<p>I came across this <a href="http://burro.astr.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html" rel="noreferrer">link</a>. 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.</p>
<p>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?</p>

CC BY-SA 4.0
2021-03-13T23:24:09.053Z
I think you're misinterpreting that page; it doesn't say that there should be an empty zone at the co-rotation radius.
Peter Erwin
CC BY-SA 4.0
2021-03-14T03:22:19.507Z
Well, no I didn't mean for it to be empty. It's just that, after all the supernova explosions & stuffs, the remains in that regions should consist of neutron stars & blackholes, overwhelmingly. So we should see that as a circle of darkness in visible light, and some bright dots in x-ray.
longtry
CC BY-SA 4.0
2021-03-14T11:31:20.133Z
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.)
Peter Erwin
CC BY-SA 4.0
2021-03-14T11:34:49.890Z
"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.
Peter Erwin
CC BY-SA 4.0
2021-03-14T13:17:03.063Z
Have a look at the initial mass function, https://en.wikipedia.org/wiki/Initial_mass_function corresponding to what PeterErwin said.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-03-15T08:40:41.947Z
That doesn't invalidate my point. With the bright, blue stars dead, the remaining puny stars and dwarfs won't be *any* significant when we see them from such distances. So the result will still be a circular disk of muddy-yellow, almost dark ring that interrupt the spiral arms at a certain distance from the core.
longtry
CC BY-SA 4.0
2021-03-15T08:43:39.897Z
And while some stars can have peculiar orbits that sometimes enter the region we're talking about, AFAIK most stars follow a somewhat circular orbit around the core. The occasional addition of other stars most likely won't contribute much to the brightness of the region.
longtry
CC BY-SA 4.0
2021-03-18T11:36:14.280Z
@longtry Stars less massive than the core-collapse-supernova limit (say, $< \sim 10$ solar masses) will still become luminous red giants towards the end of their lives, and *will* contribute significant amounts of light. (The brightest galaxies in the universe are giant ellipticals, which are nothing but light from relatively low-mass red giant stars.)
Peter Erwin
CC BY-SA 4.0
2021-03-18T11:51:06.817Z
Most stars do not follow perfectly circular orbits, and the older stars get, the less circular their orbits become (due to cumulative gravitational perturbations from giant molecular clouds, spiral arms, etc.). There is also the possibility of ["radial migration"](https://ui.adsabs.harvard.edu/abs/2018ApJ...865...96F/abstract), where stars on relatively circular orbits move inward or outward in radius. So your hypothesized region will have plenty of stars formed elsewhere in it or moving through it.
Peter Erwin
CC BY-SA 4.0
2021-03-19T07:07:26.197Z
@Peter Thanks! According to that article, most stars move outward, therefore they actually follow a spiral path around the core, not unlike the spiral arms themselves. For that, we'll have to look at the arms in a quite different light.
longtry
CC BY-SA 4.0
2021-03-19T07:11:45.423Z
Regarding giant ellipticals, I believe 1 big factor that contribute to their intensity is their shape, in other words, their depth. Even weak yellow-red light from feeble stars can be quite impressive when we look at a hundred thousand ly depth of them. But in spiral ones, the region is relatively thin, so while I retract my guess of it being dark, I remain the speculation that it should have a muddy yellow-brown color, which contrast significantly with the blue-and-pink spiral arms.
longtry
CC BY-SA 4.0
2021-08-26T15:44:27.850Z
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?
pela
<galaxy><galactic-dynamics><density><spiral-arms><theories>
Did the density wave theory for spiral arms take account of this phenomenon?
longtry
# Did the density wave theory for spiral arms take account of this phenomenon?

(Asked by: username_0 on 2021-03-13)

I came across this [link](http://burro.astr.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html). 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?

**Tags:** galaxy, galactic-dynamics, density, spiral-arms, theories
> **Comments:**
> **username_1**: I think you're misinterpreting that page; it doesn't say that there should be an empty zone at the co-rotation radius.  
> **username_0**: Well, no I didn't mean for it to be empty. It's just that, after all the supernova explosions & stuffs, the remains in that regions should consist of neutron stars & blackholes, overwhelmingly. So we should see that as a circle of darkness in visible light, and some bright dots in x-ray.  
> **username_1**: 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.)  
> **username_1**: "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.  
> **username_2**: Have a look at the initial mass function, https://en.wikipedia.org/wiki/Initial_mass_function corresponding to what PeterErwin said.  
> **username_0**: That doesn't invalidate my point. With the bright, blue stars dead, the remaining puny stars and dwarfs won't be *any* significant when we see them from such distances. So the result will still be a circular disk of muddy-yellow, almost dark ring that interrupt the spiral arms at a certain distance from the core.  
> **username_0**: And while some stars can have peculiar orbits that sometimes enter the region we're talking about, AFAIK most stars follow a somewhat circular orbit around the core. The occasional addition of other stars most likely won't contribute much to the brightness of the region.  
> **username_1**: @longtry Stars less massive than the core-collapse-supernova limit (say, $< \sim 10$ solar masses) will still become luminous red giants towards the end of their lives, and *will* contribute significant amounts of light. (The brightest galaxies in the universe are giant ellipticals, which are nothing but light from relatively low-mass red giant stars.)  
> **username_1**: Most stars do not follow perfectly circular orbits, and the older stars get, the less circular their orbits become (due to cumulative gravitational perturbations from giant molecular clouds, spiral arms, etc.). There is also the possibility of ["radial migration"](https://ui.adsabs.harvard.edu/abs/2018ApJ...865...96F/abstract), where stars on relatively circular orbits move inward or outward in radius. So your hypothesized region will have plenty of stars formed elsewhere in it or moving through it.  
> **username_0**: @Peter Thanks! According to that article, most stars move outward, therefore they actually follow a spiral path around the core, not unlike the spiral arms themselves. For that, we'll have to look at the arms in a quite different light.  
> **username_0**: Regarding giant ellipticals, I believe 1 big factor that contribute to their intensity is their shape, in other words, their depth. Even weak yellow-red light from feeble stars can be quite impressive when we look at a hundred thousand ly depth of them. But in spiral ones, the region is relatively thin, so while I retract my guess of it being dark, I remain the speculation that it should have a muddy yellow-brown color, which contrast significantly with the blue-and-pink spiral arms.  
> **username_3**: 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?  

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## 0 answers

