Glossary main content.



The scientific study of the Universe.


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

binary star system

Two stars orbiting their common center of gravity.


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


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


When two waves simultaneously travel through the same medium at one time, they combine and interfere. The shape of the resulting wave is determined by the sum of the separate amplitudes of each wave.

kilometer (km)

A unit of length equal to 1,000 meters, or 0.62 miles.


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.opticalRelating to or using light in visible wavelengths.


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.

reflecting telescope

A telescope that uses mirrors to bring light to a focus.


The degree of detail of an image.


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.


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