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SPIRAL WAVE rotates with galaxy, but
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slower than individual stars
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Like moving traffic jam after an accident has
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been cleared
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Gas (and stars) catch up with wave, move
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through it, eventually reach front
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Just like cars catching up with moving traffic
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jam, eventually get through it
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Gas is more crowded in wave – clouds
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collapse to form new stars
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More collisions in the traffic jam
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There are slightly more old stars in the arm
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too, because they speed up slightly coming
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into it and slow down slightly moving out of it.
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But the best tracers are the things that mark
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recent cloud collapses:
O,B stars, etc.
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