Globular Clusters

[BuzzLY]

Since from what I read, globular clusters are quite old (most of them anyway) so how do they stay spherical for so many billions of years? I would think they would collapse under their own gravity unless they rotated, in which case they would flatten in much the same way that a spiral galaxy flattens, but they remain sperical. The other posibility would be that all the stars have their own random orbits about the center, much like the stars near Sagitarius Alpha, but in a dense cluster nearly 13 billion years old I would think that most would have collided with other stars. There don't seem to enough blue stragglers to account for this either.

 

[MeteorWayne]

It's the last one. The stars are in very high speed, independent orbits. As for them colliding, even in a dense globular cluster, there's a long distance between the stars

 

[origin]

Good question Buzzly!

Wayne - so the stars are buzzing around a center of gravity - is it assumed to be a black hole?

And another quesiton, as I recall globular clusters are kind of in a 'halo' around the galaxy, is there any theory on why they are not in the galactic plane with most of the other stars?

Thanks

 

[MeteorWayne]

No, no cluster is known or suspected to have a black hole yet.

They are orbiting the barycenter...the center of mass of the whole cluster.

As for the location, one theory is that they are the remnant centers of small galaxies that the Milky Way has devoured. Such dwarf galaxies, stripped of their outer stars would be unlikely to orbit in the plane of the MW, rather they would be in orbits far from the center and scattered in every direction.

There are other theories, but that's the one I lean toward.

Wiki's article is pretty good, with lots of detail BTW:

http://en.wikipedia.org/wiki/Globular_cluster

 

[eburacum45]

No, no cluster is known or suspected to have a black hole yet.

Here's one suspect:
http://www.newscientist.com/article/dn10879

Collisions between stars in globular clusters form strange remnants, like luminous blue stragglers, pulsars and x-ray binaries of types that are rarely found elsewhere. Another strange and slightly disturbing thing about globular clusters is that they plunge through the Galactic Disk twice on each orbit - but they are so small compared to the disk that they cause comparatively little damage. I wonder what it would be like to be nearby when they do, though.

 

[chebby]

Here's one in our galaxy, in omega centauri

http://www.hulu.com/watch/16406/spaceri ... -discovery

 

[MeteorWayne]

Well, chebby, the clip says it suggests a black hole, but I've seen nothing in the journals about it that would come close to proof. I look forward to it though! It's the first I heard of it, so I'll do some poking around.

 

[neilsox]

We are pretty sure at least half of the large galaxies have super massive black holes. Small galaxies and clusters likely have several black holes of ten or less solar mass, but typically we can't tell for sure. With lots of dust clusters shrink into disk shape, but stop shrinking when most of the dust is swept up.
This is because near misses and collisions are as likely to increase the orbit length as decrease the orbit length. So it seems to me. Neil