Galaxies

Part of Hall of the Universe.

Galaxies_HERO AMNH/D. Finnin

Galaxies are the basic building blocks of the universe. Even in a tiny patch of sky, many hundreds can be seen. By surveying such regions, astronomers estimate that there may be 100 billion galaxies in the observable universe. These galaxies are not uniformly spread through space — they commonly occur in pairs, groups, clusters, and superclusters.

In This Section

What is a Galaxy? Galaxies are titanic swarms of tens of millions to trillions of stars, orbiting around their common center of gravity. Gravity and Rotation Everything in a galaxy orbits around the galaxy’s center. Spiral Galaxies Spiral galaxies have three visible parts: a disk of stars; a central bulge of older stars; and a halo of the oldest stars. Elliptical Galaxies Elliptical galaxies have smooth, rounded shapes because the orbits of their stars are oriented in all directions. Irregular Galaxies Irregular galaxies have a chaotic appearance, and are usually small. Other Galaxy Classifications The classification of galaxies by shape relies upon images made with visible light. Formation and Evolution of Galaxies In the early universe, there were no galaxies. Today, there are many billions. How did they form? Interacting Galaxies When galaxies pass close to each other or collide, gravity pulls them apart, twisting and distorting their shapes. The Milky Way and Its Neighbors The Milky Way Galaxy resides in a neighborhood of a few dozen galaxies called the Local Group.