What is a galaxy?
A galaxy is a huge system of stars, gas, dust and dark matter held together by gravity. Our galaxy, the Milky Way, contains hundreds of billions of stars, including the Sun.
Galaxies range from small dwarf galaxies with a few million stars to giants with trillions. Gravity keeps everything in orbit around the galaxy's center, where most large galaxies hide a supermassive black hole. The Milky Way's central black hole, Sagittarius A*, has about four million times the mass of the Sun and was first imaged in 2022.
Astronomers sort galaxies by shape. Spiral galaxies, like the Milky Way and Andromeda, have curving arms around a bright center. Elliptical galaxies look like smooth, round or oval blobs of older stars. Irregular galaxies have no clear shape, often because they were pulled apart by the gravity of a neighbor or a collision.
How many galaxies are there?
Deep images from the Hubble and James Webb space telescopes suggest the observable universe contains hundreds of billions to perhaps two trillion galaxies. Most are too faint to see without a telescope. From dark places, however, you can spot the Andromeda Galaxy with the naked eye as a faint smudge of light about 2.5 million light-years away.
Did You Know?
The Milky Way and Andromeda galaxies are moving toward each other and may collide and merge in several billion years, although recent studies suggest it is not certain they will.