteroid or spacecraft, can remain. The Sun-Earth L1 point, a location used by spacecraft monitoring the Sun, is about 930,000 miles (1.5 million kilometers) closer to the Sun than Earth. Sun-Earth L2, located at the same distance in the opposite direction, is used by the James Webb Space Telescope and other astronomy missions. Perhaps as many as a million asteroids have collected in the Sun-Jupiter L4 and L5 points; called trojan asteroids, they occupy two regions that lead and follow the planet by 60 as it orbits.

laser

A device that can emit a very intense, narrow, parallel beam of highly monochromatic and coherent light by using light to stimulate excited atoms or molecules to emit more light of the same wavelength and phase. Natural lasers have been found where irradiated planetary or stellar gases similarly amplify light, such as in the atmospheres of Mars, Venus, and the edge-on disk of the binary system MWC 349. Similar phenomena occurring at microwave wavelengths are called masers.

light

Without qualification, the common term for electromagnetic radiation, usually referring to that portion visible to the human eye (visible light). However, other forms of electromagnetic radiation are often referred to as different forms of light.

light curve

A graph showing how the brightness of an object, such as a star or galaxy, varies over time.

light-year

A unit of length equal to the distance light travels in a year. In a vacuum, light travels at 671 million mph (1.1 billion kph), so 1 light-year equals about 5.8 trillion miles (9.5 trillion kilometers).

limb

The edge of the apparent disk of a celestial body.

Local group

The group of galaxies that includes our Milky Way, the Andromeda galaxy (M31), the Triangulum galaxy (M33), all their satellite galaxies, and numerous other dwarf galaxies. Some 80 members are currently recognized.

luminosity

The total amount of energy radiated into space every second by a star, galaxy, or other celestial object.

Lyman lines

A set of ultraviolet emission or absorption lines in the spectrum of hydrogen that arise from electron transitions between the atom's ground, or lowest energy, state and those at higher energies, discovered by the American physicist Theodore Lyman IV (1874–1954). The first three lines are identified by combining Lyman with the spelled-out name of the first Greek letters — alpha, beta, or gamma — or by combining the abbreviation Ly with the respective lowercase Greek letter — α, β, or γ. The Lyman-alpha line (121.56 nm) is of particular importance in cosmology, where it can be used to identify and confirm the distances of galaxies with high redshifts, to measure how galaxies and the gases within and surrounding them evolve, and to probe reionization.

M

Magellanic Clouds

Two irregular galaxies that share a gaseous envelope and orbit each other and our Milky Way galaxy. They lie about 21° apart in the southern sky near the south celestial pole and are visible to the unaided eye from the Southern Hemisphere. The closest of the pair, the Large Magellanic Cloud (LMC) is located about 163,000 light-years away and measures about 32,000 light-years across, about twice the size of its companion. On February 24, 1987, supernova 1987A — the closest supernova seen in more than 400 years and the only one from which neutrinos have been detected — exploded in the LMC. The Small Magellanic Cloud (SMC) lies about 206,000 light-years away and is about 18,000 light-years across. They have undergone tidal interactions with the Milky Way and with each other and remain valuable laboratories for studying star formation, stellar evolution, and galaxy formation. While studying variable stars in the SMC, Henrietta Leavitt (1868-1921) discovered the period-luminosity relation for Cepheids, which remains an important tool for establishing cosmic distances. The LMC and SMC were named for the Portuguese navigator Ferdinand Magellan (1480-1521), whose crew brought them to the attention of European astronomers following the first voyage around the world (1519-1522).

magnetar

A type of neutron star possessing an unusually intense magnetic field between 100 and 1,000 times stronger than that of a typical pulsar. Astronomers have identified 30 magnetars.

magnetic field

A description of the strength of the magnetic force exerted by an object. Bar magnets have dipolar fields, as the force is exerted from the two ends of the bar. In simple terms, Earth, the Sun, and stars have dipolar magnetic fields.

magnetic pole

Either of two limited regions in a magnet at which the magnet's field is most intense. The two regions have opposing polarities, which we label "north" and "south," after the two magnetic poles on Earth.

magnetosphere

The region of space in which the magnetic field of an object (e.g., a star or planet) dominates the radiation pressure of the stellar wind to which it is exposed.

magnetotail

The portion of a planetary magnetosphere which is pushed in the direction of the solar wind.

magnitude

The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest star visible without a telescope is of magnitude 6. A decrease of one magnitude represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.

main asteroid belt

A formal name for the asteroids that lie between Mars and Jupiter. They are airless rocks that failed to adhere to one another to become larger bodies as the solar system was forming 4.6 billion years ago.

main sequence star

A star that produces energy primarily by fusing hydrogen to helium in its core, as does the Sun. Main sequence stars, also called dwarfs, make up around 90% of the stars in the universe. They range in luminosity, color, and mass — from 8% to 200 times the Sun’s — and remain in this stage for most of their lives, billions of years in the case of the Sun, millions of years for stars at the high-end of the mass range.

mass

The quantity of matter an object contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field.

matter

Any substance that occupies space and has mass.

megaparsec (mpc)

Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between galaxies.

megaton

A unit of energy typically used to describe the yield of nuclear warheads, equivalent to the explosive force of 1 million tons of TNT. The explosion associated with the meteor airburst of the Tunguska event in 1908 has been estimated at between 3 and 50 megatons. The impact of fragment G of Comet Shoemaker-Levy 9 with Jupiter in 1994 was estimated to have released an energy equivalent of 6 million megatons.

meson [MEH-zon]

An unstable subatomic particle in the hadron family composed of a quark and an antiquark. More than 200 types are known, and all decay in less than one ten-millionth (10–7) of a second. Mesons were predicted in 1935 by the Japanese physicist Yukawa Hideki (1907-1981). The first to be discovered, the charged pions, were found through cosmic ray interactions in 1947.

metal

In astronomy, any element heavier than helium. Metallicity is the abundance of heavy elements in celestial objects.

meter (m)

The fundamental SI unit of length, defined as the length traveled by light in vacuum during a period of 1/299,792,458th of a second, and equal to 39.37 inches. A kilometer (km) is equal to 1,000 meters (0.6214 miles).

methane

A chemical compound consisting of one atom of carbon and four of hydrogen (CH4). On Earth, methane is a gas produced both naturally and through human activity, but it is also common elsewhere in the solar system. The atmospheres of Uranus and Neptune have 2% and 1.5% methane, respectively. Saturn's moon Titan, the largest moon in the solar system and the only one with a dense atmosphere, also contains methane in its atmosphere, and methane rain even pools into lakes on its surface. In interstellar space, frozen methane is one of the ten most abundant ices detected, and methane gas has also been detected in the atmospheres of exoplanets.

metric system

The decimal measuring system based on the meter, liter, and gram as units of length, capacity, weight or mass, and time. The system was first proposed by the French astronomer and mathematician Gabriel Mouton (1618–1694) in 1670 and was standardized in France in the 1790s.The modern International System of Units (SI) derives from and extends the metric system.

micron (µm)

A unit of length equal to one millionth of a meter (0.000001 meters, or 10-6 meters). A micron is equal to 1,000 nanometers. Also called a micrometer.

microquasar

An object of stellar mass in our own or a nearby galaxy that displays in miniature some of the properties of distant quasars, including radio jets and strong emission across a broad range of wavelengths from radio to X-rays. A microquasar is a binary system that pairs a black hole with a normal star. Matter pulled from the companion forms a hot, X-ray emitting accretion disk around the black hole. Narrow streams (jets) of subatomic particles blast outward from the inner part of the disk at substantial fractions of the speed of light, generating strong radio wave emission.

microwave

Non-visible light with wavelengths between 1 millimeter and 30 centimeters, between shorter infrared and longer radio regions in the electromagnetic spectrum.

mid-infrared light

The region of the infrared spectrum that is between near-infrared and far-infrared light. Mid-infrared light has longer wavelengths and lower frequencies and energies than both visible and near-infrared light. Webb's Mid-Infrared Imager (MIRI) detects wavelengths ranging from 5 to 28 microns.

Milky Way (galaxy)

1. The galaxy in which Earth resides. The Milky Way is a barred spiral galaxy with a diameter estimated at about 100,000 light-years and a main disk about 1,000 light-years thick. It contains 100 to 400 billion stars and is estimated to hold at least as many planets. At its center is a supermassive black hole named Sagittarius A* that contains about 4.3 million solar masses.
2. The faintly luminous band encircling the celestial sphere formed from numerous stars too faint to be seen individually. It corresponds to our galaxy's central plane, in which most of its stars are located.

molecule

Two or more atoms bound together, either by sharing electrons (covalent bond) or by the electrostatic attraction between oppositely charged ions (ionic bond). For example, water (H₂O) has two hydrogen atoms bound to one oxygen atom. A molecule is the smallest fundamental unit of a chemical compound that can take part in the compound's characteristic chemical reactions.

momentum

A quantity equal to an object’s mass multiplied by its velocity that is always conserved in closed physical systems. The speed of two objects colliding head-on, for example, will change as a result of the impact, but their total momentum before and after the collision will remain the same. Even light and other electromagnetic radiation has momentum, defined as Planck's constant divided by its wavelength. Angular momentum is a related concept applying to rotating systems.

moon

A large body orbiting a planet. On Earth's only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.

muon

An elementary particle similar to the electron but 207 times heavier. Muons are leptons with a negative charge, and their antimatter partner (antimuons) have the same mass and spin but a positive charge. Muons are the chief constituent of cosmic radiation at Earth's surface. Muons reaching Earth are formed in the atmosphere when high-energy protons and atomic nuclei moving near the speed of light (cosmic rays) collide with air molecules. Muons exist for only 2.2 microseconds before decaying into an electron and two kinds of neutrinos, but because they move at nearly the speed of light, they travel far and can deeply penetrate Earth's surface — hundreds of meters or more — before decaying. Some 10,000 muons strike every square meter of the Earth's surface each minute.

N

nanometer (nm)

A unit of length in the International System of Units (SI), equal to one billionth (10⁻⁹) of a meter. It is often used to express dimensions on an atomic scale, such as the diameter of a helium atom (0.06 nm), and to specify the wavelength of visible light, which ranges from 400 to 700 nm.

NASA

A federal agency created on July 29, 1958, after President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958. NASA coordinates space exploration efforts and conducts aeronautical research.

near-infrared light

The region of the infrared spectrum that is closest to visible light. Near-infrared light has slightly longer wavelengths and slightly lower frequencies and energies than visible light. Webb's near-infrared instruments (NIRCam, NIRISS, and NIRSpec) detect wavelengths between 0.6 and 5 microns, which includes orange and red visible light as well as the full range of near-infrared light.

nebula (plural: nebulae)

A cloud of dust and gas located between or around stars.

neutrino

The most abundant particle with mass in the universe. Neutrinos have extremely small masses (less than a millionth that of an electron), are electrically neutral (unaffected by magnetic fields), and travel at almost the speed of light. Their neutral charge makes them important cosmic “messengers.” Since they are unaffected by magnetic fields, scientist can trace them straight back to their sources, unlike with light or cosmic rays. They come in three "flavors" and can change from one to another as they travel. Neutrinos are difficult to study because they interact so rarely with other matter that trillions of them pass through our bodies each second without leaving a trace. But this means they can escape dense, opaque environments — like the core of a star — that light cannot. Neutrinos have been detected from the Sun, from a supernova (SN 1987A), and from a distant blazar during a powerful flare (TXS 0506+056).

neutron

An electrically neutral particle slightly more massive than a proton, commonly found in the nuclei of all atoms other than hydrogen.

neutron star

The compact remnant core of a massive star that exploded as a supernova. Neutron stars host more than the Sun's mass in a ball about 12 miles (20 kilometers) across. Neutron stars are the second-smallest and second-densest class of compact objects known, surpassed only by black holes. Neutron stars are thought to be composed almost entirely of neutrons because extreme pressure causes the electrons and protons present in normal matter to merge, forming additional neutrons. Neutron stars may be isolated or members of binary systems, and can be observed as radio pulsars, X-ray or gamma-ray pulsars, millisecond pulsars, X-ray bursters, and magnetars.

Newton's law of universal gravitation

Every mass attracts every other mass by a force acting along the line intersecting them. The force of this mutual attraction is proportional to the product of the two masses and inversely proportional to the square of the distance between them. Named for English physicist Sir Isaac Newton (1643-1727), who derived it.

Newton's laws of motion

Three fundamental laws of classical physics developed by English physicist and mathematician Sir Isaac Newton (1642–1727).

1. A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
2. For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
3. In a system where no external forces are present, every acting force is always opposed by another of equal magnitude in the opposite direction.

noise

Random fluctuations that are always associated with a measurement that is repeated many times over. Noise appears in astronomical images as fluctuations in the image background. These fluctuations do not represent any real sources of light in the sky, but rather are caused by the imperfections of the telescope. If the noise is too high, it may obscure the dimmest objects within the field of view.

nova (plural: novae)

A star that experiences a sudden, temporary outburst that increases its luminosity by up to 100,000 times. Novae originate from eruptions on the surfaces of white dwarf stars in close binary systems.

nuclear fission

A reaction in which the nucleus of a heavy element splits spontaneously or on impact with another particle, creating lighter nuclei with the release of energy. Nuclear power plants generate electricity through such reactions.

nuclear fusion

A process whereby several small nuclei combine to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E = mc². This is the source of the Sun's energy.

nucleosynthesis

The creation of new atomic nuclei, either by combining two smaller nuclei (fusion) or by breaking a larger nucleus into multiple smaller ones (fission). Nucleosynthesis first occurred in the first minutes of the big bang and continues today within stars (including the Sun) and other cosmic sources. In the early universe, protons (the hydrogen nucleus) and neutrons combined to make nuclei of deuterium, an isotope of hydrogen. Deuterium nuclei then fused together to make helium nuclei. Further reactions produced lithium. Nuclear reactions within stars and during explosive events at the ends of their lives (such as mergers and supernovae) have formed most of the other nuclei in the universe.

nucleus (plural: nuclei) [NOO-klee-us, NOO-klee-eye]

1. The positively charged core of an atom, composed of protons and (usually) neutrons, that makes up nearly all of its mass and around which electrons orbit.
2. The bright central emitting region of active galaxies (AGN).
3. The solid body of a comet.

O

observable universe

The portion of the entire universe that can be seen from Earth.

observatory

A structure designed and equipped for making astronomical observations. Observatories are located on Earth and in space.

occultation

The concealment of one celestial body by another passing between it and an observer. The Moon may occult a planet or star, for example.

Oort cloud

This is the most distant region of our solar system and is theorized to be a giant spherical shell made of icy pieces of space debris. The Oort Cloud might contain billions, or even trillions, of objects. It ranges from less than half a light-year (about 2,000 AU) to over 1 (10,000 AU) light-year from the Sun. This is where long-period comets originate in our solar system.

opacity

A property of matter that prevents light from passing through it. The opacity of a given material, such as a layer in a star's atmosphere, depends on its chemical composition (some elements absorb light more easily than others), its temperature, and its density.

open cluster

Also known as a galactic cluster, an open cluster consists of numerous young stars that formed at the same time within a large cloud of interstellar dust and gas. Open clusters are located in the spiral arms or the disks of galaxies. The Pleiades is an example of an open cluster.

orbit

1. The regularly repeated elliptical course of a celestial object or spacecraft about a star, planet, or moon; a complete circuit around an orbited body; or the state of being in or moving in an orbit.
2. The path of an electron around an atomic nucleus.

organic compound (molecule)

An organic molecule is normally found in living systems. They are made up of at least one carbon, attached to oxygen, nitrogen, hydrogen, or another element.

P

pair production

The physical process whereby a gamma-ray photon, usually through an interaction with the electromagnetic field of an atomic nucleus, produces an electron and an anti-electron (positron). The original photon no longer exists, its energy having gone into the two resulting particles. The inverse process, pair annihilation, creates two gamma-ray photons from the mutual destruction of an electron/positron pair.

parallax

The apparent motion of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle a star appears to move as Earth orbits from one side of the Sun to the other.

parsec

A unit equal to the distance needed for an object to have a parallax of 1 arcsecond, equal to 3.262 light-years. A kiloparsec (kpc) equals 1,000 parsecs. A megaparsec (Mpc) equals 1 million parsecs.

particle accelerator

Any natural process or device that can accelerate charged particles, such as electrons, positrons, or protons to speeds approaching that of light. Examples include thunderstorms, the Large Hadron Collider in Switzerland, and the shock waves of supernovae.

periapsis

The closest point in the path of an orbiting body; its opposite is apoapsis. The concept has special names in commonly used systems:

• periastron, for a body orbiting a star
• perigee, for a body orbiting Earth
• perijove, for a body orbiting Jupiter
• perihelion, of a body orbiting the Sun

period

The time interval between successive occurrences of the same state in a cyclic phenomenon, such as a wave, rotation, vibration, or brightness changes in a light curve. In astronomy, it commonly refers to the time taken by a celestial object to rotate around its axis or to make one circuit of its orbit around another body.

phase

Regularly occurring changes in the appearance of the Moon or a planet. Phases of the Moon include new, full, crescent, first quarter, gibbous, and third quarter.

phase curve

A graph that shows the change in observed brightness of a star-planet system as the planet orbits the star. A phase curve is a type of light curve that includes the entire orbit and therefore all phases of the planet. (A planet's phase is the portion of its dayside that faces the telescope.) Phase curves can be used to figure out how a planet's surface or atmosphere varies with longitude.

photodissociation

The breakdown of molecules into their component atoms due to the absorption of light. In the photodissociation region in the Orion Nebula known as the Orion Bar, for example, ultraviolet light from young, massive stars strongly influences the gas chemistry of its regions and acts as the most important source of heat. This intense flood of radiation violently breaks apart molecules, ionizes atoms and molecules by stripping their electrons, and heats the gas and dust.

photoelectric effect

A phenomenon in which electrically charged particles (typically electrons) are released from or within a material when it absorbs light; it is the principle that makes solar cells work. The details of the interaction cannot be explained by the classical view of light as an electromagnetic wave. The effect was explained in 1905 by Albert Einstein (1879-1955), who postulated that light is made up of particles called photons. Light with a frequency below a certain threshold, at any intensity, will not trigger the effect, but above that frequency, electrons are emitted in proportion to the intensity of incident light (the number of photons striking the surface). Einstein was awarded the Nobel Prize for physics in 1921 for this explanation.

photometry

Measurement of the properties of light, especially its luminous intensity and how it changes over time. Transit photometry is a method to detect exoplanets by measuring the change in a star's brightness as an orbiting planet passes in front of it.

photon

An elementary particle representing the smallest amount (a quantum) of light and the carrier (gauge boson) of the electromagnetic force. Photons have zero rest mass, no electrical charge, always travel in a vacuum at the speed of light, and carry energy equal to their radiation frequency multiplied by Planck's constant.

photon splitting

A process in which a single very high-energy gamma-ray photon can borrow energy from an external source, such as a strong magnetic field, and split into two lower-energy photons.

pi (π) [pie]

A mathematical constant, approximately equal to 22/7 or 3.14159, that is the ratio of a circle's circumference to its diameter. It is an irrational number — meaning that it cannot be expressed exactly as a ratio of two integers — so its decimal representation neither ends nor enters a repeating pattern.

pion [PIE-on]

Any of three unstable particles — the π⁺, π^-, and π⁰ — making up the lightest mesons. The charged pions (π⁺, π^-) are antiparticles and were discovered in 1947 through cosmic ray collisions in Earth's atmosphere. The neutral pion (π0), discovered in 1950, is its own antiparticle. While charged pions decay into muons and neutrinos, the neutral pion decays into a pair of gamma rays, making it important in high-energy astrophysics.

Planck constant (h), equation, law

A fundamental constant that defines the quantum nature of energy. It relates the energy (E) of a photon, the quantum of electromagnetic radiation, to its frequency (ν) through the equation E = hν (the Planck equation). Its value is defined as 6.62607015 x 10-34 joules per hertz. Named for German theoretical physicist Max Planck (1858-1947), the constant was formulated as part of his work to mathematically describe the frequency pattern emitted by a blackbody when there is no net flow of matter or energy (now known as Planck's law). The law describes how, with increasing temperature, the total radiated energy of a body increases and the peak of its emitted spectrum shifts to shorter wavelengths.

planet

An object that orbits a star. Although smaller than stars, planets are relatively large and shine only by reflected light. Planets are made up mostly of rock or gas, with a small, solid core. In our solar system, the inner planets (Mercury, Venus, Earth, and Mars) are the rocky objects, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) are the gaseous ones.

planetary nebula

Expanding shells of gas ejected from stars like our Sun at the ends of their lives. The gas is ionized by the remaining hot stellar core, a white dwarf, and emits light in the process. The misleading name originates from the round planet-like appearance of these nebulae as seen through early telescopes.

planetary ring (ringed planet, ring system)

A disk or ring orbiting an astronomical object that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets.

planetesimal

A solid object arising from the accretion of other orbiting objects composed of rocks, dust, and ices via gravitational forces. Planetesimals correspond to objects larger than approximately 1 kilometer (0.6 miles) in circumstellar disks like the one from which the solar system's planets formed, and they are regarded as the building blocks of planets.

plasma

Often regarded as the fourth state of matter, it is the universe's most abundant form of ordinary matter and makes up the Sun and most stars. It is an ionized gas, typically at low pressure (as in the upper atmosphere or fluorescent lamps) or at very high temperature (as in stars), consisting of positive ions and free electrons in proportions resulting in more or less no overall electric charge. Its properties, in particular its sensitivity to external and internal electric and magnetic fields, allow plasmas to spontaneously form complex, intermittent structures across a wide range of spatial scales, making it a distinct form of matter.

pointing

The act or process of directing a telescope to a sky position or the precision of a telescope's ability to be directed to a particular sky coordinate.

polarization

The state of transverse waves in which the wave vibrations have been restricted, or the action of such restriction. In astronomy, polarization carries information about the processes that emitted light, its interaction with magnetic fields, and the shape of the source. Light reflected by a shiny surface generally obtains a degree of horizontal polarization, and filters in polarized sunglasses reduce reflected glare by blocking horizontally polarized light. Like all forms of light, gravitational waves also exhibit polarization.

positron

An elementary particle with the same mass and spin as an electron but opposite electrical charge (positive). It is the electron's antimatter counterpart.

proper motion

The apparent motion of a star across the sky (not including a star's parallax), arising from the star's velocity through space with respect to the Sun.

proton

A particle with a positive charge found in the nuclei of all atoms. The nuclei of each chemical element hold a characteristic number of protons, which is known as its atomic number.

protoplanet

A body formed by the gradual accumulation of planetesimals in a solar system's protoplanetary disk.

protostar

A mass of interstellar gas and dust in the process of collapsing to form a star.

pulsar

A rapidly rotating neutron star that generates regular pulses of light. Pulsars were discovered by observations at radio wavelengths but have since been observed in visible light, X-rays, and gamma rays. The pulses arise from light-emitting features that rotate in and out of our line of sight.

pulsar wind nebula

An extended cloud formed as the rapid outflow of charged particles (pulsar wind) generated by a neutron star expands into its surroundings, the most famous example being the Crab Nebula. Such nebulae have been detected across the electromagnetic spectrum (from radio waves of less than 100 MHz in frequency to extremely high-energy gamma rays (more than 100 TeV). Observations reveal the presence of jets and wind termination shocks, compact emission structures that change with time, shocked supernova ejecta, and newly formed dust. The detection of very high-energy gamma rays in these sources requires particles that have been accelerated to even higher energies, suggesting that pulsar wind nebulae are important sources of some of the highest-energy particles in the galaxy.

Q

quasar

A class of active galactic nuclei, quasars are enormously bright objects in the distant universe that emit immense amounts of energy from a compact source — thought to be an accreting supermassive black hole — in the form of light ranging from radio waves to gamma rays. In visible light, quasars appear point-like, similar to stars. They typically emit thousands of times more light than a galaxy like our own. More than a million quasars have been catalogued, with the nearest ones several hundred million light-years away. Often abbreviated QSO, the name is a contraction of "quasi-stellar radio source."

quantum mechanics

The fundamental physical theory that describes the behavior of matter and light and whose unusual characteristics occur at atomic and smaller scales. It is a mathematical description of the motion and interaction of atoms and subatomic particles, incorporating concepts about the quantization of energy, quantum tunneling and entanglement, wave-particle duality, and the uncertainty principle. The predictions of quantum mechanics have been verified experimentally to high accuracy.

R

radial velocity

The component in the velocity of an object's motion that is moving away or toward an observer. By observing Doppler shifts in spectral lines, astronomers can derive the radial velocity and determine how fast objects are moving away from or toward us. Measuring such shifts in the light of a star can reveal the presence of exoplanets and brown dwarfs orbiting them.

radian (rad)

A unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57.3°, and 2π radians equals 360°.

radiation

Energy moving through space in the form of electromagnetic waves (light) or as moving subatomic particles (such as electrons or protons).

radiation belt

A region surrounding a planet where charged particles accumulate under the influence of the planet's magnetic field, that is, a region in its magnetosphere. Around Earth, particles become trapped in two donut-sha[ed regions, known as the Van Allen Belts. The outer belt is made up of billions of high-energy particles that originate from the Sun, and the inner belt results from interactions of cosmic rays with Earth’s atmosphere.

radiation pressure

A minute pressure exerted on any surface by electromagnetic radiation. It is caused by the exchange of momentum between the surface and any photons absorbed, reflected, or emitted by it.

radio

Light with the lowest frequencies and longest wavelengths, produced by charged particles moving back and forth. Radio wavelengths range from 11.8 inches (30 centimeters) to larger than Earth.

radio galaxy

A type of active galaxy with giant regions of radio emission extending to millions of light-years, far beyond the galaxy's its visible structure. These structures are formed and powered by jets from a supermassive black hole in the galaxy's center. The sizes of these structures imply that the jets have been operating for tens to hundreds of millions of years. Cygnus A was the first radio galaxy identified (1951) and is one of the strongest radio sources in the sky.

Rayleigh scattering

The scattering of light by particles that are small compared to its wavelength, with the amount of scattering dependent on the color of the light. Air molecules, for example, are 1,000 times smaller than visible light wavelengths and scatter blue light about four times more strongly than red light, which is why a clear sky looks blue. Named for British physicist John William Strutt (1842–1919), also known as Lord Rayleigh.

red dwarf (M dwarf)

A small, low-mass, dim, and cool star. Many have big flares and mass ejections on their surfaces. The habitable zone of red dwarf stars is closer to the star than stars like our Sun, making it easier to observe potentially habitable planets.

red giant star

A low- or intermediate-mass star (born with a mass between about 0.5 and 5 Suns) in an advanced phase of stellar evolution. Due to internal changes related to the depletion of hydrogen fuel in their cores, the outer envelopes of such stars expand greatly, making them hundreds of times larger than the Sun. This causes their surfaces to cool and redden and increases their luminosity.

redshift (z)

The apparent shift toward longer wavelengths of light or spectral lines emitted by an object. In astronomy, there are three forms: Doppler redshift, due to the motion of sources away from us at the instant the light is emitted; gravitational redshift, seen as light escapes massive objects, such as the Sun or a white dwarf; and cosmological redshift (often symbolized as z) caused when the wavelength of the emitted light is lengthened during its travels by the stretching of space in the expanding universe.

reflected light

Light that has bounced off an object or material. The Moon is visible in the night sky because it reflects sunlight. Most objects and materials on Earth are visible because they reflect light from the Sun or from artificial lights.

reflection law

For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.

refracted light

Light that bends, or changes direction, as a result of passing from one material into another. For example, light refracts when it moves between air and water. Refraction causes optical effects such as the apparent bending of objects placed in water, and the formation of rainbows. Eyeglasses and telescope lenses work by refracting light.

reionization

This is the process where the neutral hydrogen that filled the universe was converted to ionized hydrogen by the first stars, galaxies, and black holes. As these objects formed and radiated energy, the universe reverted from being neutral to once again being an ionized plasma. This occurred between 150 million and 1 billion years after the big bang, and is formally known as the Era of Reionization.

relativity

Theories developed by German-born physicist Albert Einstein (1879-1955) that are central to modern science. The special theory of relativity (1905) is based on the principles that all constant, straight-line motion is relative, that light has the same speed in a vacuum for all observers, and that space-time is a four-dimensional continuum. From it emerged the equivalence of matter (m) and energy (E) as described by the famous equation E=mc², where c is the speed of light. The general theory of relativity (1916) extends the special theory to systems that are accelerating with respect to one another, covering gravitation and the curvature of space-time. It predicted the existence of black holes, gravitational waves, and gravitational lensing.

resolution (spatial)

The ability of a telescope to differentiate between two objects in the sky that are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the resolution of the telescope.

resolution (spectral)

The ability of a telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope.

resonance

A relationship in which the orbital period of one body is related to that of another by a whole-number fraction, such as 1/2, 2/3, or 3/5. The dwarf planet Pluto has a 2:3 mean motion resonance with Neptune such that for every two orbits of Pluto, Neptune orbits three times. Jupiter's moons Io, Europa, and Ganymede share a triple resonance of 4:2:1, meaning that Io orbits four times for every two orbits of Europa and each orbit of Ganymede.

retrograde

The actual or apparent motion of a body in a direction opposite to that of the (direct) motions of most members of the solar system or of other astronomical systems with a preferred direction of motion. Retrograde motion in the solar system is clockwise when viewed from the north pole of the ecliptic.

revolution

The movement of an object in a circular or elliptical course around another or about an axis or center, or a single orbit of one object around another or about an axis or center.

right ascension

A coordinate which, along with declination, may be used to locate any position in the sky. Right ascension is analogous to longitude for locating positions on Earth. 

Roche limit

The closest distance from the center of a planet that a satellite can approach without being pulled apart by the planet's gravitational field, named for French mathematician Édouard Albert Roche (1820-1883).

Roche lobe

A region around a star in a binary system within which orbiting material is gravitationally bound to that star, the apex of the teardrop (which is the L1 Lagrangian point of the system) pointing towards the other star. Named for French mathematician Édouard Albert Roche (1820-1883).

rotation

The action of rotating around an axis or center, the spin n of a celestial body on its own axis.

S

satellite

A body, either natural or artificial, that revolves around a larger body. The Moon is a natural satellite orbiting Earth, while the International Space Station is an artificial one.

Schwarzschild black hole

A black hole resulting from the complete gravitational collapse of an electrically neutral and nonrotating body, having a physical singularity at the center to which infalling matter inevitably proceeds and at which the curvature of space-time is infinite. The Schwarzschild radius — the radius of such a black hole's boundary or event horizon — is determined solely by the object's mass. First published in 1916 by German physicist Karl Schwarzschild (1873-1916) and derived from Einstein's equations of general relativity.

Schwarzschild radius

The radius of the event horizon for a Schwarzschild black hole.

scientific notation

A compact format for writing very large or very small numbers, most often used in scientific fields. The notation separates a number into two parts: a decimal fraction, usually between 1 and 10, and a power of ten. So 1.23 x 10⁴ means 1.23 times 10 to the fourth power, or 12,300, and 5.67 × 10^–8 means 5.67 divided by 10 to the eighth power, or 0.0000000567.

scintillation

In astronomy, the twinkling of stars, caused by the passing of light through small-scale fluctuations in air density of Earth's turbulent atmosphere. Scintillation is always more pronounced near the horizon than near the zenith because incoming light rays near the horizon pass through a more of the atmosphere before reaching the observer.

season

A periodic change in weather conditions on a planet caused by the tilt of its rotational axis and its orbit around the Sun. Earth has four seasons in one year. Mars has similar seasons, but they last about seven months since its year is approximately two Earth years. Jupiter's seasons last three Earth years and Saturn's last about seven Earth years.

second (s)

The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. A nanosecond is equal to one-billionth (10^–9) of a second, the time it takes light to travel 29.98 cm (11.8 inches).

secondary eclipse

The portion of a planet's orbit where it passes behind its star as viewed from an observer; a type of occultation. During a secondary eclipse, heat emitted from the planet does not contribute to the total light seen from the system. Light detected during the eclipse can be compared with the total light detected right before and after the eclipse to reveal contributions from the planet’s surface and atmosphere.

semimajor axis

One half of the longer (major) axis of an ellipse, a line from one side of the ellipse to the other passing through both foci. The semimajor axis of a planetary orbit is also the average distance from the planet to its star. The minimum (periapsis) and maximum (apoapsis) distances of the orbit can be calculated from the semimajor axis and its eccentricity (e):

• Min. = a × (1 – e)

• Max. = a × (1 + e)

sensitivity

A measure of how bright objects need to be in order for a telescope to detect them. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.

Seyfert galaxy

The most commonly observed type of active galaxy, with about 1% of spiral galaxies classified as Seyferts. They have exceptionally bright, star-like central regions and spectra dominated by strong emission lines from highly ionized gases. They were first described in 1943 by the American astronomer Carl K. Seyfert (1911-1960). Type 1 Seyfert galaxies are strong ultraviolet and X-ray emitters and display two sets of emission lines. Narrow lines come from low-density gas moving at a few hundred thousand mph (a million kph), while broad lines arise from higher-density gas moving at up to 22 million mph (36 million kph). Type 2 Seyfert galaxies show only narrow spectral lines, and while weak in ultraviolet and X-rays, they are very bright in the infrared. Astronomers think both types have similar structures but appear different due to the way we view their central "engine," an accreting supermassive black hole. A galaxy is a Seyfert 1 if we see clearly into the back hole; it's a Seyfert 2 if we view it from the side, where thick dust obscures some features. Astronomers also recognize additional subclasses between the two main types.

shock wave

A propagating disturbance, characterized by an abrupt change in pressure, temperature, and density, that moves faster than the medium's speed of sound. Shock waves, often shortened to shocks, are common in astrophysics. Solar flares result in interplanetary shocks that traverse the solar system, rapidly moving planets and stars form bow shocks as they move through gaseous environments, and supernova remnants form shocks as they expand into the interstellar medium.

singularity

The point where matter is crushed to infinite density at the center of a black hole. This may be either a physical structure or a purely mathematical one, but right now astronomers don't know. The prediction of a singularity may signal the limits of relativity, where quantum effects not included in the theory become important in a more complete description of gravity. For rotating black holes, scientists believe the singularity takes the shape of a ring around the center of the black hole.

solar eclipse

When the Earth's moon is precisely between the Earth and the Sun, blocking the light of the Sun. During a total solar eclipse, the Sun is only completely blocked by the Moon for seven and a half minutes.

solar flare

A sudden, intense increase in electromagnetic radiation from the atmosphere of our Sun. Solar flares are typically associated with sunspots. Flares develop in seconds to minutes and may last up to several hours. High-energy particles, X-rays, and radio bursts are typically emitted. Flares are often accompanied by coronal mass ejections, which may propel a large quantity of particles into the solar wind and into interplanetary space. Particle fluxes can arrive at Earth minutes, hours, or days after the flare and can disturb radio communications and power line transmissions. X-ray outbursts similar to but far more powerful than those seen from the Sun, called stellar flares, have been observed from young stars.

solar luminosity (L_⊙ or L_Sun)

The total power output of the Sun radiated to space, equal to 3.85 × 10²⁶ watts or 3.85 × 10³³ ergs/sec. It is a convenient unit for describing the luminosities of other stars, star clusters, galaxies, and other objects. For example, the star Sirius shines with 25.4 L_⊙, or 25.4 times the Sun's energy output.

solar mass (M_⊙ or M_Sun)

The mass of our Sun, a convenient unit for describing the masses of other stars, star clusters, galaxies, and other objects. One solar mass equals 1.988 x 10³⁰ kg, or about 333,000 Earth masses, and the unit name is often shortened to Suns.

solar radius (R_⊙ or R_Sun)

The radius of the Sun, equal to 695,700 km (432,300 miles). It is a convenient unit for describing the sizes of stars. The star Sirius is 1.71 R_⊙, or 1.71 times larger than the Sun.

solar system

The Sun and its surrounding matter, including asteroids, comets, planets, and moons, held together by the Sun's gravitational influence. Other stars that are known to have planets orbiting them are referred to as other solar systems or planetary systems.

solar wind, stellar wind

The solar wind is a constant outflow of charged particles, especially protons and electrons, and embedded magnetic fields streaming off of the Sun at high velocities in all directions. The solar wind's composition, density, and speed vary with solar activity. It typically moves at about 895,000 mph (1.4 million km/h) in Earth's vicinity, but structures called coronal holes can generate streams traveling twice as fast. These streams interact with Earth's magnetosphere and play a role in space weather. Other types of stars, such as hot, luminous O- and B-type stars like Eta Carinae, can produce far denser and faster stellar winds than the Sun.

snow line (ice line, frost line)

The distance from a star where is it cold enough for volatile compounds such as water, carbon dioxide, or methane to form solid ice grains. The snow line will be different for each compound. For example, the water-ice snow line will be closer to a star than the methane snow line.

space-time

A mathematical model, originally described in 1908 by Hermann Minkowski (1864–1909) to better explain Einstein's special theory of relativity, that combines time and the three dimensions of space into a single four-dimensional continuum. It is often popularly described as the "fabric of the universe."

space weather

Variable conditions on the Sun, in the solar wind, and near Earth (within its upper atmosphere and magnetosphere) that can degrade the performance and reliability of technology and can endanger human health. Strong events are sometimes called solar storms. Solar activity is the main source of space weather. These include coronal mass ejections — immense fast-moving clouds of plasma launched into space — and brief, sudden eruptions of X-rays from solar flares. Severe space weather events pose a risk to astronauts and can damage satellites, disrupt communications, and, by inducing additional currents in electrical transmission wires, disrupt power systems on the ground.

spectral line

In a spectrum, light emitted or absorbed at a specific frequency by an atom or molecule. Each ion, atom, and molecule emits and absorbs light at specific wavelengths, making it possible to identify the makeup of a star or other celestial body. Emission lines produce bright features, absorption lines dark features, and each line represents light given off or absorbed by one or more substances.

spectrometer

The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum. A dispersive spectrometer is like a prism, scattering light of different energies to different places. Astronomers measure the energy by noting where the X-rays go. A non-dispersive spectrometer measures the energy directly.

spectroscopic observations

Observations made using a spectrograph.

spectroscopy

The study of spectral lines from different atoms and molecules and the interpretation of suck lines observed in the spectrum of a celestial object. Spectroscopy is an important part of studying the makeup of astronomical sources.

spectrum (plural: spectra)

In astronomy, this term can refer to:

1. An image or plot showing the distribution of components of light according to wavelength, as captured an instrument called a spectrometer or spectrograph.
2. The entire range of wavelengths of electromagnetic radiation in all forms, from radio to gamma rays.

speed of light

The speed at which light in all its forms moves through a vacuum, defined as 299,792,458 meters per second (186,282 miles per second). Put another way, light takes a nanosecond to travel 11.8 inches (29.98 centimeters). Light travels at slightly slower speeds when it moves through materials like glass or air.

spiral galaxy

A spiral-shaped system of stars, dust, and gas clouds. A typical spiral galaxy has a spherical central bulge of older stars surrounded by a flattened galactic disk that contains a spiral pattern of young, hot stars, as well as interstellar matter.

star

A huge, incandescent ball of gas held together by gravity — like our Sun — that shines because of energy produced in its core by nuclear fusion. Despite their names, neutron stars and white dwarfs are stellar remnants rather than true stars because they no longer generate energy in their centers.

star cluster

A collection of stars, ranging in number from dozens to millions, that formed at roughly the same time and place. Open star clusters contain up to a few thousand stars, and because they are loosely bound together by their mutual gravity, typically disperse over hundreds of millions of years. Globular clusters contain tens of thousands to millions of tightly packed stars, including some of the oldest known, and remain intact for billions of years.

star formation

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars.

starburst galaxy

A galaxy undergoing an exceptionally high rate of star formation relative to that seen in most others. The Milky Way galaxy forms stars at a rate of about three Sun-like stars per year, while starburst galaxies can form 100 or more Sun-like stars a year. Starbursts episodes may be triggered by a close encounter or merger with another galaxy.

Stefan-Boltzmann constant (σ)

The total radiation emitted by a blackbody is proportional to the radiating surface area and the fourth power of its absolute temperature. The proportionality constant, σ, equals 5.670374419 x 10-8 watts per square meter x K-4. Named for physicists Josef Stefan (1835-1893) and Ludwig Boltzmann (1844-1906).

Stefan-Boltzmann law

The total radiation emitted by a blackbody is proportional to the radiating surface area and the fourth power of its absolute temperature. The proportionality constant, σ, equals 5.670374419 x 10-8 watts per square meter x K-4. Named for physicists Josef Stefan (1835-1893) and Ludwig Boltzmann (1844-1906).

stellar classification

A designation consisting of a letter and a number according to the nature of a star's spectral lines, which correspond roughly to surface temperature. The classes are: O, B, A, F, G, K, and M. O stars are the hottest; M the coolest, with the letters L, T, and Y later added to extend the scheme to even cooler substellar objects called brown dwarfs. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long before astronomers understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.

stellar evolution (stellar life cycle)

The process of change that occurs during a star's lifetime from its birth to its death.

stellar-mass black hole

This black hole forms when a massive star explodes and leaves behind a small, heavy core. If this core is massive enough, it will collapse on itself and form a black hole. Stellar-mass black holes are typically between 3 and 10 solar masses.

steradian (sr)

The supplementary SI unit of solid angle, defined as the solid central angle (circular cone) of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.

submillimeter

Light in the transition region between the microwave and far-infrared parts of the spectrum that can be regarded as either, with wavelengths between 1 and 0.1 millimeter.

sunspots

Cooler (and thus darker) regions on the Sun where the magnetic field loops up out of the solar surface.

super-Earth

An exoplanet with a mass higher than Earth's, but substantially below the masses of the solar system's ice giants, Uranus and Neptune, which have masses of 15 and 17 times Earth's, respectively.

supermassive black hole (SMBH, SBH)

A black hole possessing as much mass as a million or a billion stars. Supermassive black holes reside in the centers of galaxies and are the engines that power active galactic nuclei and quasars. Sagittarius A* (pronounced Sagittarius A-star) is the supermassive black hole at the center of our Milky Way galaxy.

supernova (plural: supernovae)

The explosion death of a massive star, resulting in a sharp increase in its brightness followed by gradual fading over weeks or months. At peak light output, supernovae can outshine a galaxy. The star's outer layers blast outward in a radioactive cloud. Visible long after the initial explosion fades from view, this cloud forms a supernova remnant. In type II, Ib, and Ic supernovae, the explosion is triggered when the star's core ran out of fuel and collapsed. Such stars may leave behind neutron stars or black holes. In type Ia supernovae, an Earth-sized remnant called a white dwarf explodes, trigger either by merging with a binary companion or by accumulating enough gas from a companion star to exceed its stability limit. Type 1a supernovae exhibit roughly the same intrinsic brightness and play an important role in determining cosmic distances.

supernova remnant (SNR)

The glowing, expanding, gaseous remains of a supernova explosion.

synchronous rotation

Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (like the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.

synchrotron radiation

Light, characteristically highly polarized, emitted by charged particles moving at speeds close to that of light when their paths are altered, as by gyrating in a magnetic field. The stronger the magnetic field and the higher the energy of the particles, the greater the intensity and frequency of the emitted radiation. High-energy electrons spiraling in Jupiter's magnetic field emit radio waves through this process, and the Crab Nebula emits radio, visible, and ultraviolet light. Active galaxies, kilonovae, and other jet-producing cosmic objects also emit this radiation.

Systéme Internationale d'Unités (SI) - International System of Units

The international decimal system of weights and measures in common use in today, derived from and extending the metric system. The fundamental SI units of length, time, and mass are, respectively, the meter, the second, and the kilogram.

T

telescope

An instrument for making distant objects appear nearer and larger, often containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified. Astronomical telescopes collect light emitted by celestial objects across the electromagnetic spectrum. At energies lower than and beyond those of visible light, telescopes may take the form of a large dish (radio) or a particle detector (gamma rays).

temperature

A measure of the heat of an object and the average kinetic energy of the randomly moving particles within it. Astronomers use the Kelvin scale in their research. Also see Absolute Zero.

terminator

The boundary between the day and night sides of a planet, moon, or other planetary object; also called the "twilight zone." On an object with no atmosphere, the terminator appears as a sharp line between light and dark. On an object with atmosphere, which scatters light, the terminator can appear broader and less distinct, grading from light to dark. On planets that rotate relative to their star, the terminator moves over the course of the day. But on a planet that is tidally locked, the terminator remains stationary.

terrestrial gamma-ray flash (TGF)

Millisecond pulses of gamma rays associated with lightning and produced by electrons accelerated to high energy within thunderstorms. Terrestrial gamma-ray flashes were discovered in 1994 by an experiment aboard NASA's Compton Gamma Ray Observatory.

terrestrial planets

These rocky planets are between half of Earth's size to twice its radius. Exoplanets with a mass higher than Earth's may be rocky as well, but those are considered super-Earths. In our solar system, Earth, Mars, Mercury, and Venus are terrestrial planets.

thermal emission

Heat energy given off by an object in the form of electromagnetic radiation (light). Thermal emission is directly related to an object's temperature.

thermal emission spectrum

The pattern of light (the brightness of different wavelengths) that is given off by an object, material, surface, or region of space as a result of its temperature. The thermal emission spectrum is used to determine temperature, which can be combined with other information to infer composition and other properties. A blackbody spectrum is a type of thermal emission spectrum.

tidal heating

A phenomenon in which a planet or moon experiences internal heating as a result of being physically distorted (e.g., stretched, compressed, or flexed) by a continuous change in gravitational pull. The heating is a result of internal friction that occurs as the object changes shape. An object in an eccentric (non-circular) orbit will experience tidal heating as the changing distance between it and its parent body (the star or planet it is orbiting) causes a change in the gravitational pull between the two objects.

tidally locked (or tidal locking)

An effect caused by the gravitational interactions between an orbiting celestial object and the object it orbits, causing the same side of the orbiting object (usually an exoplanet) to face the host star. The gravitational interactions slow the objects' rotations (or spins) over many millions of years.

time dilation

The slowing down of the passage of time as measured between a clock in a frame of reference that is moving relative to the observer's (special relativity) and/or at a different height in a gravitational field than the observer's (general relativity). Both effects must be included for satellite navigation systems like GPS to work.

time-series observation

A set of measurements taken over a period of time, typically at regular intervals (cadence) of seconds, minutes, or hours. Time-series observations are used to detect and record changes in an object's position, brightness, and other properties over time. Common examples of time-series observations include those of variable stars, eclipsing binary stars, variable brown dwarfs, and exoplanet transits.

tracker

The part of a high-energy gamma-ray detector that is used to determine the arrival direction of the incoming gamma ray. When a high-energy gamma ray passes through a target material, it can convert into an electron and a positron, whose paths through the tracker can be detected. Backtracking along this trajectory allows the direction of the original gamma ray to be determined.

transient

A source that appears and then quickly disappears, such as a gamma-ray burst.

transit

The passage of a celestial body or satellite across the face of a relatively larger body, such as a planet crossing the face of its host star. Thousands of exoplanets have been discovered through transits because the events measurably and regularly dim the star's light. Transit photometry is the measure of the light changes caused by transit events.

transit technique/transit method

A technique for detecting and studying exoplanets. When a planet passes directly between a star and its observer, it dims the star's light by a measurable amount. Transits can help determine a variety of exoplanet characteristics, including its orbit or period, the size of the planet, and details about its atmosphere.

transiting planet

A planet that transits its star, or appears to cross the face of the star as it orbits. Whether or not a planet is "transiting" depends on the orientation of the planet's orbit with respect to the viewer. For example, we can observe a transit when the planet passes between its star and the telescope.

transmission spectroscopy

When light (also known as radiation) passes through a material such as a planet's atmosphere, some of the light is absorbed by the material and some is passed through or is transmitted. The resulting spectrum shows both the molecular absorption and transmission of the material.

transmitted light

Light that has passed through a material. Transparent and translucent materials like water, glass, and air transmit visible light. Most materials transmit some forms of light and block (absorb or reflect) others.

trojan asteroid

These asteroids orbit at the same distance from the Sun as Jupiter, both leading and following Jupiter's orbit. They are airless rocks that failed to adhere to one another to become larger bodies as the solar system was forming 4.6 billion years ago. These are not part of the main asteroid belt, which orbits the Sun between Mars and Jupiter.

T Tauri star

A star that is in the process of evolving onto its mature phase, known as the main sequence on the Hertzsprung-Russell Diagram. Many T Tauri stars have a circumstellar or protoplanetary disk.

two-dimensional spectrum

Data about an object spread out by wavelength (color) by a spectrograph. For a point source, like a star, this creates a rainbow-like image of the brightness of the object at different wavelengths. For an extended object like a galaxy, this creates an image of the brightness of the object at different points along one axis and wavelength on the other axis. Explore this in a visual example.

U

ultramafic

An igneous rock made of silicate minerals that have more iron and magnesium, and less silica, than basalt. Earth's mantle (the layer beneath the crust) is ultramafic.

ultraviolet (UV)

Light at wavelengths shorter than the violet visible light, ranging from 100 to 400 nm. Earth's atmosphere blocks out most ultraviolet light from astronomical sources, so studying them requires telescopes in the upper atmosphere or in space. An exception is the Sun, whose ultraviolet light can quickly damage unprotected skin (sunburn).

universal constant of gravitation (G)

The constant of proportionality in Isaac Newton's law of universal gravitation plays an analogous role in Albert Einstein's general relativity. It is equal to 6.6743×10⁻¹¹ meter³ per kilogram per second².

universe

Everything that exists, including the totality of time and space and all of the matter and energy in it; the entire cosmos.

V

vacuum

Ideally, a space in which all matter is absent, but in reality, no vacuum is perfectly free of matter. Even in the most remote part of the universe, there's roughly one atom or ion per cubic centimeter, plus other particles, such as neutrinos. And since normal matter only makes up about 5% of the total energy of the cosmos, dark matter and dark energy, which make up 27% and 68% respectively, must be present as well.

variable star

A star whose luminosity (brightness) changes with time.

visible light

Light at wavelengths the human eye can see, ranging from red (about 700 nanometers) to violet (400 nanometers).

volcanism

The release of molten material, known as magma, from the interior to the surface of a planet or a planet's moon or satellite. Magma cools to become lava. Volcanoes have been identified throughout the solar system, including Mars, Venus, and Jupiter's moon Io.

W

wave

A vibration in some media that transfers energy from one place to another. Sound waves are vibrations passing in air. Light waves are vibrations in electromagnetic fields.

wavelength

The distance between two successive peaks or crests on a wave. Human eyes register wavelength differences in visible light as differences in color.

wave-particle duality

A concept in quantum mechanics that fundamental entities, like photons and electrons, exhibit either particle or wave properties according to the experimental circumstances. An example is the photoelectric effect, which can be explained only by light behaving as a particle.

weight

The force acting on an object due to gravity. Weight changes due to location. Also see Mass.

white dwarf

A hot, dense, compact remnant of a Sun-like star. Typically, a white dwarf packs a mass roughly equal to the Sun's into a ball just 1% the Sun's size, comparable to Earth's. Such stars no longer produce their own energy, having exhausted their core fuel supply, but are much hotter than Sun when they form and cool very slowly.

Wien's displacement law

The blackbody radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. As temperature rises, the peak of the radiant energy shifts toward shorter wavelengths (higher frequency and energy). The color of a star is determined by its temperature, with the hottest stars emitting more blue light and the coolest emitting more red light.

WIMP (weakly interacting massive particle)

Theoretical subatomic particles that do not respond to electromagnetism or interact through the strong nuclear force, but interact only through the weak nuclear force and gravity. Because of these properties, they are difficult to detect, and are therefore considered "dark" — hence, WIMPs are a possible form of dark matter.

Wolf-Rayet star [wolf reye-ay]

A class of very hot, intensely luminous stars whose spectra show prominent broad emission lines of ionized helium and highly ionized nitrogen or carbon, depletion of hydrogen, and strong stellar winds. A typical Wolf-Rayet star is several times the diameter of the Sun, 10 or more times its mass, and thousands to hundreds of thousands of times more luminous. Such stars may end their days as type Ib and type Ic supernovae. Only a few hundred are known in the Milky Way. Named after French astronomers Charles Wolf (1827-1918) and Georges Rayet (1839-1906).

X

X-rays

Invisible light of short wavelength and high energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays, spanning 0.01 to 10 nanometers. X-rays can pass through materials that visible light cannot. Soft X-rays are generally those with energies less 10 keV (wavelengths longer than 0.12 nm), while hard X-rays have higher energies and shorter wavelengths.

X-ray binary

A binary system in which one member is a collapsed object such as a white dwarf, neutron star, or black hole. The separation between the stars is small enough so that matter flows from the normal star (the donor) to the collapsed star, producing X-rays in the process. In low-mass X-ray binaries, the compact object is either a neutron star or a black hole and is more massive than the donor. In high-mass systems, the donor has a mass of 10 or more Suns and is typically either a supergiant O/B star or a Be star, both of which produce strong stellar winds and experience high rates of mass loss. The compact object is typically a neutron star, which accretes directly from the stellar wind.

Y

year

The length of time a planet takes to orbit its star or sun. Citations of this term are often compared to Earth's year or hours for easier comparison. For example, Mercury's year is 88 Earth days and Jupiter's year is 11.86 Earth years.

yellow dwarf star (F star, G star)

An intermediate mass star, like our sun. Yellow dwarfs lead quieter, longer lives over billions of years.

Z

z

The ratio of the observed change in the wavelength of light emitted by a moving object to the rest wavelength of the emitted light. In general, with the velocity of the object (v), the speed of light (c), the rest wavelength (lambda), and delta-lambda for the observed change in the wavelength, z is given by

z = (delta–lambda)/lambda = (sqrt(1+v/c) / sqrt(1–v/c)) – 1.

If the velocity of the object is small compared to the speed of light, then

z = (delta–lambda)/lambda = v/c

The farthest observed gamma-ray burst with a redshift measurement is GRB 090423 (z = 8.2), which corresponds to the event occurring nearly 13.1 billion years ago.

zodiacal light

A faint, elongated, triangular glow seen extending from the horizon before sunrise or after sunset. It is caused by sunlight scattered off of dust particles in the inner solar system, most of which is thought to have been ejected by Jupiter-family comets. For both hemispheres, zodiacal light is best seen in evenings in the spring and in autumn before dawn.