| Spiral Galaxies |
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| � | Already
discussed many observational |
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| properties
(e.g., frequency, types, |
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| luminosity
functions, rotation curves, light |
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| profiles,
masses, etc.). |
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| � | Now
want to spend some time discussing |
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| their
most notable feature � the spiral arms! |
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| It
hasn�t been such an easy thing to |
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| understand.� Why not? |
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