Glossary main content.


The rate of change in velocity over time.

active galactic nucleus (AGN)
A galaxy with an unusually strong output of energy, thought to be powered by a supermassive black hole in its core.

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.

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

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.

cosmic rays
Fast-moving, high-energy subatomic particles, mainly protons, that permeate the galaxy.

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

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

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.

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

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.

kilometer (km)
A unit of length equal to 1,000 meters, or 0.62 miles.

Anything that takes up space.

Northern Hemisphere
The half of Earth north of the equator.

nucleus (pl. nuclei)
The “core” of an atom, containing the atom’s protons and neutrons.

A positively-charged subatomic particle. Every atomic nucleus contains one or more protons.

The emission of energy by waves (including light) or particles.

The emission of energetic subatomic particles and/or gamma rays from the decay of unstable atomic nuclei.

Southern Hemisphere
The half of Earth south of the equator.

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.

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

The speed and direction of an object’s motion.

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.

Electromagnetic radiation with wavelengths shorter than ultraviolet light but longer than gamma rays.