The Ages and Metallicities of Dwarf Ellipticals James Schombert Univ. Oregon A galaxy is a collect of stars, gas and dust bound together by their common gravitational pull. Galaxies have certain features in common.
Gravity holds their stars together.
The densest region is in the center, called a core or bulge. Some galaxies are disk shaped with pinwheel arms, called spirals. Others are smooth and elliptical in shape. Most of the mass of a galaxy is in dark matter and stellar reminants, but almost all of the optical luminosity is due to photons emitted from normal stars. Thus, a study of the appearence and total luminosity of galaxies with time is a question of the behavior of the stellar population of which they are composed.