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Astronomy and Cosmology
Astronomy and Astrophysics
Orientation of the Earth, Sun and Solar System in the Milky Way
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[QUOTE="Janus, post: 5865700, member: 4"] The present model for the spiral arms is that they are caused by density waves moving through the galaxy independently of the orbits of the stars. Stars on the inner part of the galaxy orbit faster than the density waves move and those far out orbit slower. Thus stars move in and out of the spiral arms over time. The relative brightness of the arms is caused by the increased density causing greater star formation. By the time the stars formed in the density wave leave it, the bright massive stars have died, leaving only the cool dim stars with longer lifetimes. So the brightness of the spiral arms isn't about a difference in the number of stars in the spiral arms but rather that there is a greater population of young bright stars [/QUOTE]
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Astronomy and Cosmology
Astronomy and Astrophysics
Orientation of the Earth, Sun and Solar System in the Milky Way
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