Galaxies
Galaxies have sizes of about 100,000 ly and contain some 100 to 200 billion stars. They can be subdivided into two different kinds, namely spiral and elliptical galaxies. Spiral galaxies exhibit the shape of a thin disc consisting of stars and gas, which rotate about a common gravitational centre over a period of about 250 million years. Elliptical galaxies appear structureless, with no evidence for systematic rotation. Stars in them move on stretched elliptical orbits about the common gravitational centre. Elliptical galaxies have formed via gravitational merging of larger galaxies, in which a large fraction of their gas has been turned into stars. The remaining gas forms hot corona around the elliptical structure of the stellar component. Apart from these large spiral and elliptical galaxies there are numerous so-called dwarf galaxies, which have masses of only one tenth to one hundredth of that of massive galaxies. Most galaxies come in pairs, groups, or even clusters. The latter often have thousands of galaxy members. This grouping or clustering of galaxies results from hierarchical structure formation, and represents the distribution of density fluctuations after the Big Bang.