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Astronomy

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    $\begingroup$ Why do you say that it "must have some insane gravitational pull"? Stuff orbits the barycenter because it's the center of mass, it doesn't have to be a big concentration of mass. FWIW, for galaxies with a central BH, the mass of that BH is a tiny fraction of the whole galaxy's mass. It's not like the BH dominates the galaxy's gravitational structure, although of course it has a fairly big effect in the immediate vicinity of the BH. $\endgroup$
    – PM 2Ring
    Commented Oct 14, 2018 at 23:36
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    $\begingroup$ Where's the black hole in the middle of every binary system that makes the stars orbit the barycenter? The mass of the stars is more than enough, you don't need them to orbit something else. Solve for a binary star system, and you'll see the same mechanism still works for n-bodies. $\endgroup$
    – Luaan
    Commented Oct 15, 2018 at 7:22
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    $\begingroup$ @PM2Ring: While that's a fairly well-known physical result, it ignores the fundamental question: Would all that mass even be there concentrated in a galaxy without a black hole? Or are black holes essential to the formation of starts and their clustering into a galaxy? $\endgroup$
    – MSalters
    Commented Oct 15, 2018 at 11:44
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    $\begingroup$ @MSalters Sure, central black holes might be important in galaxy formation, but I don't see how they are relevant to star formation. And then you have to explain spiral galaxies without a central BH. Did they lose their BH, eg in a collision? If so, how did they manage to retain their spiral structure? FWIW, we still don't have a good theory to explain how super massive BHs are so massive. I suspect that we need to learn more about the role of dark matter to address these questions properly. $\endgroup$
    – PM 2Ring
    Commented Oct 15, 2018 at 12:37
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    $\begingroup$ @AndrewMorton If you only account for gravity and ignore gravitational waves, no - the orbits are stable. The real system is trickier, because stars aren't inert, perfectly solid balls of stuff - they're losing large amounts of mass to solar winds, for example. I'm not sure, but I think that main sequence binary stars are actually slowly expanding their orbits as the orbiting mass decreases. It gets much more complicated once you add things like solar evolution, of course, but the "spiraling down to the center" thing needs some way of losing massive amounts of orbital energy. $\endgroup$
    – Luaan
    Commented Oct 15, 2018 at 13:22