Just wondering how spiralling galaxies maintain their shape in terms of the 'arms' of a galaxy that spiral out from the centre?
I ask this because (at least to my level of understanding, which is bery basic) I would have thought that the parts of the arm closer to the centre would spin faster than the ends of the arms in the outer regions. Eventually this would 'stretch' the arms until they're so thin and congested that they aren't visible anymore or just start to look like a disk.
If someone is able to explain, whether my thinking is right or wrong, it would be greatly appreciated. 🙂
by xXGTA5L3G3NDXx
5 Comments
Gravity. Have you not seen the [“gravity waves” animation](https://www.ligo.caltech.edu/video/gravitational-waves)? I figure the stars form in the troughs, as it emanates from the large black hole in the galactic center.
I can also not help too much, since I only “know” a bit there, but you’re right in that they should. That is, as far as I know, the main reason, why dark matter is hypothesized. If there is a bubble of dark matter reaching far beyond the visible matter around galaxies, the increased spin in the visible part could be explained.
Dark matter provides the additional mass to keep the matter we can see working as we expect it to.
The issue we see, without adding dark matter, is that galaxies should be spinning faster [edit: slower], but they aren’t. The velocity curve at the outer edge should be faster [edit: slower] than it is and we can only account for that if we add dark matter to flatten out the velocity curve.
I mean…they don’t, right? It’s constantly change. It’s just on a much larger scale than what we humans typically can comprehend.
[Density Wave Theory](https://en.wikipedia.org/wiki/Density_wave_theory?wprov=sfti1) is a big part of this! Essentially the arms in grand design spiral galaxies are standing waves that stars move in and out of. So each star is on a relatively normal orbiting path, just the system as a whole shows the structure.