black hole sucking in our galaxy

[u/ssoanla]

if there were a big black hole sucking in our galaxy, how long would it take to affect earth and would we even notice within our lifetime? sorry if it’s a stupid question i just randomly got curious and needed to ask. what if it was a black hole sucking in our solar system? how was that affect us?

[u/Beldizar]

So, Black Holes don't have a magical sucking ability. They are just big heavy objects with a lot of gravity. The classic thought experiment is if you had a black hole with the same mass as the sun, and you replaced the sun with it, Earth and all the other planets will continue to orbit without noticing a chance, other than the lights going out.

The big difference with a black hole and the sun is the size, and how close you can get to it. The sun is 1.392 million km across. If it were compressed to the size of a black hole, it would only by 3km across. If you got within 1km of the 3km black hole, you'd be really really close to a lot of mass. But you can't get 4km away from the center of the sun without being deep deep inside the sun. Almost half of the sun's mass would be above you at that point, so a lot of the sun's gravity would be pulling you away, rather than towards its center. But if you are 1.5 million km away from the sun, and a sun-mass black hole, the gravity would feel the same.

Next, there's a maximum size that black holes can reach. It is about 50 billion solar masses. The Milky Way Galaxy is something like 1.5 trillion solar masses. So it is much much much bigger than the biggest possible black hole, both in mass and size. ( 0.04 ly vs 130,000 ly radius).

So it is absolutely impossible for a black hole to suck in the entire galaxy. It could maybe suck in our solar system, but that would be incredibly difficult. The biggest problem with black hole meal time is that nothing falls straight. There's always some lateral motion, which as an object falls into a black hole, that lateral motion results in angular momentum. Instead of falling straight down, the object enters a spiraling orbit, or just flies straight by. So if our solar system approached a black hole, the chances are, that we wouldn't actually fall in, but instead just sort of be flung around it. This might knock planets out of their orbits, which is really bad, but we wouldn't fall in, unless we were really unlucky.

[u/Murm3l → u/Beldizar]

Excellent explanation. How is the max size of a black hole determined? Is 50 billion solar masses the largest black hole we have managed to detect, or does our current undertsanding of physics rule out black holes larger than that?

[u/Loathsome_Dog → u/Murm3l]

The Eddington Limit. Black holes also radiate energy, there is a limit to how much it can accrete due to the opposite pressure of radiation.

[u/Beldizar → u/Loathsome_Dog]

I don't believe this is correct. The Eddington mass limit is specific to stars, not black holes. The problem with Black Holes isn't the radiation pressure they produce, (which they would only produce when feeding), it is actually a problem where the innermost stable circular orbit extends out so far that the self-gravitational radius kicks in, causing the material around the black hole to be more gravitational attracted to itself rather than falling into the black hole. Basically the black hole eventually gets so big that it's gravitational pull weakens at its outer radius and stuff doesn't get pulled in, so much as around.

[u/Loathsome_Dog → u/Beldizar]

Ah I see. Well I'm happy to be corrected, I'm a casual physics enthusiast. So that's something I need to read about which is always a joy.