Why don't stars in a cluster clump?

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Hi all,
I have this question:
Stars in a cluster have gravity influence on each other, so why they do not clump together?
Bigger scale : stars in a galaxy.

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Borek
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Sounds simple. You mean they orbit around a center of mass?. But in a global cluster, the ones in the axis have no centrifugal force.

Staff Emeritus
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And a cluster is a clump.

DaveC426913
Gold Member
Sounds simple. You mean they orbit around a center of mass?. But in a global cluster, the ones in the axis have no centrifugal force.
A globular cluster of stars does not have stars orbiting in a plane; they form a sphere, each star in its own plane.

If the cluster were left alone long enough and managed to stabilize, it would flatten out into a disc, like the solar system or spiral galaxies.

sylas
Sounds simple. You mean they orbit around a center of mass?. But in a global cluster, the ones in the axis have no centrifugal force.
Check out the "virial theorem". Basically, in a stable gravitationally bound cluster of particles, the velocities of particles are balanced with the size of the cluster, so that the time average kinetic energy is equal to half the time average potential energy (negated). The same relation is familiar to us for a simple orbit of two bodies.

If the total kinetic energy is greater, then the cluster will tend to expand. If the kinetic energy is less, then the cluster will tend to contract. The stable point is given by the Virial theorem.

The tendency to settle into a disk like shape is a distinct feature where net angular momentum is significant, but the Virial theorem holds in any case.

Cheers -- sylas

marcus
Gold Member
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Sylas you have been doing a whole run of fine clear posts, so thanks and compliments on then all.
Yes! the virial theorem!

Pixel you got your questions answered from a lot of people. Ask more if you want more explanation. I will throw in an extra observation although unnecessary.
In cases where some glob or blob does collapse, like a cloud contracting to form a star, it has to have a mechanism to radiate off the extra energy.

And this is real.
when you see stars in the process of formation you actually see ways that the extra energy is being chucked out or radiated off.

A little tight blob of matter has less energy than the same matter more spread out. Less gravitational energy. Like a brick on a stool represents more energy than when it is closer to the center of the earth, like on the ground.
Or if we wanted to bring the moon in closer to us we would have to find a way to send an enormous amount of energy off into space, which would be the gravitational potential energy that she has when she is far away like now.

Globular star clusters have almost no way to get rid of energy. They are very clean. No gas No dust No friction No viscosity. Only extremely rare collisions between stars, or near-collision tidal rip-offs could be imagined to give them some way to dissipate the energy they are stuck with. With no way to give it away, the gravitational energy acts as a preservative. So they stably persist for billions of years. Fossils, kind of.

Virial means of or pretaining to energy (vis, vires is Latin for strength or energy)

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DaveC426913
Gold Member
Globular star clusters have almost no way to get rid of energy. They are very clean. No gas No dust No friction No viscosity. Only extremely rare collisions between stars, or near-collision tidal rip-offs could be imagined to give them some way to dissipate the energy they are stuck with. With no way to give it away, the gravitational energy acts as a preservative. So they stably persist for billions of years. Fossils, kind of.
I did not know this.

Is this why they don't tend to reform into discs like galaxies and solar systems do?

turbo
Gold Member
I did not know this.

Is this why they don't tend to reform into discs like galaxies and solar systems do?
Globular clusters are remarkably free of residual gases and dust, and are populated by old stars. With no easy way to shed angular momentum or channel it, they remain very stable (and globular).

marcus
Gold Member
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Is this why they don't tend to reform into discs like galaxies and solar systems do?
I have only rudimentary knowledge about globular clusters. I know some of them are very old. They have old Herzsprung-Russell diagrams. They are often composed of stars from old populations. An acquaintance from many years back was studying globular clusters partly because (according to him) they were like fossils. But I have only this vague secondhand information and must not speak with any sense of authority. I think if you do a googlesearch you can find out 10 times more than I know in about 5 minutes. I want somebody to do this.
All I know is they are very interesting.

Yes! That is a great question! why don't glob clusters form pancakes? Why do globular clusters stay globular for billions of years. What complicated orbits the stars in the cluster must have. The N-body problem in spades! Why do galaxies have viscosity, apparently, and globular clusters not have viscosity, apparently. Hope somebody will answer!

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Oooops! I see that Turbo already did. I had to leave the computer and missed that. Thanks Turbo. Your explanation makes sense.

heavier stars sink to the center of globular clusters and lighter stars can be expelled from them.

Chronos