Universe Glossary T-Z

A-G | H-M | N-S | T-Z

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.


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.

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.


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


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.

visible (or optical) light

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


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.

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.


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.


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.