I came across this link. In it the author speculates that in the outer region of a galaxy, the spiral arms overtake the stars, and vice versa for the inner region. That means there must be a middle region where the speed of the arms and the stars are similar. Since the stars there are big, blue and short-lived, they must die quickly. But because no other stars or clouds are moving into this region because of the same speed, it will turn dark quite fast in cosmic terms.

The result is, every spiral galaxy should have a distinctive dark, round disk of 'nothingness' somewhere between the central core and the outer rim. Looks like it's not the case. How can we explain this discrepancy?

  • $\begingroup$ I think you're misinterpreting that page; it doesn't say that there should be an empty zone at the co-rotation radius. $\endgroup$ Mar 13, 2021 at 23:24
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    $\begingroup$ No, the "remains" will include all the other, lower-mass stars that were formed at the same time. (As well as all the older, lower-mass stars whose orbits carry them through that region.) $\endgroup$ Mar 14, 2021 at 11:31
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    $\begingroup$ "Star formation" involves the formation of many stars of different masses. This includes massive, bright, blue stars, but it also includes many more lower-mass stars which are much longer-lived, including those with main-sequence lifetimes of billions or even (for the lowest masses) trillions of years. $\endgroup$ Mar 14, 2021 at 11:34
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    $\begingroup$ Have a look at the initial mass function, en.wikipedia.org/wiki/Initial_mass_function corresponding to what PeterErwin said. $\endgroup$ Mar 14, 2021 at 13:17
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    $\begingroup$ I think this is a good question. Despite diffusion of stars, it still seems to me that the surface brightness around the corotation radius would be somewhat lower than inside and outside, at least in the B band and/or if stacking. Would you mind writing up a "formal" answer, @PeterErwin? $\endgroup$
    – pela
    Aug 26, 2021 at 15:44


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