Cosmos: Glossary / Terms

Here are some basic terms used for describing star patterns, galaxies and nebulae. Find more terms in the Glossary.

Asterism: A named group of stars that is part of a constellation; the Big Dipper is an example of one.
Celestial Equator: The imaginary line around the sky directly above the Earth's equator.
Celestial Pole: Points above the Earth's north and south poles.
Celestial Sphere: An imaginary sphere surrounding the Earth where the sun, planets and stars are, a 3 dimensional map of the universe. Click here for more about the celestial sphere.
Constellation: A pattern of stars usually named after animals or people in stories. Now used to designate an area in the celestial sphere.
Ecliptic: The path the sun seems to follow in the sky.
Emission Nebula: A cloud of glowing gas.
Globular Star Cluster: A group of stars that look like a shape of a ball.
Light-year: The distance light travels in a year. About 10¹³ km or 6 trillion (thousand billion) miles.
Nebulae: A glowing clouds of gas or dust reflecting the light of nearby stars.
Open Star Cluster: A group of stars that look close together in the sky.
Orbit: The path an object takes as it moves around another object.
Planetary Nebula: An expanding ring of gas around a star.
Reflection Nebula: Dust and gas reflecting light from stars close by.
Rotation: An object spinning about its center.
Zenith: The point in the sky directly overhead.

Terms