Glossary main content.



Electromagnetic radiation emitted by a gamma-ray burst after the initial explosion. Afterglows can last for hours or days.


The scientific study of the Universe.


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


A gaseous envelope surrounding a star, planet, or satellite, and bound to it by gravity.

black hole

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


Of or relating to the Universe as a whole.


The Universe regarded as a whole, including all matter, energy, and space.


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

gamma-ray burst

A brief cosmic explosion of gamma-rays. Gamma-ray bursts are thought to arise when a star collapses into a black hole or from the merger of two neutron stars.


Invisible electromagnetic radiation (light) with wavelengths shorter than X-rays, or less than 1 picometer (one millionth-millionth of a meter). Gamma-rays are the highest-energy radiation on the electromagnetic spectrum.

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.modelA theoretical system that represents scientific processes using a set of variables and the quantitative relationships between them.

neutron star

An extremely dense collapsed star consisting mainly of neutrons. A neutron star is what often remains after the supernova explosion of a massive star.


The path of one celestial body moving around another under the force of gravity.


The emission of energy.

radio waves

Low energy electromagnetic radiation, with long wavelengths and low frequencies.satelliteA body that orbits around a larger body.


Where matter is not.

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.

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.