Star Cluster Gravity

1. Nov 21, 2004

DB

What happens to the forces of gravity and orbital forces of stars when they are so close together in a star cluster?

2. Nov 21, 2004

Physik

The gravity of the largest star would attract objects, with the gravity of the surrounding stars slightly interfering with an object's orbit, I suppose.

3. Nov 21, 2004

tony873004

They orbit the center of mass of the cluster. I'm not sure, but I think a star cluster needs one body to be much more massive than the other bodies or the whole thing files apart. Perhaps every star cluster has a parent black hole.

4. Nov 21, 2004

Physik

Well, if the type of clusters is an open cluster, then all the stars have mutual attraction to each other. Then the orbit of an object would be quite irregular.

Look at this model of a star cluster:

http://img126.exs.cx/img126/6231/starcluster.gif

The stars are red, and their gravitational fields are the blue rings. Now imagine an object in the orbit of one of those stars, the object would hit another star's gravity field and would jerk around.

EDIT: The stars would probably move, as they would be attracted to each other. Just as Jupiter's gravity moves the sun around.

Last edited: Nov 21, 2004
5. Nov 22, 2004

Nereid

Staff Emeritus
A belated welcome to Physics Forums DB!

Whether there are just two massive objects or trillions, gravity is still the same. For star clusters, Newtonian gravity provides pretty good answers (some exceptions, of course) ... inverse square, m1 x m2, etc ... between all pairs. Not possible to derive a complete analytical solution to the equations of motion (hey, even the general 3-body problem isn't soluble!), but with good computer codes, and lots of memory, your own PC will do a pretty nice job of simulating what happens.

In the absence of external masses (e.g. a nearby galaxy ... OK, pretty unrealistic) ... the answer is quite simple: the 'cluster' will 'evaporate' unless the initial motions of the stars are insufficient for them to escape the combined gravity of the whole cluster. The cluster's evolution then becomes largely determined by how much 'cooling' it can undergo ... e.g. close encounters of a tight binary by a lone star may result in one star being flung out of the cluster (and all the other stars responding accordingly); or a collision which results in a merger and subsequent supernova (some gas may be expelled from the cluster, resulting in a loss of mass).

However, the evolution of most clusters are probably determined more by interactions with galaxies than internal dynamics ... (there's a lovely SDSS result showing a small globular on the outskirts of our Milky Way being shredded by 'tidal interaction' with the mother ship; too, a recent SDSS finding of a strange, halo object that might be a very low density cluster left over from MW formation times).