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>I always assumed that most stars kept approximately the same position relative to the rest of the galaxy. That was the original assumption. However, basic [0]
by gizmo686 10y ago
>I always assumed that most stars kept approximately the same position relative to the rest of the galaxy.
That was the original assumption. However, basic [0] orbital mechanics says that objects with smaller orbits orbit faster. If this were applied to the galaxies spiral arms, then as they continue to orbit, the inside ones would orbit faster than the outside ones, and the galaxy would become increasingly spirally. By this point, the galaxy would be so tightly wound that we would not even be able to see the spiral any more.
A more modern hypothesis is that the arms are more like waves of high density. Individual stars move through the the arm.
Since this high density region exerts gravity, stars that are moving out of it are slowed down, and stars that are moving into it are sped up. The end result is that orbiting stars tend to spend a little bit more time in the arm region then they would under a pure orbit. In doing so, they contribute to the mass of the arm, causing other stars to also spend more time in the arm then they otherwise would. The end result is a stable wave as all the individual stars move into and out of it throughout their orbits.
https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory
[0] As in, just applying Newtonian gravity.
- mirimir 10y agoRight, density waves. But as rtkwe and the Wikipedia article note, there's also increased star formation in the higher-density arms. So more stuff hitting Earth also seems plausible.
- LoSboccacc 10y agoa visualization of the current model orbital path http://m.imgur.com/Eup0Yoa http://m.imgur.com/Eup0Yoa
- PhantomGremlin 10y agoIt's hard to be sure. But observing both the Wiki animation and this GIF, it appears that stars don't orbit at a constant radius from the galactic center. Rather, they appear to move significantly closer and further. Does that seem right? If so, that's much different than planets orbiting our solar system. To a first approximation our orbits appear circular. Yes I know there's a certain eccentricity but for most planets it's not much. I don't like the idea of our solar system getting too close to the galactic center. It gives me a vague sense of dread, like "bad things can happen there".
- LoSboccacc 10y agoThe orbit in the model are supposedly eccentric, but the period for a complete prbit is quite long, allegedly around 220milliom years, and yes the paper is all about bad things happening periodically as we change our local neighboors. But not something we'll have to worry about in quite a while
- iand 10y agoI think that's just a side effect of the compressed scales used by the animation. If they didn't do that then you would not be able to see the wave effect as clearly.
- Keysh 10y agoThat's an exaggerated, simple model of spiral density waves; it's not a model for the Sun's orbit, which probably only varies in radius by about 10-15%.
- alkonaut 10y ago> If this were applied to the galaxies spiral arms, then as they continue to orbit, the inside ones would orbit faster than the outside ones, and the galaxy would become increasingly spirally. By this point, the galaxy would be so tightly wound that we would not even be able to see the spiral any more. I thought that problem was the same as the galaxy rotation curve problem? If we solve (erm) that issue by adding in some dark matter - can't we then also accept a spiral structure to rotate with the galaxy without "winding up" the spiral?