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How did globular clusters get their properties?

  1. Mar 10, 2005 #1
    As I understand it, the stars in a globular cluster have orbits with a whole range of eccentricities and directions of motion, giving the cluster an overall spherical shape, with a greater density of stars toward the center.

    How did the overall cloud out of which the globular cluster arose ever get such properties? How did clumps of the cloud ever get such a range of momentums? One would think that the overall cloud would have had a net rotational inertia, and that gravitational collapse would have occurred in the direction parallel to the axis of rotation... at least to some degree.

    Is there a theory or conjecture that the overall cloud originated in some colossal explosion, and perhaps pre-existing complex, twisted magnetic fields gave rise to the range of momentum--both magnitude and direction--of the clumps that collapsed within the cloud to form the population of stars we see today?

    Please excuse me if this question is worded poorly, but I am strictly an amateur/layman with an ongoing interest in cosmology.
     
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  3. Mar 11, 2005 #2

    Chronos

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    Perhaps there were globs of primodial gas clouds that got shocked by early supernova.
     
  4. Mar 11, 2005 #3

    SpaceTiger

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    Globular clusters are nearly collisionless systems, so their properties can be relatively easily described by N-body simulations. When you take a simple spherical, collisionless system and evolve it in time, we find that it tends to undergo mass segregation (more massive objects moving to the center) and core collapse (ever increasing central density, decreasing outer density).


    There are a lot of interactions going on in the history of a globular cluster. Some of it is interactions within the cluster (stars on stars) and some of it comes from galactic tidal forces. In addition, the stars in it probably formed coevally (at the same time), so the initial burst of star formation would have had an impact on the dynamics.


    Globular cluster formation is still an unanswered question in astronomy, so it's not clear exactly how large the initial object (probably a molecular cloud) was. You're right that if it were huge, one would expect collapse to lead to a more flattened system.


    There are a lot of theories on globular cluster formation, including merger remnants, pre-galactic collapse (we expect small objects to form first in the cosmological "bottom-up" scenario), and substructure from the inital collapse of galaxies.

    If you're still curious and you're comfortable with the lingo, I suggest a search of Astro-ph.
     
  5. Mar 11, 2005 #4
    Thanks, SpaceTiger, for your illuminating reply!

    I knew from previous readings that globular clusters were [nearly] collisionless systems, and I have some appreciation of how the multiple-star gravitational interactions would result in highly complex orbits and orbital evolution, but I'm still boggled at how a system with orbits and momentum in every direction, that is so finely balanced in each dimension, can come into existence from a single molecular cloud.

    The evolution of globular clusters would seem to require initial conditions that imparted a wide range of momentum to clumps of gas in each of the three dimensions of space, and yet have an overall momentum of close to zero.

    I have read that magnetars have incredibly twisted magnetic fields. I wonder what would happen if mass accretion to a magnetar occurred to the point where the magnetar suddenly collapsed into a black hole, ejecting some mass (and super-twisted magnetic fields?) in the process. Is there evidence of black holes at the center of globular clusters?
     
  6. Mar 11, 2005 #5

    Labguy

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    There probably are as there are some strong observations indicating this. See:

    http://arxiv.org/PS_cache/astro-ph/pdf/0110/0110016.pdf

    And for quite a few papers see:

    http://www.google.com/search?as_q=b...occt=title&as_dt=i&as_sitesearch=&safe=images

    M15 seems to be a most likely candidate for aBH in a globular cluster.
     
  7. Mar 12, 2005 #6

    Chronos

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    SpaceTiger knows whereof he speaks. The big issue, to me, is that globular clusters appear extremely ancient - they evidently formed while this galaxy was just begining to take shape.
     
  8. Mar 12, 2005 #7

    Labguy

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    Yes, nothing wrong with SpaceTiger's summary, and about anyone ever looking into anything about stellar evolution would already know that Globulars are very ancient.(??) Globulars and galaxy cores are where we find the "older" Population II stars. It was this "population error" (on Cepheids) that led Hubble to first, and wrongly, estimate M31 at a distance of ~1.1 million ly.

    This being the case, some old stars must have formed to be massive enough to have gone supernova by now. If so, there could easily be supernovae remnants, including black holes, in globular clusters. From another source:



    So, in simply answering the question of whether or not black holes can exist in globular clusters, it would seem to be yes, it is likely. That was what I posted above.

    Chronos;
    Your post was very short. Did (do) you have another point to make on this subject?? If not, I don't understand your post.. :confused:
     
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