Astronomy Glossary

Astronomical Unit – The average distance between Earth and the Sun: about 93 million miles. Note: It is often used to measure distances within our Solar System or in multiple-star systems; for example, Jupiter is more than 5 AU from the Sun.

Astronomy – The branch of science that deals with celestial objects, space, and the physical universe in general. Note: It is not the same as astrology, which is related to fortune-telling.

Big Bang – A secular, naturalist theory of the origin of the universe which proposes that all mass, energy, and space were contained in a point that rapidly expanded to become stars and galaxies, et cetera, over billions of years. Note: A viable alternative can be found in Creationism.

Black Hole – A place in space where gravity is so strong, and matter so dense, that nothing can escape it…not even light. It cannot be directly observed, only indirectly from the material and activity around them. It ranges in size from ten to billions of solar masses. Note: Most galaxies are believed to have “supermassive” central black holes.

Constellation – Visual groupings of stars that divide the night sky into shapes and sections for easier identification and location of night sky objects; many have ancient and mythical origins. Note: Some famous ones are Andromeda, Gemini, Leo, Orion, Scorpio, Ursa Major, and Virgo.

Creationism – A theological, supernatural view of the origin of all things, whereby God created everything with value and purpose, and without Him nothing was made (see Genesis 1 and John 1 in the Bible). Note: It need not oppose science, and science need not oppose it.

Galaxy – A system containing millions to trillions of stars, along with gas and dust, held together by gravitational attraction. Note: Scientists think the universe contains 10^10 ~ 10^12 galaxies, with most being spiral, elliptical, or irregular in shape. (Andromeda is a famous, bright galaxy.)

Globular Cluster – A dense ball of stars held together by gravity; typically tens or hundreds of thousands within a radius of a 100 light years or less. Note: Some are viewed as possible failed galaxies, and most of the stars appear yellow or red. (Hercules is a famous, bright globular.)

Light Year – The distance light travels in 365 days: approximately 5.9 trillion miles / 9.5 trillion kilometers. Note: Light from the Sun takes about 8 minutes to reach Earth.

Local Group – A cluster of a few dozen nearby galaxies, and of which our galaxy is a member. Note: The Andromeda and Milky Way Galaxies dominate the Local Group.

Magnitude – A logarithmic measure of the brightness of celestial bodies. Lower numbers – even negative numbers – mean brighter objects. Note: Magnitude 6.0 is roughly the naked-eye limit.

Messier Catalog – The most popular catalog of astronomical objects. It contains 110 items that range from galaxies to star clusters to nebulae. It was chronicled by French astronomer Charles Messier in the late 18th century. Note: It is referenced by amateurs and professionals alike today.

Milky Way – A faint, cloud-like band of light crossing the night sky; the entire galaxy hosts 100-500 billion stars (including our Sun) and spans some 100,000 light years. Note: It also has 150+ globular clusters, 1100+ open clusters, many famous nebulae, and 15-30 dwarf satellite galaxies.

Nebula – A cloud of interstellar gas and dust, often illuminated by nearby stars. Note: Its variant forms include emission, reflection, absorption / dark, planetary, and supernova remnants. (Orion is a famous, bright nebula.)

Open Cluster – A group of stars loosely held together by gravity; generally tens, hundreds, or a few thousand stars lie within a radius of 20 light years, and many are white or blue. Note: Some were known in ancient times. (Pleiades is a famous, bright open cluster.)

Planet – A large, spherical celestial body that orbits a star on a fairly unobstructed path. It may or may not have moons, and it comes from the Greek word meaning “wanderer.” Note: We have identified about 2000 planets so far, but the total number is likely exponentially greater.

Solar System – The nearby area in space over which our Sun has the dominant mass and gravity. It includes the Sun, the four inner rocky planets (including Earth), the four outer gaseous planets, several dwarf planets, 100-200 moons around the planets, an asteroid belt, meteors, comets, and distant Kuiper Belt objects. Note: Most stars can be said to have “solar systems” of some kind. 

Star – A huge celestial sphere of hot, luminous gas, mostly hydrogen and helium. Its own gravity holds it together, while nuclear fusion (which forces lighter elements into heavier ones) produces its energy. Note: Scientists think the universe contains 10^20 ~ 10^24 stars, and they vary widely in size, heat, color, brightness, and distance. (Sirius is a famous, bright star.)

Virgo Cluster – A huge cluster of about 2000 galaxies behind the stars of the Virgo constellation.