How do galaxies form

As you may know, galaxies are mainly of two kinds: spiral galaxies and elliptical galaxies (there are also irregular galaxies).

Spiral galaxies form from the collapse of a protogalactic cloud. Spiral galaxies consist of three components: a rotating disk, a bulge and a halo. When the protogalactic cloud collapses, the stars in the bulge and halo form first. These stars have rather random orbits around the galactic center. The remainder of the cloud forms a disk due to the conservation of angular momentum (the same effect as the spinning up of the dancer when she pulls her arms inside). The stars in the disk form later and thus the disk population of stars are younger than those in the bulge and the halo. Further, the stars in the disk rotate around the center of the galaxy in a collective, well defined way unlike the stars in the bulge and halo.

Elliptical galaxies are thought to be formed as a result of a merger of two disk galaxies. When two spiral galaxies merge, then the orbits of all the stars are randomized. As a result, all the stars in an elliptical galaxy have random orbits and there is not much collective motion of stars.

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