How do spirals form?

Answer 1

Spiral galaxies form from the collapse of a protogalactic cloud. Spiral galaxies consist of three components: a rotating disk, a bulge and a halo. Spiral galaxies, like the Milky Way, owe their shape to stars inside the protogalaxy developing at different intervals. The gas between forming stars continues to be compressed, and the resulting gravitational differences manhandle the protogalaxy's stars, dust and gas. 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 galactic center probably contains a supermassive black hole, which likely exerts some gravitational influence on the formation of a spiral galaxy.

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). This motion forces everything into a rotating disk, and additional differences in gravity build the spiral arms. Oppositely, when a protogalaxy's stars develop at the same time, you have an elliptical galaxy on your hands. 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.