Star clusters why do they not collapse ?

[Gerald Sargent]

When one see's increasingly high resolution images of star clusters
one has to question what keeps them apart - why does their individual
masses not result in a gravitational collapse - Gerald

[Robh]

Gerald,
I have often pondered this question myself, especially with globular clusters like Omega Centauri where the stars are close together. I think the answer is that the cluster has a rotational movement so that the stars are essentially in orbit about the cluster centre (like the planets about the Sun). Because of the distance and size of most clusters, this rotational movement is hard to detect.
However, astronomers can measure this movement. Apparently, with Omega Centauri the stars near centre orbit faster than those further out. (See Omega Centauri, Wikipedia, 3rd paragraph).
There are probably many open clusters with no apparent rotation, whose stars are weakly bound and moving through space together but don't collapse because of the larger distances between the stars.
Regards, Rob.

[sjastro]

The cores of globular clusters do in fact eventually collapse.
It's the case of it happening later than sooner.

A large percentage of a globular's core stars are in binary star systems.
Newtonian physics predicts that a binary system can impart kinetic energy on neighbourhood stars that perturb the system.

This can delay the eventual collapse.

For the outlying stars the effect can be opposite. The kinetic energy gained by stars can strip the stars from the globular.

Regards

Steven

[jjjnettie]

Maybe they just disintegrate rather than collapse?
We know that globs are held together by gravity but could entropy also play a part in this scenario?
The stars movements/speed increasing within the cluster. The stars then starting to interact with each other like particles, colliding and anihilating each other.
We need that illusive unified theory to explain precisely how it may work.

[Robh]

Not a lot is known about the formation of globular clusters let alone their eventual outcome. It is thought however that they are the oldest objects in the Galaxy. If they were going to self-destruct, they probably would have done so long ago.
"Core collapse" for globulars is not to be interpreted as the same type of event as the core collapse of a star, which might produce a neutron star or black hole. In any event, core collapse does not signal the end of the cluster.
With globular clusters, core collapse simply means that more stars, and perhaps more massive ones, congregate at the clusters centre. Some collisions may occur but the cluster remains basically intact. Overall, the stars are essentially in equilibrium, attracted to each other gravitationally in many directions.
Just as our Galaxy of stars does not collapse because of its rotation,
I stand by my original view that the overall cluster is in equilibrium also due to its rotation.

Regards, Rob.

[sjastro]

Core collapse can either be a redistribution of the kinetic energy of stars in which the more massive stars are closer to the centre of mass, or as the result of the presence of intermediate mass black holes (IMBH).

There is evidence of IMBH in certain globular clusters.

IMBM are much more massive than the gravitational collapse of single stars and are therefore most likely formed by collisions between stars in the core.

Once an IMBH is formed, neighbouring stars in the core can provide an accretion source for the black hole.

If core collapse results in a segregation of mass, a process known as evaporation occurs where the lighter mass stars have sufficent kinetic energy to escape the cluster. This is an on going process which results in globulars and open clusters having finite life spans.

Steven

[Robh]

Steven,
This is a great topic of debate. I wonder if anyone else with some ideas can add to it.
The scenarios you've painted are quite feasible and I've read your points with much interest. A globular cluster is a fascinating but by no means simple system; witness the complexity of n-body problems. And I certainly agree with you that globular and open clusters have finite life spans. Eventually globular clusters as well as the host galaxy will collapse as their elements lose their rotational energy. The outer stars may well be lost to space, whether by evaporation or being stripped by other massive bodies nearby.

However, globular clusters remain gravitationally bound for a very long time, maybe 10 billion years and more. We cannot even say how many, if any, have ceased to exist.

I think the question is why clusters don't collapse much sooner but appear to be, more or less, in equilibrium. I made the point about the rotational energy of the cluster as a whole. Maybe we can also think of a globular cluster as an enormous multiple star system with each star following some complex trajectory within the cluster (elliptical, figure eight etc). Eventually, as the heavier stars congregate to the center through energy re-distribution, the scenario you've painted might kick in. As far as I know, no black holes have been established in any globular cluster yet, not even 47 Tuc. Proposed maybe, but may indeed not be.

Regards, Rob.

[g__day]

Maybe you're thinking of this the wrong way - because we can readily image clusters maybe our brains automatically intuit that's what's present. Additionally we aren't seeing these clusters in motion (centrifugal force + spacetime expansion defying gravitational collapse).

So to my first point - imagine space is full of atomic Hydrogen, now let say 1/5 th of this matter gravitationally collapse, becoming increasinly opaque and visible to us - particularly if it collapses into a star. The rest of the massive - but diffuse cloud of hydrogen that hadn't yet collapsed would have a significant gravitational effect retarding the further collapse of the gravitational bound cluster (and imparting inertia I guess).

Now all this matter has residual momentum and kinetic energy from the big bang itself - gravity has to overcome this force too.

Finally non gravitationally bound spacetime is generally expanding - so the space between filaments in super cluster must be expanding - which again would complicate any general gravitational collapse scenario.

Great thread!

[Enchilada]

Read this thread today.
The brief answer is that globulars don't collapse due to the significant angular momentum of the cluster and the energy the stars contain...
Yet to satisfy the curiosity I have attached some useful text I've written in a pdf file explaining the role of soft and hard core binaries within globulars and open clusters.

Hope that it is of assistance to you.

[Robh]

Most interesting read!
Nicely explained theory of interactions. I must admit I lean towards this concept of a globular as a system of interacting stars whose angular momentums continuously change as they weave around each other. The significance of the core binaries should make Steven happy, as he mentioned them in an earlier post. It's also a dynamic wonder that collisions, if they occur at all, aren't more common.

Thanks for the contribution, Rob.

[mental4astro]

Enchilada,

I found your pdf article most interesting, along with this entire thread.

I do have a few questions to put forward:

*you mentioned little gaseous residue has been found in globular cores, is there any evidence of a residual 'tail' of hot gas behind the orbit of these?

*Open clusters are often filled with gas and dust, can we see much evidence of its discipation, or is it eventually just 'swallowed up' by the component stars?

*would Eliptical galaxies be 'over grown' globulars? I can follow the thinking of the dynamics holding GC's together, even irregular galaxies, but elipticals escape me.

*I believe that a planetary nebula has been found in a GC, if so wouldn't it be exibiting tell-tale signs of gravitational influence from neighbouring stars to guide the theories of GC structure?

Mental

[Enchilada]

Odd question. No. Stars don't make comet-like contrails.

* The huge distance of the globular clusters makes seeing any hot gases (plasma) near impossible.

* If you mean mass loss, like seen in the solar wind, the quantity of gas is absolutely miniscule. Even for the highest mass-loss red giants stars, the quantities are not overly significant.

* If you mean gases as nebulosity, well the age of the globular (8 to 12 billion years) suggests the material has collected the gas into the stars or has been lost long ago - discarded by strong winds during the stars initial formation now spread into now long distant intergalactic (or extragalactic) space.



discipation ?? Do you mean dissipation ??

This is because most open clusters are relatively new - being millions and not billions years old like the more ancient globulars. Being recently formed en masse out of some emission nebulae as new big hot blue stars.
Star losses also do occur in open clusters and probably at a more rapid rate. Mergers are even less likely than predicted in the globular systems.



Elliptical galaxies are not the same as globulars - structurally or dynamically. Behaviours and evolution are totally different. They are just different kinds of objects.



Planetary nebulae exist in only three known globulars. (one each) They are a natural part of stellar evolution, and observations of their structure would not be influenced be grossly by other nearby stars. Understanding globulars actually structure would not be likely because of the very small numbers of planetaries. :thumbsup:

Note : New version of dynamics attached, including some online links, and some comments on angular momentum pertaining to globulars.

[Enchilada]

Espérez que c'est d'aide à vous!

Surprised no one asked what this means!!!! :shrug:

"Hope that it is of assistance to you."

Like those who are curious about how things work in astronomy!

Ne pensez-vous pas?

[Robh]

I knew what it meant. I did some French at school years ago.
Can't believe I still remember any of it!
Regards, Rob

[mental4astro]

Muchas gracias, :).

Your reply has been a big help.

With respects to my first Q, your third point answered my intention.

Where would I be able to source some info on Eliptical galaxies?

Alex

[Enchilada]

I'll have to have a look for you if you want more detail.

GENERAL SUMMARY

We know very little about the true origin and evolution of any galaxies, let alone elliptical galaxies themselves. It is generally assumed that they have been either formed by galaxy mergers or that the evolution was swift in their formation where the nebula gas was taken out by a massive amount of star formation (starburst). Nebulae are now devoid in most of the known ellipticals.

They are different than globulars, as they;
-Orbital velocity of the stars are forty to fifty times faster (200 to 300 km each second)

- They move in orbits around the centre and not 'randomly' throughout the object (like the globulars do)

- They contain hundreds of billion stars, globulars merely one to ten million

- They are so very much larger. I.e. 20 parsecs for globulars versus 10000 to about 20000 parsecs i.e. 500 or 1000 times more across each object !
:whistle:

Globulars dynamically are generally unrelated to elliptical galaxies - sharing just similar old ages.

HTH

[mental4astro]

Thanks again for your replies.

I was not aware of the void in knowledge in galactic evolution as a whole.

Just one more Q. this time on classification. I know galaxies are classified according to the structure, but ellipticals, are they lenticular in shade or cigar? I have not been able to clarify this.

Their lack of nebulae comes as a great revelation to me. Most intriging.

Alex

[Robh]

Elliptical galaxies are featureless and classified according to their degree of ellipticity. E0 being virtually spherical, E7 very elongated. E4 is something like the shape of a rugby football. A lenticular galaxy (S0) has a bright central bulge and disk like a spiral galaxy but has no visible spiral structure in the disk. Galaxies don't actually have the exact shape of a cigar. The Cigar Galaxy (M82) looks somewhat like a cigar so it got the tag. Classified as irregular, it is probably a distorted disk galaxy.

Regards, Rob.

[mental4astro]

Aha, now I've got it. Very curious indeed.

Thankyou all for your help. And Gerald for hijacking your thread.

Alex.

[xelasnave]

Consider... if gravity does not work via attraction and if gravity works via push:D or as a universal pressure from all the particles flying about say maybe like HB and or neutrinos etc we would get what we see I feel:whistle:.

alex:):):)