Glossary main content.



The rate of change in velocity over time.


The scientific study of the Universe.


The branch of astronomy that deals with the physics of astronomical objects and phenomena.

Big Bang

The birth of the observable Universe, about 13.7 billion years ago, in an explosion of space itself. The Universe started expanding at the Big Bang and continues to do so to this day.

charge-coupled device (CCD)

A device that uses a light-sensitive silicon chip to detect and quantify photons of light. The light signal is collected into pixels, or picture elements.


Of or relating to the Universe as a whole.cosmic horizonThe apparent edge of the observable Universe. The cosmic horizon is visible in all directions, and its distance is as far as light has traveled in the Universe.


The astrophysical study of the Universe as a whole, including its origin, evolution, structure, and dynamics.

dark energy

A theoretical form of energy that may be causing the rate of expansion of the Universe to increase.


A relatively massive assembly of stars, interstellar clouds, and dark matter bound together by gravity.


The force of attraction acting between any two masses (according to Isaac Newton); the curvature of space by matter (according to Albert Einstein).


The distance that light travels in one year (63,000 astronomical units, or 9.46 trillion kilometers), a convenient unit of measurement for interstellar distances.

observable Universe

The part of the Universe lying within our cosmic horizon.


The lengthening of light waves due to the motion through space of a light source and an observer away from each other (the Doppler red shift), or due to the expansion of space itself (the cosmic redshift). It is so-named because red light is at the long wavelength end of the visible spectrum.


Where matter is not.

spectrograph (or spectroscope)

An optical instrument designed to divide light into its many colors.


The catastrophic explosion of a star, which blows off most of its mass, increasing in brightness by as much as a billion times. A Type I supernova is due to the thermonuclear detonation of a compact white dwarf star which becomes unstable by accreting mass from an orbiting companion star. A Type II supernova results from the gravitational collapse of a massive star that has exhausted its nuclear fuel.


An instrument designed to gather and focus electromagnetic radiation (light) to study celestial objects and events.


The distance between successive wave crests, or troughs. Light of different wavelengths has different properties, such as color.