Answer to Question #202926 in Astronomy | Astrophysics for Prachi

Question

Explain Hubble’s scheme of galaxy classification with the help of a diagram. Describe the prominent features of spiral galaxies.

Answer 1

This diagram is taken from en.wikipedia.org

Hubble's classification consists of three large types of galaxies. Early type galaxies are elliptical (E0 to E7, the larger number is, the more oblate galaxy is) and lenticular (S0), and late-type galaxies are spiral with (S-type) or without (SB-type) central bar. Letters after S and SB show the tightness of the spiral arms and the prominence of the central bulge, letter a corresponds to the tight arms and huge bulge.

The spiral galaxies are disc galaxies with prominent spiral arms consisting of young massive stars, gas and star formation regions. The mechanism of spiral formation is not exactly known. Moreover, some spiral galaxies have a large ellipsoid called bar in their central regions. Such galaxies are called SB-type galaxies (spiral-with-bar) and make a distinct branch on the diagram. Numerical simulations show that bar and maybe spiral arms are transient features.

Also not all material is situated in a central bulge/bar and disk. A huge dark halo is thought to exist in spiral galaxies, that influences the rotation curve and makes a plato on it.