| Spiral Galaxies |
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| � | Opening
considerations: |
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| � | Disk
Stability Issues.� Most of the mass
of the |
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| galaxy
is in a dark halo, and stars are treated as |
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| particles
in the overall potential, so should disks |
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| even
be stable? |
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| � | Note
that nearby interactions are unimportant: |
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| Trelax/tc
= (h/R)(N/(8logN)) � �collisionless� if |
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| large.� For N = 1011, the ratio of
relaxation to |
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| crossing
time is about 108. |
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