How many Shapes of Galaxies are there?
Hubble, in the Mt. Wilson days, devised a classification system for galaxies. The three basic shapes of galaxies are elliptical, spiral, and barred spiral. Elliptical galaxies appear as flattened discs of light with no arms. This type far outnumbers the other two in the known universe. Ellipticals are thought to be much older, evolved galaxies where there are few irregularities in interstellar gas density, and therefore few star births taking place. Spirals like our own, on the other hand, have gigantic regions of low and high particle density which are conducive to star birth. When the separated regions of particles and gas orbit at slower angular speeds for greater distances from the nucleus, spiral arms are formed naturally. Within the spiral group, galaxies differ based on how tight the spiral arms are (see Hubble’s diagram). In barred spirals, the material that would normally form regular spiral arms is somewhat faster-moving, that is, the angular speed decreases less farther from
