Why don't stars in a cluster clump?

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In summary, globular clusters are very stable and don't tend to reform into discs like galaxies and solar systems do. They are composed of old stars and are often studied because they are like fossils.
  • #1
pixel01
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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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  • #2
Whay doesn't Earth fall on the Sun?

Why doesn't Moon fall on the Earth?
 
  • #3
Borek said:
Whay doesn't Earth fall on the Sun?

Why doesn't Moon fall on the Earth?

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.
 
  • #4
And a cluster is a clump.
 
  • #5
pixel01 said:
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.
 
  • #6
pixel01 said:
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
 
  • #7
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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  • #8
marcus said:
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?
 
  • #9
DaveC426913 said:
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).
 
  • #10
DaveC426913 said:
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!

================

Oooops! I see that Turbo already did. I had to leave the computer and missed that. Thanks Turbo. Your explanation makes sense.
 
  • #11
heavier stars sink to the center of globular clusters and lighter stars can be expelled from them.
 
  • #12
Globular clusters appear to be older than their associated galaxies. They are also nearly devoid of dust or gas, suggesting they evolved very differently. Virial theorem is a good try, but not quite good enough. This is an unsolved mystery in cosmology.
 
  • #13
Chronos said:
Globular clusters appear to be older than their associated galaxies. They are also nearly devoid of dust or gas, suggesting they evolved very differently. Virial theorem is a good try, but not quite good enough. This is an unsolved mystery in cosmology.

Oh, I wasn't trying to explain how the evolved. Only why they don't "clump".
 

1. Why don't stars in a cluster clump?

Stars in a cluster do not clump together due to a combination of factors, including their relative distances from each other, their individual gravitational pulls, and their random velocities. As they orbit around the center of the cluster, the stars' gravitational forces keep them from colliding and forming a clump.

2. Can stars in a cluster eventually clump together?

While stars in a cluster do not typically clump together, there are cases where they may form binary or multiple star systems through gravitational interactions. Over time, these systems may merge into larger stars, but this process is relatively rare.

3. Do stars in a cluster have different compositions?

Yes, stars in a cluster can have different compositions, just like stars in our own galaxy. This is because they are formed from the same interstellar clouds, which have varying levels of heavier elements. However, since they are all formed from the same cloud, they will generally have similar compositions compared to stars in other clusters.

4. How do scientists study the movement of stars in a cluster?

Scientists use various techniques to study the movement of stars in a cluster, including spectroscopy and astrometry. Spectroscopy allows us to measure the Doppler shifts of stars as they move towards or away from us, while astrometry involves tracking the positions of stars over time to determine their velocities.

5. Can stars in a cluster change their positions over time?

Yes, stars in a cluster can change their positions over time due to gravitational interactions with other stars and objects, such as gas clouds or black holes. This can cause some stars to be ejected from the cluster or to form new binary systems.

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