A galaxy with spiral arms. Edwin Hubble divided spiral galaxies into two broad groups: those with a central bar (SB galaxies) and those without (S). Each group he further subdivided into three categories, a, b and c. Sa and SBa galaxies have tightly wound arms and a relatively large central bulge. Sc and SBc galaxies have loose arms and a small central bulge, while types Sb and SBb are intermediate between the two extremes.
Our own Galaxy (the Milky Way) is a spiral, possibly with a small central bar. Its structure is fairly typical: young stars and interstellar material are concentrated in a disc, particularly in the spiral arms, and there is a surrounding spherical halo containing old stars and globular clusters.
The spiral arms are not permanent rigid structures but are thought to have the character of ensity waves. As stars and interstellar material orbit around the centre of the galaxy, they create regions of enhanced density in a spiral pattern. The arms are actually made up of different stars and gas clouds at different eras of time.
