In astronomy, a bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies.
The bulge in galaxy spirals is usually composed of Population II stars, small, red and old. This is because all stars were born together with the galaxy, thus at least several billion years ago. Only small and reddish stars can live for this long time.
Most bulges are thought to host a supermassive black hole at their center. Such black holes have never been directly observed, but many indirect proofs exist.
Some galaxies have bulges with Population I blue, young stars, or a mix of the two populations. While far from clearly understood, this is usually taken as evidence of interaction with another galaxy (such as galaxy merging ), that sends new gas to the center and promotes star formation.