Glossary main content.



The scientific study of the Universe.


The smallest individual particle that retains the distinctive properties of a given chemical element.

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.b

black hole

A region in space where gravity is so strong that space closes back on itself, allowing nothing, not even light, to escape.

celestial hemisphere

One-half of the celestial sphere.

celestial sphere

An imaginary sphere in space with Earth at its center. All space objects we see appear to lie on the sphere, yet in reality are varying distances away from Earth.

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 horizon

The 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.


The Universe regarded as a whole, including all matter, energy, and space.dark matterUnseen matter that is detected only by its gravitational pull on visible matter. Most of the Universe is evidently made of dark matter. Its nature is yet to be determined.


Information, often in the form of measurements or observations, which can be analyzed.


The arrangement of objects in space.

electromagnetic spectrum

The complete array of electromagnetic radiation (light). In order of increasing wavelength (decreasing frequency and energy), the spectrum ranges from gamma rays through X-rays, ultraviolet light, visible light, infrared radiation, microwaves to radio waves.


The capacity of a physical system to do work. Energy can be converted among its various forms (motion, light, mass, etc.) but the total amount of energy remains constant.exposureThe presentation of photographic film or plates to light.

field of view

The area that is visible through an instrument.


A relatively massive assembly of stars, interstellar clouds, and dark matter bound together by gravity.gravityThe force of attraction acting between any two masses.


A modification of Big Bang theory which proposes that the infant Universe went through a very brief period of extremely rapid expansion. Inflation suggests that the Universe is vastly larger than its observable part.

infrared (IR)

Invisible electromagnetic radiation (light) with wavelengths longer than red light and shorter than microwaves. Infrared light occupies the spectral band extending from 0.75 to about 200 micrometers.


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.


A measure of the total amount of matter on a body. It can be defined either by the body's inertia (resistance to altering its motion) or by its gravitational influence on other bodies.


Anything that takes up space.

observable Universe

The part of the Universe lying within our cosmic horizon.


Relating to or using light in visible wavelengths.


A quantum of light, the energy of which is related to the wavelength of the radiation.


An astronomical body with enough mass for its gravity to make it spherical but not enough to generate nuclear energy. Planets have nonintersecting orbits around stars. Some might drift freely in space.


The highly luminous core of a remote galaxy, thought to be powered by a supermassive black hole. Quasars look like stars on an ordinary photograph but have very different spectra.


The emission of energy.spaceWhere matter is not.spectrograph (or spectroscope)An optical instrument designed to divide light into its many colors.


The practice of using a spectrograph to obtain a spectrum.

spectrum (pl. spectra)

The range of electromagnetic radiation (light) expressed in terms of frequency or wavelength. A rainbow displays the spectrum of visible light.


A self-luminous body held together by its own gravity and with a central temperature and pressure sufficient to generate nuclear energy.


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

theorist (or theoretician)

An astrophysicist who forms theories of how space systems work, relying on other researchers√É  observations to test whether the theory is correct.


A detailed explanation of the behavior of a physical system based on experiments. Theories may be revised if new data warrant.

ultraviolet (UV)

Invisible electromagnetic radiation (light) with wavelengths shorter than violet light and longer than X-rays. Ultraviolet light occupies the spectral band extending from 300 nanometers down to about 10 nanometers.


The physical system that encompasses all matter, energy, and space that exists.

visible light

The portion of the electromagnetic spectrum corresponding to the visible colors, with wavelengths longer than ultraviolet light and shorter than infrared radiation. Visible light occupies the spectral band extending from 300 nanometers to about 750 nanometers.


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