Universe Glossary T-Z

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telescope

An instrument used to observe distant objects by collecting and focusing their electromagnetic radiation. Telescopes are usually designed to collect light in a specific wavelength range. Examples include optical telescopes that observe visible light and radio telescopes that detect radio waves.

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 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 increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both special and general relativity, and is quite pronounced for speeds approaching the speed of light and in regions of high gravity.

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.

transit

The passage of a celestial body or satellite across the face of a relatively larger body as seen from Earth, such as distant planet crossing the face of its host star, slightly but measurably dimming it.

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.

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)

Electromagnetic radiation at wavelengths shorter than the violet end of visible light, ranging from 100 to 380 nm. Earth's atmosphere blocks most UV light from reaching the surface.

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 Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.

vacuum

A region containing no matter. The space between planets and stars is mostly empty, with small amounts of gas, dust and particles, so it is a near vacuum.

variable star

A star whose luminosity (brightness) changes with time.

visible (or optical) light

Electromagnetic radiation at wavelengths the human eye can see, with ranging from red (about 750 nanometers) to violet (380 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.

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 adjacent peaks in a series of periodic waves. See also: electromagnetic spectrum.

wave-particle duality

A principle of quantum mechanics that implies that light (and, indeed, all subatomic particles) sometimes act like a wave, and sometimes act like a particle, depending on the experiment performed. For instance, low-frequency electromagnetic radiation tends to act more like a wave than a particle, and high-frequency light tends to act more like a particle than a wave.

weight

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

white dwarf

A star that has exhausted most or all of its nuclear fuel and has collapsed to a size comparable to Earth's. Typically, a white dwarf packs a mass roughly equal to the Sun's into a ball just 1% the Sun's size.

Wien's displacement law

For a blackbody, the product of the wavelength corresponding to the maximum radiance and the thermodynamic temperature is a constant. As a result, as the temperature rises, the maximum of the radiant energy shifts toward the shorter-wavelength (higher frequency and energy) end of the spectrum. Named for German physicist Wilhelm Wien (1864-1928).

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

This massive, aging star is typically more than 25 times the mass of our sun. Due to its strong stellar winds, the star consistently loses mass at a very high rate.

X-ray

Electromagnetic radiation of very short wavelength and high energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.

X-ray binary

A system in which a normal star and a compact object are gravitationally bound and orbit their common center of mass. The compact object may be a white dwarf, a neutron star, or a black hole. The separation is so small that matter from the star flows toward the compact object, collects into an accretion disk around it, and becomes hot enough to emit X-rays.

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

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

The inner Solar System is filled with tiny dust particles from disintegrated asteroids and comets. Sunlight reflects off those particles, creating a glow called the zodiacal light that can be observed from the ground on Earth. For both hemispheres, spring is the best time to see the zodiacal light in the evening and autumn is the best time to see it before dawn.