Spiral Galaxy

A galaxy consisting of a flattened rotating disk of stars, a central bulge and a surrounding halo. The disk is prominent due to the presence of young, hot stars which are often arrayed in spiral patterns. The characteristic appearance of these bright spirals gives the galaxy type its name.

Figure: The great galaxy in Andromeda. This is an example of a Spiral Galaxy, probably similar to our own Milky Way. The discovery by Hubble of Cepheid variables in Andromeda allowed the distance to this galaxy to be determined, thus demonstrating the existence of independent galaxies outside the Milky Way.