2 upvotes, 2 direct replies (showing 2)
View submission: Ask Anything Wednesday - Engineering, Mathematics, Computer Science
Why hasn't the supermassive black hole in Sagittarius A destroyed the entire Milky Way galaxy, if it's so strong that even light can't escape?
How does that work if almost every galaxy has a supermassive black hole in or near the center?
Have we seen any evidence for micro black holes?
What happens to matter and energy expelled from a black hole? I read that it doesn't come from the black hole itself, it comes from an accretion disk around the black hole.
So there's something around the black hole that accumulates all the matter and energy sucked into the black hole, and at what point is it expelled? And what happens to it?
Comment by 199_Below_Average at 26/06/2024 at 16:28 UTC
12 upvotes, 0 direct replies
There's nothing about a black hole, even a supermassive one like Sagittarius A*, that magically sucks in objects from all over. A black hole is just an extremely dense, strong source of gravity. In the same way that planets can orbit the sun without falling into it, stars can also orbit a black hole. And indeed, all the stars in our galaxy basically orbit that supermassive black hole, and will continue doing so unless something else directly pushes them closer to it.
The part of a black hole from which nothing can escape is called the Event Horizon, and it only exists at a certain distance from the center of the black hole. Anything that crosses closer than that distance can never come back out, but anything outside that distance can continue orbiting undisturbed, and can move away from the black hole given enough of a push. The accretion disk is a region around the black hole, just outside the event horizon, where lots of matter (mostly gas and dust) is getting all smushed together as it orbits very fast. Because it's getting smushed together, it gets hot and emits light (in the same way that a lightbulb or a "red hot" piece of metal does). This is the light that we can observe "coming from" black holes, but as you say it's not coming from within the black hole's event horizon itself (because it can't), but rather from all the stuff nearby.
We don't really know what happens to all the stuff that does fall into a black hole and cross the event horizon, but it doesn't get expelled in a traditional sense. It just becomes part of the black hole, adding to its mass, in the same way that a meteorite which strikes Earth becomes part of the planet. If something in the accretion disk gets pushed around enough that it can escape from that close orbit, nothing particularly special happens to it; it will probably either enter a new, higher orbit around the black hole, or keep flying away until it hits something else.
Comment by nivlark at 26/06/2024 at 19:42 UTC
6 upvotes, 0 direct replies
Black holes aren't vacuum cleaners; they don't "suck" matter in. From a distance, they behave no differently than any other object of the same mass would.
Sagittarius A* makes up about a millionth of the total mass of the Milky Way, so except for the relatively small number of stars that orbit close to it, the gravitational influence it has is negligible. For our Sun, even tiny Pluto exerts a greater gravitational force than A* does.
No, there is no evidence for "micro" black holes. Extensive searches have been made, but outside of a few narrow ranges of black hole mass we've been able to rule them out.
Matter falling into an accretion disk reaches very high speeds, and so gains a lot of energy. This energy is released through collisions and friction within the accretion disk, and produces a lot of light and drives some matter outwards in powerful jets. For supermassive black holes, those jets can expel matter into intergalactic space, from where it will eventually, over many millions of years, fall back toward the galaxy.
Most matter in the accretion disk continues to spiral inwards though, until it eventually crosses the event horizon, the "point of no return" beyond which escape is impossible. That matter simply adds to the mass of the black hole.