| Density Waves |
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| • | Barred
or spiral kinematic waves = density waves. |
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| Lin
and Shu tackled |
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| this
problem in the |
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| 1960s,
using self- |
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| gravity
to make the |
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| precession
rates |
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| coincide
such as to |
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| produce
long-lived |
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| spiral
structures. |
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