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.
Galaxies are titanic swarms of tens of millions to trillions of stars, orbiting around their common center of gravity.
Everything in a galaxy orbits around the galaxy’s center.
Spiral galaxies have three visible parts: a thin disk composed of stars, gas, and dust; a central bulge of older stars; and a spherical halo of the oldest stars and massive star clusters.
Elliptical galaxies have smooth, rounded shapes because the orbits of their stars are oriented in all directions.
Irregular galaxies have a chaotic appearance, and are usually small.
The classification of galaxies by shape relies upon images made with visible light. But galaxies observed in other wavelengths, or with longer exposure times, or at earlier cosmic epochs, show the need for additional categories.
In the early universe, there were no galaxies. Today, there are many billions. How did they form?
Galaxies are grand yet fragile structures. When they pass close to each other or collide, their mutual gravity pulls them apart, twisting and distorting their shapes.
The Milky Way Galaxy resides in a neighborhood of a few dozen galaxies called the Local Group.