Thread: Galaxies' central blackholes

So I was just curious if anybody could answer this question for me. Do all galaxies have a central supermassive black hole? I only wonder this because obviously the black hole had to collapse from something, and I was wondering if that happens before or after it "accumulates the galaxy" around itself. I guess another way of asking this is if there are any galaxies that have an actual star at its center. I figured since black holes have the same gravity as the star that it came from (up until the event horizon), it should be possible for a star to have enough gravity to hold a galaxy stable.

Most are believed to have SMBH's in their centres. We don't really know for sure. A SMBH is usually hundreds of thousands to billions of times of a solar mass so would exert a lot more gravity than a star. It would have to be one hell of a big star to keep a Galaxy in orbit. This also makes sense as a galaxy is a big collection of stars.

Wow, I don't know why I didn't think that a blackhole's gravity increased as it "sucked" more mass in. That kind of leads me to another question though. I don't know if there's even an accepted theory about this, but how do galaxies form? I assume there's a very large nebula in which a very large star becomes a black hole and begins growing due to the amount of matter around it. Then once it becomes large enough it starts drawing more and more matter around it into a spiral and forms the beginnings of a galaxy? It's weird, this is the first time I've thought about the intermediate between the beginning of the universe and the current visible universe...

I don't actually know the answer as to how the galaxies were formed. If what you propose is correct, that might explain it for spiral shaped galaxies but there are different shapes of galaxies that it wouldn't account for.
This might help http://en.wikipedia.org/wiki/Galaxy_..._disk_galaxies

According to one model, in the early universe dark matter “clouds” formed. Something like a hundred million years after the Big Bang clusters of stars formed within them. These stellar nurseries hosted several huge stars. The larger a star is, the quicker it fuses its hydrogen into heavier elements. Once the core fuses to iron, this process stops. It takes more energy to fuse (or split) iron nuclei than you get from the action. So the core “cools” and the outer layers of the star collapse from gravity and crush the core. The outer layers then bounce off this compressed core and the shock wave becomes a supernova. Sometimes the resulting crushed core is a black hole. Over time, due to gravity, the clusters become conglomerates and the black holes in them merge into ever larger black holes until SMBH are eventually formed in galaxies. FYI, a galaxy is actually not held together by the gravity of the SMBH. The massive dark matter cloud that surrounds every galaxy and to a lesser extent the mass of the stars themselves, do this.

Hmm, so then does the black hole's gravity have any effect on the outermost matter in the galaxy at all?

The effect decreases by the square of the distance, so it is almost negligible.
If the Sun were a million times farther away, then its gravity would have a million million times less effect on the Earth

Yeah, i've the show on TV a couple of times now. Still not buyin it. Massive gravitational center....yes, massive black hole with enough force that nothing escapes.... oh we are just going to explain the fact that the galaxy is not obliterated in a few hundred years with the idea that these things suddenly go "dorment". This is what they say the difference between an active galaxy and galaxy like ours is. Then they say that this massive gravity well, that not even light can escape, is showing itself to us by ejecting gaseous energy jets out each side of the galaxy (look at the pics folks, that is how they are saying they are seeing these things). I will admit that I've been out of the science loop for a bit now, but, unless there's some new physics in the universe that I'm not aware of, gravity, i.e. black holes, doesn't shut down and stop pulling things toward it. There are just masses that are out of its current reach. In the case of proposing an SMBH at a galaxies center, you would see a constant reduction in the makeup of the galaxy. Not buyin this idea that a black hole just gets its fill of food... anyone care to enlighten me on this subject? Been wanting to mull this over with sombody for awhile, maybe someone can shed some new light on this for me.

Originally Posted by darkmayne122

yes, massive black hole with enough force that nothing escapes.... oh we are just going to explain the fact that the galaxy is not obliterated in a few hundred years with the idea that these things suddenly go "dorment". .

You aren't thinking this through, are you? Imagine the sun were to become a black hole. Do you think the Earth would be drawn into it? Of course not. It would continue to orbit just as it does now. Exactly the same is true for galactic black holes. The majority of objects in the galaxy are in stable orbits. The cessation of feeding reflects a time when all of the unstable objects have been absorded. Clearer?

Yes, if the distance from the Black Hole is large relative to the Schwarzschild radius, there is no difference between a star and a Black Hole when it comes to gravitational attraction.

Example:
Mass of the sun:
Radius of the sun:

Schwarzschild radius:


Schwarzschild radius of the sun:

So, if the sun were to be compressed to a sphere with a radius below 3 km, it would be Black Hole. Anything close to that radius would experience something strange, i.e. string tidal forces. The similar situation exists for the Milky Way. There is just not much there that comes close to the event horizon, i.e. the Schwarzschild radius to "feed" the galactic Black Hole. Active galaxies possess large amounts of gas that can be pushed into the central regions, e.g. via spiral arm interactions.