Globular clusters

[Stu]

Just wondering if anyone can explain why and how globulars clusters exist. My main question is how they are stable as a ball of stars, should they not collapse down into themselves under gravity? If they are stable because they are spinning, why they aren't elliptical or disk-like in structure?

Anyone know?

Cheers

Stu

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[ollypenrice]

Like elliptical galaxies, globulars have stars in random orbits. There is no requirement for the orbits to be in the same plane or same direction. In fact the older 'population two' stars that orbit the milky way's central region are also on randomly distributed orbits. An orbit is an orbit, it doesn't matter where the other orbits lie.

I think the thinking behind globular formation has changed recently so I'll look forward to seeing what comes up on this thread.

Olly

[Telrad]

I have been reading about an apparent process known as 'core collapse' that is thought to control or strongly influence the internal dynamics of Globs, and that these stars at the centre form rapidly orbiting pairs. These act as a reservoir of energy which other stars are drawn into and increases their velocities. The least massive stars end up with the highest velocities and tend to migrate to the outer regions of clusters while more massive stars move more slowly and appear to sink toward the centre of the Globs..

[ollypenrice]

I have been reading about an apparent process known as 'core collapse' that is thought to control or strongly influence the internal dynamics of Globs, and that these stars at the centre form rapidly orbiting pairs. These act as a reservoir of energy which other stars are drawn into and increases their velocities. The least massive stars end up with the highest velocities and tend to migrate to the outer regions of clusters while more massive stars move more slowly and appear to sink toward the centre of the Globs..

I heard something along these lines at an Astrofest lecture a few years ago. Supercomputer models even suggested that low mass stars might be ejected and that stellar mass black holes might sink.

All this from memory though.

Olly

[FraserClarke]

Yes, this is a mass segregation process that happens all clusters of objects (globulars are a relatively extreme example, where it is clear to see). The heavy objects end up sitting in the middle, and the lighter objects end up getting whipped around by them and sent out on long orbits.

One of the OP's questions: "should they not collapse down on themselves under gravity?". No, not really. For that to happen, you need to have some way of getting rid of the gravitational potential energy of the system. If you have a gas cloud, you get rid of the energy by collapsing the gas, which knocks the gas molecules together hence heating things up. Then you can get rid of that energy by radiating it away. The equivalent doesn't work in star clusters, because all the stars are moving around collisionlessly so you never get that transfer of gravitational energy to some other form that you can get rid of. So a star plunges into the centre, converts it's gravitational energy to kinetic energy -- but then all it can do is shoot back out the other side and convert it's kinetic energy back into gravitational energy --- ad infinitum. Basically, there is no friction in there, so you can never lose energy. Same reason the Earth doesn't fall into the Sun, fortunately

[Scosmico]

Thanks for the lecture guys! I had the same thought looking at M13 other day.

[Stu]

Thanks very much for the input guys, I think I am getting it now. It makes sense that they don't collapse because there is nothing to collapse in on effectively, they just keep orbiting

A few more questions...

If all the stars have their own individual orbits, what are they orbiting around, the gravitational centre of the glob? Or, as was suggested are they paired up our linked in some other way?

Olly, to confirm, do elliptical galaxies operate in a similar manner ie all the stars on their own distinct orbits in any plane which is why they don't have a spiral or disc shape?

Why are elliptical galaxies elliptical and globulars spherical?

Cheers

Stu

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[Telrad]

Thanks for the lecture guys! I had the same thought looking at M13 other day.

Scosmico The Hercules Globular Cluster is one of my favourites !

http://stargazerslounge.com/observing-deep-sky/144439-m13.html

[FraserClarke]

If all the stars have their own individual orbits, what are they orbiting around, the gravitational centre of the glob?

Yes, they are orbitting in the overall gravitational potential of the cluster.

Why are elliptical galaxies elliptical and globulars spherical?

They're not! Ellipticals, as the name suggests, have a range of ellipicity from 0.0 (spherical) to ~0.7. You'll often see ellipticals classified in the range E0 -- E7; which relates directly to their ellipticity. Globulars are similar, though I'm not sure if they get quite so elliptical?

[ollypenrice]

...and why are they elliptical or almost?

I think a popular conjecture is that they are formed by spirals merging and, in the chaos, losing all hope of systematic orbital rotation. I'm not an expert, mind.

And, yes, there is no systematic alignment of orbits in ellipticals or in the old stars in the central bulges of spirals (which, by the way, is where we tend to find globulars which are also on random orbits themselves, as well as being made up of them).

Olly