Within it, globulars orbit the Galaxy on extremely elongated elliptical paths. Most of the time, the globulars move slowly through the halo at the outer extremes of their orbits; only briefly do they whip in and around the nucleus.
These stars exhibit the motions of the cloud from which they were formed. So the Galaxy must have been born form a gas cloud that was initially huge- at least 300,000 ly in radius.
Imagine a tremendous, ragged cloud of gas roughly twice as big as the Galaxy’s halo today. Its density is low. This proto Galaxy cloud probably is turbulent, swirling around with random churning currents.
Slowly at first, the cloud’s self-gravity pulls it together, with it central regions getting denser faster than its outer parts.
Throughout the cloud, turbulent eddies of different sizes form, break up, and die away. Eventually, the eddies become dense enough to contain sufficient mass to hold themselves together. These might be hundreds of light years in size – incipient globular clusters.
Each blob then splits up to form individual stars – all born at about the same time.
Meanwhile, the gas contracts more and fall slowly into a disk. Why a disk? Because the original cloud had a little spin, and the conservation of angular momentum requires that it spin faster around its rotational axis as it contracts.
The kinetic energy energy of the cloud slowly decreases, as gas clouds collide and heat is radiated away.
The disk rapidly flattens. As the disk forms, its density increases and more stars form. Each burst of starbirth leaves behind representative stars at different distances from the present disk.
Finally, the remaining gas and dust settle into the narrow layer as we see today. Somehow density waves appear and drive the formation of spiral arms.
During this time, massive stars were manufacturing heavy elements and flinging them back into the cloud by supernova explosions.
So as stars were born in succession, each later type had more heavy elements. That enrichment continues today in the disk of the Galaxy.
The Birth of the Galaxy
