Astronomy Glossary

A large, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter, each of which orbits a center of mass. Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology as elliptical, spiral, or irregular. Most of the galaxies in the observable universe are between 1,000 and 3,000 parsecs (3,300 and 9,800 ly) in diameter though some, including the Milky Way, are much larger.

A collective name for the four moons of Jupiter discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto.

A giant planet composed mainly of hydrogen and helium gases rather than heavier elements, e.g. Jupiter and Saturn in the Solar System.

A tight, spherical conglomeration of many thousands of stars which are gravitationally bound to each other and which orbit a galactic core as a satellite. They differ from open clusters in having a much higher combined mass, with a typical lifespan extending for billions of years.