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).
- Disk stars (population I)
- Both heavy stars and light stars.
- A good supply of heavy elements.
- Halo stars (population II)
- Only light stars.
- Not much of heavy elements.
Recall another clue: gas is in the disk, not in the halo
- Halo stars formed a long time ago.
- There probably were some heavy ones, but heavy stars don't live
long, so they are gone.
- A long time ago, the concentration of heavy elements was lower than
it is now in the disk, so the halo stars are low in heavy elements
- Disk stars formed later, and more are made continually.
- In particular, heavy stars are still made.
- After generations of star production, the heavy element concentration
has built up.
A basic hypotheses
- The Galaxy formed by gravitational attraction.
A plausible history
The broad outlines seem pretty clear. But the early stages are pretty
murky since the evidence has been erased.
- Begin with a giant gas cloud .
- In small dense clouds within the big cloud,
stars and globular
- Slowly the big cloud is pulled together by gravity.
- The big cloud forms a disk.
- Angular momentum conservation causes it to rotate.
- Recall that gas clouds can
orbit in a disk without bumping into each other.
- The disk part continues to evolve by the cycle of new star
halo stars can easily pass through the disk.
- Any gas in the halo part is removed when it
collides with the disk.
- This leaves a disk of gas and stars plus
a halo of stars without gas.
What is the time scale for this?
- How long did it take?
- Time for Sun to orbit once around the center of the Galaxy is
approximately 200 x 106 years.
- That gives us a rough idea of how long it would take the matter to fall
toward the center of the Galaxy and form a disk.
- This suggests that the Galaxy could form pretty completely in 1000 x 106 = 1 x 109 years.
- When did it happen?
- The oldest things around should be the globular clusters.
- Massive stars die quickly, so look at a globular cluster and
ask ``how massive are the most massive stars that are still alive burning hydrogen in their cores.''
- Knowing from stellar models how long such a star should last gives
an estimate of the age of the globular cluster.
- Result: the age of globular clusters is in the range 12 x 109 years to 15 x 109 years.
- So that's about how old the Galaxy must be.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA