The super-massive black hole at the center of this galaxy is estimated to be 4 million times the mass of the sun. I understand that a star only needs to be about 10 times the size of the sun to eventually collapse into a black hole. However, there are stars that are orders of magnitude larger. So could/do smaller black holes revolve around larger stars, similar to planets?
Yep. I remember reading about one a few years ago. In this case the star was spinning very fast. I have no idea if that is because of the presence of the black hole or not, but I do remember the author of the article saying that the black hole fed off of matter ejected from the larger star, and was therefore growing.
ETA: Found the article (was actually one of the first google hits):
Yes. In fact, one way scientists detect Black Holes is through that kind of gravitational relationship in a binary system where one star is visible and the other a Black Hole.
Thanks! So I assume at some point, the larger star will also collapse into a black hole, which would presumably lead to a black hole merger like those detected by LIGO. Interesting on the spinning star in that article. I guess their point was that it only sparked their interest due to the spinning star that was ejecting enough materials to form the accretion disk around the black hole.
Yes, and black holes can orbit stars, too.
Not unless the orbits work out that way, as opposed to the black hole simply remaining in orbit, for example.
My (perhaps limited) understanding is that the detection you mentioned could have been a pair of neutron stars as well (or another exotic star who’s name escapes me at the moment), but probably was a pair of black holes which can, indeed also circle each other. BTW, they don’t necessarily have to merge, they could simply continue to circle each other since if they were originally a pair of stars they wouldn’t have (necessarily) gained any mass and, presumably, were already in stable orbits. If they do gain mass over time then, certainly, they could become unstable and start circling each other in tighter and tighter orbits (or if the process of collapsing to a black hole somehow destabilized one or both of their orbits).
I think, and I could well be wrong, that once two black holes are orbiting one another, or orbiting a common center of mass, general relativity effects will cause their orbits to move closer together, and a collision is inevitable.
Orbiting is a general relativity effect itself.
Outside Innermost Bound Circular Orbit, or about 1.5 times the event horizon radius for the simplified black holes we are discussing here there is no problem with stable orbits.
Well for as much as there is a stable orbit for anything.
In GR, any closed orbit will eventually decay due to gravitational radiation and result in a collision of the orbiting bodies. This will happen regardless of the nature of the orbiting bodies: The only difference with black holes is that, since they’re smaller than anything else of the same mass, they’ll be able to get closer before colliding.
But “eventually” is a very important word, there. It will happen, but it might take an extremely long time, trillions of years or more. And in most cases, something else catastrophic will happen long before then.