What is a galaxy?
A galaxy is a large gravitationally bound system of stars, gas, dust and dark matter. The Solar System belongs to the Milky Way; galaxies are fundamental structures in the distribution of matter across the universe.
Types and structure
Spiral galaxies show disks, arms and central bulges; elliptical galaxies appear rounder and often lack a prominent disk. Irregular galaxies show no clear symmetry. These labels describe appearance alone and do not determine a galaxy’s mass, age or every star’s properties. Most stars in the Milky Way cannot be distinguished individually with the unaided eye. New stars can form in galactic gas clouds, while stars enrich surrounding gas over their lifetimes.
Interactions and observation
Galaxies may gather in clusters and interact or merge under gravity. Direct collisions between individual stars are rare; gas clouds can be compressed, affecting star formation. Because light takes time to reach us, observing a distant galaxy reveals an earlier state. Radio, infrared and X-ray observations expose different processes alongside visible light. Many large galaxies host a supermassive black hole at the center, but the galaxy is much more than that hole.