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Possible Origin of Spiral Arms
•Differential rotation smears features out into spiral patterns
•
•But can’t be whole story:
•
•Number of times Sun has orbited the galaxy:
–10 billion yr/200 million yr
= 50 times
–Spiral arms would have been wound up very tightly
–
•Something must continuously rebuild them
From our text:  Horizons, by Seeds
From Realm of the Universe by Abell et al.