Astronomy Glossary

Astronomy is a science concerned with celestial objects, phenomena and events that originate outside of Earth’s atmosphere. As such, it is an extremely complex field with an equally vast vocabulary and a significant amount of jargon. This glossary contains definitions for many of the most common astronomical terms.

Asteroid – a small, rocky celestial body that orbits a planet or minor planet and is primarily found in the asteroid belt between Mars and Jupiter or in the Kuiper belt beyond Neptune and Pluto. Asteroids are primarily made of rock and metal and have very low masses, but can occasionally contain ice. Comet – A celestial body composed of rock, dust and frozen gases that may or may not have a tail. A comet forms from the ejection of material from the surface of its parent body and, under certain conditions, can become visible to the naked eye.

Cosmic Ray – A fast-moving, high-energy radiation originating in the outer regions of the universe and traveling through the Solar System and the Milky Way galaxy. These rays are part of the cosmic background radiation and can be detected using radio telescopes.

Crater – A crater on the surface of a moon or planet that has formed from the impact of a meteor. These craters often appear to be bowl-shaped.

Constellation – A group of stars that have been given definite names, such as Ursa Major, Bootes or Cancer. Stars within a constellation are linked by the gravitational force that exists between them.

Cosmos – The Milky Way Galaxy is not the only galaxy in the universe. Other galaxies exist, but they are too far away to be seen by humans. Astronomers are working on ways to detect these other galaxies.

Luminosity – A measure of the brightness of a star that is dependant on its temperature; a hotter star is more luminous.

Hertzsprung-Russell Diagram – A plot of the relative magnitude (apparent) and spectral class/color/temperature of a star. The Hertzsprung-Russell diagram helps astronomers understand the formation of stars and other stellar bodies.

Nebula – An interstellar cloud of gas and dust that is not distinct enough to be classified as a star or planet. Examples of nebulas are the Veil Nebula and the Ring Nebula.

Radius of a Circumstellar Orbit – The distance from the center of a star to the edge of its radius. The radius of a star is an important factor when it comes to its mass, since it determines the power of its gravity.

Parabola – A type of curve in which the focus point is a line drawn through the center of the curve. Parabolic surfaces are used in astrophysics and engineering for the construction of dishes and mirrors that concentrate light to a point.

The apogee of a planet is the point in its orbit when it is at its greatest distance from the Sun. This is the reason why a full moon appears to be larger and brighter than a new moon.