Spiral Galaxies

Spiral galaxies are divided into two subclasses: ordinary (designated S or SA) and barred (designated SB). Both have spiral arms, with two arms generally placed symmetrically about the center of the axis of the rotation. In an ordinary galaxy, the arms originate directly from the nucleus of the galaxy. In the barred spirals, on the other hand, a bar of stars cuts through the center of the galaxy and the arms originate from the ends of the bar. Both ordinary and barred galaxies are further classified, starting from `a' to `c' according to how tightly the arms are wound. In `Sa' and `SBa,' the arms are tight and they form an almost a circular pattern; in `Sb' and `SBb,' they are more open and in `Sc' and `SBc' the nuclei are small and have extended arms. The intensity of the spheroidal component of the spiral galaxies, around their nucleus, follows the r^-1/4 law as in elliptical galaxies, but the intensity of the disk components falls off at a slower rate as $I(r) \propto e^{-\alpha r}$, where $\alpha$ is a constant (Zeilik, Gregory & Smith 1992).