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CORRECTION: spiral arm patterns are thought to be due to density waves travelling through the disk. The statement in the video that the stars interior to the Sun’s galactic radius move more quickly was missing the important phrase ‘relative to the spiral arm pattern speed’. Overall of course, the actual circular velocity of stars around the centre of the galaxy is remarkably constant out to large radii as with almost all spiral galaxies: leading to the important deduction that the stellar mass of the galaxy is embedded in a larger halo of ‘dark matter’.
More Messier objects: http://bit.ly/MessierObjects
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Videos by Brady Haran
Editing in this video by James Hennessy