black holes orbiting?

A link to the hubble website article is here

If black holes are such huge gravitational forces, how can they be held in an orbit? I always thought they were fairly stationary. I’ve even read that a galaxy may very well orbit around a black hole at its core.

As an orbiting black hole gets larger, does it eventually stop orbiting?

Thanks in advance.

Nothing is so massive as to be immovable. When we say one body orbits another, it is more accurate to say that they both orbit the common center of mass. It’s just that when the center of mass is within the more massive body, we say that the less massive body orbits the more massive one.

You might also want to read an essay I wrote about what if the Sun turned into a black hole.

Ah…I appreciate the explanations and thanks for the link, Bad astronomer. Very informative.

Black Holes come in all sizes…from galactic cores of millions of
solar masses, to the Planck Scale…10^-11 grams!

There could be a microscopic black hole orbiting earth right now,
and we’d never know it unless it ran into something.


Big BHs are colapsed stars, systems, galaxies etc. Nice and easy.

Small BHs are ? Presumably they are the ones supposedly created at the big bang, rather than big ones that have shrunk.

I take it the reason that we don’t notice them is that , whilst they are superdense, they have relativelty low masses compared to the Earth.

Well, we probably would notice a microhole orbiting the Earth, actually. They’re considerably hotter than their big brothers (the temperature of a hole is inversely proportional to its mass, and the power emmitted is inversely proportional to the square of the mass), so a hole small enough that we wouldn’t notice the gravitational effects would probably be large enough that we would notice the Hawking radiation. Then again, the Hawking process would probably be dramatically different for a hole down near the Planck mass, and it’s possible that such a miniscule hole would be stable (i.e., not emit any radiation). In that case, it might go unnoticed after all, although I have read of proposals to try to detect such things.

And yes, Bromley, holes smaller than stellar size would be primordial. Not only is the Universe far too young for a stellar-mass hole to have shrunk appreciably, it’s far too hot for one to have shrunk at all.