In
astronomy, a
bulge is a tightly packed group of
stars within a larger formation. The term almost exclusively refers to the central group of stars found in most
spiral galaxies (see
galactic spheroid). Bulges were historically thought to be
elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the
Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.