What prevents a globular cluster from collapsing on itself?

Question

Are the stars rotating around a central point? Are they just flying away?

If they rotate around a common center of gravity, what kind of orbits do they follow? It must be chaotic. Periods and orbital planes must be changing constantly as their gravitational pull interact with one another.

If they orbit a proposed blackhole, how come they don't follow a flat orbital plane like solar systems or galaxies?

Answer 1

Globular clusters are dynamically "relaxed" systems, whose internal random kinetic energy of individual stars prevents collapse. Individual stars are also relaxed systems, where the motions of the individual particles support the star against collapse. In both cases, the Virial Law applies and total kinetic energy equals 1/2 the potential energy.

The individual stars orbit in the gravitational potential of the entire cluster. There's some interaction between stars, but not much. They all just see the average gravitation of all the rest of the stars most of the time. Orbits are more complex than Keplerian ellipses, because the cluster potential is not generated by a point mass as in the solar system.

The system does not flatten because it's completely relaxed already to a state that isn't small enough for such processes to become important.

Answer 2

Angular momentum conservation. If the cluster is isolated, it can not change its total angular momentum and all objects are orbiting around the center of gravity of the cluster unless some of them come very close to each other in a quasi-collision. In that case one object can be deflected to go closer to the center of mass while another one will either orbit farther away OR completely leave the cluster.

So if you want to move some stars closer to the center of gravity (which could very well be near a large black hole but that wouldn't change anything at all about angular momentum conservation), you have to move other stars further out or eject them from the cluster completely.

And that is what is happening in these clusters in reality, too. Some stars get ejected and the cluster shrinks a little. Same in the solar system: a planet can share some of its angular momentum with a body going nearby. The planet moves a little closer to the sun, the small body gets accelerated and may even leave the solar system. When NASA does that with a space probe it is called a swing-by. Voyager 1 and 2 are leaving the solar system at higher speed than they had before they passed the giant planets because of planned swing-byes.

Answer 3

Basically angular momentum. The stars are all orbiting. And some globular clusters have been found to have a big black hole at their core, so the stars are orbiting a black hole. I bet they all have a big black hole in the core. It is the only way I can imagine to explain how all these stars stay in a nice neat little ball like that.

Answer 4

Each star has its own rotation around the centre of gravity. Same principle as a solar system and a galaxy.

Answer 5

There's a common center of gravity that's probably shifting quite a bit, be it black hole or what-have-you. The stars are not flying apart, although the system is probably not as stable as an elliptical planetary system. M13 is a lot of fun to look at. The motion keeps them from falling in on themselves.

Answer 6

Though they look very tightly packed from our perspective, there is a LOT of space between the stars of a globular cluster.
Gravity between the stars simply isn't strong enough to cause collapse.
It's been theorized that stellar collisions do, and will take place. We just haven't witnessed one yet.

Answer 7

gravity...