Glossary of Terms

On this page, you will find common astronomical terms and definitions.  The names behind many acronyms can be found here as well.  Underlined terms have links to pages with more detailed information.  Simply click on the link and you will be taken to that page.

Cluster of Galaxies - A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.

Dark Matter - Observations reveal that vast halos of invisible matter surround galaxies and galaxy clusters. This dark matter adds up to many times the mass of the visible stars, gas, and dust seen in galaxies.  The temperature of dark matter is a measure of how fast the particles in it are moving.   Hot refers to particles moving near the speed of light, and cold refers to particles moving much slower than that. The future of our universe depends on how much dark matter exists.  More dark matter means the universe will eventually collapse while less dark matter means an ever-expanding universe.  Hot and cold dark matter play a crucial role in understanding how galaxies formed.  If all dark matter in the early universe were cold, galaxies would form first and later congregate into clusters.

Doppler ShiftThe apparent change in wavelength of sound or light caused by the motion of the source, the observer, or both.  Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching or redshifted (elongated) if receding.   It occurs both in sound and light.  How much the frequency changes depends on how fast the object is moving toward or away from the observer.

Electromagnetic Spectrum - The full range of frequencies, from radio waves to gamma-rays, that characterizes light.

Elements - The fundamental kinds of atoms that make up the building blocks of matter, which are each shown on the periodic table of the elements.   The most abundant elements in the universe are hydrogen and helium.  These two elements make up about 80 and 20 % of all the matter in the universe respectively.   Despite comprising only a very small fraction the universe, the remaining heavy elements can greatly influence astronomical phenomena.  About 2 % of the Milky Way's disk is comprised of heavy elements.

Galaxy - A component of our universe made up of gas and a large number (usually more than a million) of stars held together by gravity.

Gravity - A mutual physical force attracting two bodies.

GSFC - Goddard Space Flight Center

Hubble's Law - The rate at which a galaxy recedes is directly proportional to its distance away from us (V=H x D, where V is velocity, H is Hubble's constant, and D is distance).  This rate can be detected by the cosmological redshift of the object.

Infrared - Electromagnetic radiation at wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns).  Almost none of the infrared portion of the electromagnetic spectrum can reach the surface of the Earth, although some portions can be observed by high-altitude aircraft or telescopes on high mountaintops.

IPAC - Infrared Processing and Analysis Center

JPL - Jet Propulsion Laboratory

Light - Electromagnetic radiation that is visible to the human eye.

Light Year - A unit of length used in astronomy which equals the distance light travels in a year. At the rate of 300,000 kilometers per second (671 million miles per hour), 1 light-year is equivalent to 9.46053 x 1012 km, 5,880,000,000,000 miles or 63,240 AU

NASA - National Aeronautics and Space Administration

Nebula - A diffuse mass of interstellar dust and gas.

Nuclear Fusion - A nuclear process whereby several small nuclei are combined 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 "Energy = Mass times the Speed of Light squared". This is the source of the Sun's energy.

Orbit - The path of an object that is moving around a second object or point.

OSC - Orbital Sciences Corporation

Quasar - A specific type of quasi-stellar source.

Quasi-stellar Source (QSS) - Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of very large redshift that is a strong source of radio waves; presumed to be extragalactic and highly luminous.

Radiation - Energy radiated in the form of waves or particles; photons.

Redshift -  An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer.  See also Doppler effect.

SDL - Space Dynamics Laboratory

Speed of Light (in vacuum), c - The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.

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

Star Cluster - A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.

Ultraviolet - Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.

USU - Utah State University

Visible - Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)

Wavelength - A property of a wave that gives the length between two peaks of the wave.

Wien's Law -    Wien's Law states that the maximum wavelength (l max) at which an object emits radiation is inversely proportional to its temperature in Kelvins.  This means that by cooling an object, its l max can be moved into a region of higher wavelength (lower frequency).


Last Updated: 1/3/99