4
Traditional
Model of Galaxy Evolution
•Stars
are stuck with their original orbits
–They
plow through gas like bullets
•Orbits
of gas can evolve
–Gas
clouds collide and only average motion (rotation) survives
•Metal
abundance grows with time
•System
starts out with little organized motion,few metals
–Halo
stars form at this time
•It
contracts, velocities average out leaving only rotation
–Gas
collapses to form the disk
–Disk
stars form after this has happened
•Some
problems with traditional model
–Globular
clusters not all same age
–Gap
in ages between halo and disk objects
–Presence
of some metals in even in oldest stars
•System
may have formed by merger of smaller galaxies
–Galactic
Cannibalism
From our text: Horizons, by Seeds