Universe Glossary N-S

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The National Aeronautics and Space Administration, founded in 1958 as the successor to the National Advisory Committee for Aeronautics.

nebula (pl. nebulae)

A cloud of interstellar dust and gas.


A fundamental particle produced in nuclear reactions, such as those occurring in stars. They are hard to detect because the vast majority of them pass completely through the Earth without interacting with other matter.


A particle with approximately the mass of a proton but zero charge, commonly found in the nuclei of atoms.

neutron star

The remnant compact core of a massive star that exploded as a supernova. Neutron stars host more than the Sun's mass in a ball about 20 km (12.5 miles) across and can be observed as pulsars and magnetars.

Newton's law of universal gravitation

Two bodies attract each other with equal and opposite forces. The magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between their centers of mass. Named for English physicist and mathematician Sir Isaac Newton (1642–1727).

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.


Random fluctuations 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.

nova (pl. novae)

A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by up to 100,000 times before fading back to its original luminosity. See also: cataclysmic variable.

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.

nucleus (pl. nuclei) [NOO-klee-us, NOO-klee-eye]

In physics, 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. The term is also used in astronomy for the solid body of a comet and for the central emitting region of active galaxies.


The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.


A property of matter that prevents light from passing through it. Opacity depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultraviolet light, but is opaque to visible light.


The regularly repeated elliptical course of a celestial object or spacecraft about a star, planet, or moon. Also, a complete circuit round an orbited body.

pair production

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


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 of the angle by which a star appears to be displaced as Earth moves from one side of the Sun to the other.


A unit equal to the distance at which an object would have a parallax of 1 arcsecond, equal to 3.262 light-years. A kiloparsec (kpc) equals to 1,000 parsecs. A megaparsec (Mpc) equals 1 million (106) parsecs.


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


A particle representing the smallest amount (a quantum) of electromagnetic radiation. 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.


Any of three unstable particles — the π+, π and π0 — comprising 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 an important tool in high-energy astrophysics.

pi (π)

The constant equal to the ratio of the circumference of a circle to its diameter, which is approximately 3.141593.

Planck constant (h)

A fundamental constant equal to the energy of any quantum of radiation divided by its frequency, with a value of  6.62607015 × 10-34 joule per hertz.

Planck equation

The quantum mechanical equation relating the energy of a photon (E) to its frequency (ν) via the Planck constant (h):E = h × ν

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.


A low-density gas in which the individual atoms are ionized, and therefore charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality. 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.


A property of light in which the vibrations of waves in a light beam are at least partially aligned. Light may be polarized by reflection or by passing it through filters that transmit vibration in one plane but not in others. Polarization measurements allow astronomers to learn much more about an object, including their 3D shapes and the strength of their magnetic fields. 


The antimatter counterpart of the electron, with the same mass and spin but carrying a positive charge.


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 the atomic number. 


Very dense regions (or cores) of molecular clouds where stars are in the process of forming.


A rotating neutron star that generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.


An enormously bright object in the distant universe that emits an immense amount of energy from a compact source – thought to be near a supermassive black hole – in the form of light ranging from radio waves to gamma rays. In visible light, they appear point-like, similar to stars, and are a type of active galaxy. Also known as a quasi-stellar object or QSO.

radial velocity

The speed at which an object is moving away or toward an observer. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky.


The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. 1 radian is equivalent to 57.296 degrees and π/2 radians equals 90 degrees.


Energy emitted and transmitted in the form of electromagnetic waves (light) or moving subatomic particles (electrons, positrons, etc.).

radiation belt

One or more regions surrounding a planet where charged particles accumulate under the influence of the planet's magnetic field. 

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.


Electromagnetic radiation with the lowest frequency and the longest wavelength, produced by charged particles moving back and forth.

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 giant

A low- or intermediate-mass star (born with a mass between about 0.5 and 5 Suns) in an advanced phase of stellar evolution, exhibiting high luminosity and low surface temperature. Due to internal changes related to the depletion of hydrogen fuel in their cores, the outer envelopes of red giants expand greatly, making them hundreds of times larger than the Sun.


An apparent shift toward longer wavelengths of spectral lines emitted by an object caused by its motion away from an observer. See also: Doppler effect and blueshift.

relativity principle

The principle, employed by Albert Einstein's (1879–1955) relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the founding principles of special relativity. 

resolution (spatial)

In astronomy, 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 or frequency)

Similar to spatial resolution, spectral resolution is the ability of the telescope to differentiate two light signals that 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.


A relationship where the orbital period of one body is related to that of another by a ratio of small integers, such as 1/2, 2/3, 3/5. This allows the bodies to regularly exert enhanced gravitational influence on each other and can alter their orbits, stabilizing or destabilizing them. 


The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic, that is, moving in the opposite sense from the great majority of solar system bodies.


The movement of one celestial body in orbit around another, often measured as its orbital period.

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 smallest distance from a planet or other body at which purely gravitational forces can hold together a body of the same mean density. Closer than this distance, tidal forces from the larger object would break up the smaller one.

Roche lobe

In a binary star system, the volume around a star within which matter is gravitationally bound to it. That is, a particle released within the Roche lobe would fall back onto the star's surface. Where the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point. If a star in a close binary system evolves to the point where fills its Roche lobe, gas from this star will overflow and stream onto the companion star via the L1 point.


The spinning motion of a celestial body around its own axis.


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

Schwarzschild black hole

A black hole described by solutions to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916. The solutions assume the black hole is not rotating, and that the size of its event horizon is determined solely by its mass.

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. Thus 1.23 × 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.

second (s)

The fundamental SI unit of time, defined as the 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).

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)


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 UV and X-ray emitters and display two sets of emission lines. Narrow lines come from low-density gas moving at a few hundred kilometers per second, while broad lines arise from higher-density gas moving at up to 10,000 km/s (36 million km/h or 22 million mph). Type 2 Seyfert galaxies show only narrow spectral lines, and while weak in UV 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, or 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. NGC 1275, which resides at the center of the Perseus galaxy cluster, is a Seyfert 1.5 galaxy. NGC 7742, the Fried Egg galaxy, is a Seyfert 2.

shock wave

A strong compression wave where there is a sudden change in gas velocity, density, pressure, and temperature.


General relativity predicts that the very center of a black hole contains a point where matter is crushed to infinite density, called the singularity. This may be either a physical structure or a purely mathematical one, but right now astronomers don’t know which is true. 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.

solar flares

Violent eruptions on the Sun's surface.

solar luminosity (L or LSun)

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 MSun)

The mass of the Sun, a convenient unit for describing the masses of other stars, star clusters, galaxies, and other objects. One solar mass equals 1.988 × 1030 kg, or about 333,000 Earth masses. The star Sirius weighs 2.02 M.

solar radius (R or RSun)

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/stellar wind

The constant outflow of charged particles, especially protons and electrons, and embedded magnetic fields streaming off of the Sun (and other stars) at high velocities in all directions. The solar wind's composition, density, and speed vary with solar activity. It typically moves at about 1.4 million km/h (895,000 mph) 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.

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.

special relativity

The physical theory of space and time developed by Albert Einstein (1879–1955), based on postulates that all laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence. See also: general relativity.

spectral line

Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 GHz (a wavelength of 2.7 mm).


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.


The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry and make-up of astronomical sources.

spectrum (pl. spectra)

A plot of the intensity of light at different frequencies, or the distribution of wavelengths and frequencies.

speed of light

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


A large ball of gas that creates and emits its own radiation.

star cluster

A collection of stars (ranging in number from a few to hundreds of thousands) that are bound to each other by their mutual gravitational attraction.

Stefan-Boltzmann constant (σ)

The constant of proportionality present in the Stefan-Boltzmann law. It is equal to 5.6704 x 10-8 watts per square meter per degree kelvin to the fourth power. Named for Austrian physicists Josef Stefan (1835-1893) and Ludwig Boltzmann (1844-1906).

Stefan-Boltzmann law

The power (P) of electromagnetic energy radiated by a hot body is proportional to the radiating surface area (A) and the fourth power of the thermodynamic temperature (T). The constant of proportionality is the Stefan-Boltzmann constant. Named for Austrian physicists Josef Stefan (1835-1893) and Ludwig Boltzmann (1844-1906).

stellar classification

Star types are designated by a letter and a number according to the nature of their spectral lines, which correspond roughly to surface temperature. The classes are O, B, A, F, G, K, and M, with O stars the hottest and M the coolest. The numbers represent subdivisions of the major classes. The class sequence is odd because they were assigned long ago before astronomers understood how they related to temperature. O and B stars are rare but very bright. M stars are the most numerous but quite dim. The Sun is designated G2.


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

supernova (pl. supernovae)

1. The explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, this type of supernova (called type II) 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. 2. The explosion of a white dwarf that has either merged with another white dwarf or has accumulated enough material from a companion star to reach the Chandrasekhar limit. These types of supernovae (called type Ia) have roughly the same intrinsic brightness, and play an important role in determining distances.

synchronous rotation

Said of a satellite if the period of 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, such as 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 in the opposite direction.

synchrotron radiation

Electromagnetic radiation emitted when very high-energy electrons encounter magnetic fields.

Systéme Internationale d'Unités (SI)

The coherent and rationalized system of units, derived from the MKS system (which itself is derived from the metric system), in common use in physics today. The fundamental SI unit of length is the meter, of time, the second, and of mass, the kilogram.