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I still don't quite understand what the stars in galaxies without SBHs revolve around. I mean, the stars must orbit something. If the Sun suddenly disappeared, the planets wouldn't just continue in their orbits as there's nothing to orbit. E.g. M33 is thought to not have an SBH in its center. If the barycenter is in the very center of the galaxy wouldn't that require that all stars revolving around it have the same mass relative to their distance to each other, forming a gargantuan multiple-star-system? And how would such a system be stable over the course of thousands of years? Perhaps we are wrong and there is an SBH or another massive star at the center of each galaxy?

Giovanni
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  • Globular star clusters don't have a massive central object but their stars orbit each other all the same. – Kristoffer Sjöö May 22 '21 at 06:24
  • @KristofferSjöö The globular clusters I explored have an intermediate black hole in their center, with a mass inbetween that of a stellar and a supermassive black hole. If any clusters don't have them, my question applies to globular clusters too. How can such systems be stable throughout the millenia? – Giovanni May 22 '21 at 06:54
  • Does this answer your question? Orbiting supermassive black hole or galactic center of mass? (the accepted answer there is wrong) or https://astronomy.stackexchange.com/questions/28012/do-all-galaxies-have-a-black-hole-in-the-center. The stars orbit in the mass of the dark matter, The blackhole has neglible effect, except for stars very near the core. Also https://astronomy.stackexchange.com/questions/24064/why-do-black-holes-in-the-middle-of-galaxies-not-suck-up-the-whole-galaxy – James K May 22 '21 at 07:32
  • @JamesK So it's a bit similar to how galaxies revolve around the Great Attractor. – Giovanni May 22 '21 at 07:49
  • The matter and dark matter in a galaxy are self gravitating. The stars orbit in this gravity field generated by the matter and dark matter. The black hole is only a very very small part of this. – James K May 22 '21 at 08:10
  • @JamesK The linked answers don't mention dark matter, it's just that the mass of all the more interior stars lets the exterior ones orbit it. This is unstable and it's likely stars can be ejected into intergalactic space I guess. Dark matter may in fact be primordial black holes btw. – Giovanni May 22 '21 at 08:24
  • @Giovanni It's technically correct that stars can be ejected into intergalactic space. But the timescale for this to occur on a significant scale is many times the current age of the universe. So on timescales of billions (or hundreds of billions) of years, it's very stable. (https://astronomy.stackexchange.com/questions/35196/what-is-the-characteristic-time-of-the-evaporation-of-the-galaxies) – Peter Erwin May 22 '21 at 09:00
  • The accepted answer on that question is wrong. I've already noted this. Scroll down to ProfRob's answer. Stars orbit in the combined mass of the galaxy. Most of that mass is dark. We don't know what it is (but its not probably not black holes, because we would have detected them by microlensing) – James K May 22 '21 at 09:12

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