Whats the reason for different types of orbits in a galaxy?
The different parts of a spiral galaxy like the Milky Way- bulge, disk and halo – resulted from the processes of its formation (I’ll say more about that next lecture). The bulge and halo formed first, from a “protogalactic cloud” of gas (no stars yet); stars then formed in these spherical regions with pretty much random orbits. After these stars formed, and the cloud contracted more, it started to form a disk, which “spun up” due to angular momentum (the process is somewhat similar to the formation of an accretion disk, but on a galactic scale). Clouds of gas ended up orbiting all in the same direction, and the stars that then formed in the disk from these moving clouds ended up moving in mostly the same direction too. See questions above for reasons for the bobbing motion of disk stars.
