How did the Galaxy get to be the way it is?

Recall what the Galaxy looks like.


A clue: Population I and Population II stars

The characteristics of the population of stars in the galaxtic disk are not the same as the characteristics of halo stars (e.g. stars in globular clusters).

Recall another clue: gas is in the disk, not in the halo


A basic hypotheses

A plausible history

  1. Begin with a giant gas cloud .
  2. In small dense clouds within the big cloud, stars and globular clusters form.
  3. Slowly the big cloud is pulled together by gravity.
  4. The big cloud forms a disk.
  5. The disk part continues to evolve by the cycle of new star formation.
  6. Meanwhile, halo stars can easily pass through the disk.
  7. Any gas in the halo part is removed when it collides with the disk.
  8. This leaves a disk of gas and stars plus a halo of stars without gas.
The broad outlines seem pretty clear. But the early stages are pretty murky since the evidence has been erased.

What is the time scale for this?

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA