Glossary main content.

Glossary

astronomy

The scientific study of the Universe.

astrophysics

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

atmosphere

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

binary star system

Two stars orbiting their common center of gravity.

cosmic

Of or relating to the Universe as a whole.

cosmology

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

cosmos

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

galaxy

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

Northern Hemisphere

The half of Earth north of the equator.

optical

Relating to or using light in visible wavelengths.

radio waves

Low energy electromagnetic radiation, with long wavelengths and low frequencies.

seeing

The steadiness of the earth's atmosphere affecting the sharpness of astronomical observations. The steadier the atmosphere, the better the seeing.

Southern Hemisphere

The half of Earth south of the equator.

spectrograph (or spectroscope)

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

star

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

telescope

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

Universe

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