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Why are Globular Clusters so Metal poor?

  1. Feb 26, 2006 #1
    I do not understand why Globular clusters, with ages around 10 Gigayears, are so metal poor. Their stars are surely undergoing nucleosynthesis, right? Are all the stars in such cluster only of such low mass that they never explode as supernova and scatter their elements?

    TIA

    Sterling
     
  2. jcsd
  3. Feb 26, 2006 #2
    (Disclaimer: I am only a student)

    I believe the reason why globular clusters are so metal poor is because, like elliptical galaxies, they do not have regions of new star formation, so all the massive stars have gone nova long ago. In contrast, in the main disks of spiral galaxies with their regions of new star formation, the continual process of creating new stars means that you get high mass stars created all the time so that more heavier elements are accumulated.
     
  4. Feb 27, 2006 #3

    They do produce metals (which in astronomy means anything bigger than helium). However these are produced in the core of the stars, not the surface. When we examine the spectrum of the star, we measure the composition of the surface, which typically remains the composition that the star originally formed with (not entirely, but pretty close).
     
  5. Feb 27, 2006 #4

    tony873004

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    Globular clusters are very old, and I do not believe that new star formation is very common in them. Old stars are metal-poor since they condensed from clouds that had not yet been seeded by the heavy metals produced in earlier-generation stars.
     
  6. Feb 27, 2006 #5

    Garth

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    And what have these old stars been doing for the last 12 Gyrs?

    Garth
     
  7. Feb 27, 2006 #6

    SpaceTiger

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    Some good answers so far. We think that globular clusters formed at around the same time as the galaxy itself, so the gas from which its stars formed would not have had many heavy elements. It's likely the gas had been enriched by ~1 generation of stars (perhaps Population III stars). Since the time of their formation, many stars in globular clusters have either exploded (supernovae) or ended their lives as a white dwarf. Although both of these processes will have enriched the small amount of gas within and near the globular cluster, the already formed stars will not accumulate any (or, at least, a negligible amount of) further heavy elements.

    Most of the stars that remain in globular clusters today are of relatively low mass -- the majority less massive than our sun. Although some of them will be burning helium in their cores (and, in fact, these are among the brightest stars in globular clusters), the majority will not have yet created metals. As franz pointed out, the few that have formed metals will not be able to transport them to their surface where we can see them. The stars with the lowest masses, which are the longest-lived, will probably never even create significant numbers of elements heavier than helium in their core.

    As for your last question, you're right, the majority of the remaining stars will never end their lives as supernovae. Most will end up as a white dwarf. There is, however, a very tiny fraction of stars in globular clusters that will have recently accreted mass from a binary companion and settled onto the main sequence as short-lived massive stars. Many of these stars, which are called "blue stragglers", will eventually explode in a supernova.
     
    Last edited: Feb 27, 2006
  8. Feb 27, 2006 #7
    Some great answers so far indeed. But I think that many of them just beg the question. Why would a globular cluster evolve in such a way to begin with? What happened to the metals from the original stars that were big enough to go supernova? And why are there only such smaller stars in them now?

    It seems to me that the answer must lie either in the way the Milky Way's gravity effects the evolution of Globular clusters and in their very odd "plunging" orbits that take them sometimes close to the galactic center, or in the gravitational field of a more spheroid shape.

    In my text, it notes that that in a typical galactic arm, collisions (or more precisely close encounters) would be extraordinarily rare (~ once every 20 million galactic orbits), but that near the center of a globular cluster, close encounters would indeed have enormous gravitational influence on a star, and presumably its evolution. Could it be that larger stars, having a larger "cross section" if you will, get weeded out during a globular cluster's swing through the center? But then again, why would supervnova gas be weeded out in such swings?

    Or alternatively is there something about the shape of globular clusters that creates a gravitational field that is not conducive to large star formation or to keeping gas around? I don't see off hand any reason why that would be so. But my theory in the paragraph above wouldn't apply to elliptical galaxies which also seem to be metal poor.

    It seems to me that the issue of galactic metallicity structure is rather perplexing, and rather glossed over in the textbooks I've seen. (of course, I'm far from that well read!). I think it needs a lot more discussion here!

    TIA

    Sterling
     
  9. Feb 27, 2006 #8

    SpaceTiger

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    These metals get dispersed into the interstellar and intergalactic media. Globular clusters would have a tough time holding this material (their gravity isn't very strong), so most of it would be expelled into the Milky Way or Local Group.


    It turns out that massive stars burn their nuclear fuel much more quickly than low-mass stars, so they have shorter lifetimes. The most massive stars only live about 10 million years, while globular clusters are ~10 billion years old. The least massive stars can live a trillion years or more!


    The Milky Way's gravity does have an impact on globular clusters, but not a very big one. Perhaps most importantly, tidal forces result in an outer "tidal radius" for the globular cluster -- stars beyond this distance from the globular cluster center are stripped by the Milky Way's gravity.


    They do have a larger cross section (more massive stars are also larger), but this would be a small effect compared to their rapid evolution.


    What your book is talking about is collisions between stars in the globular cluster, not collisions between globular cluster and Milky Way stars. These collisions can result in supernovae (eventually), but again, it's a small effect.
     
    Last edited: Feb 27, 2006
  10. Feb 27, 2006 #9

    tony873004

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    Is it speculated that globular clusters have a black hole in the middle? I guess evaporation rates would give a clue, since a black hole significantly higher than a stellar mass would help hold the cluster together better.

    When I look at a picture of M13, I see a central part that is very dense, and at the fringes, I can make out individual stars. Are these stars still bound to the cluster, or are they stars that got ejected, but haven't had a chance yet to distance themselves from the cluster.

    The reason I ask is that I simulated a mini cluster of 200 stars, and as it evaporated, it looked quite similar to M13, with the dense cluster and density dropping with distance. But the stars not concentrated in the core were all on escape trajectories. I can't trust the simulation since 200 stars doesn't do a good job representing a few million, and I suspect in the real M13 that the fringe stars are still bound. Any thoughts?
     
  11. Feb 27, 2006 #10

    SpaceTiger

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    There is some speculation about intermediate mass black holes living in some globular clusters, but I don't know of any firm evidence to support it. Some interesting discussion here:

    http://xxx.lanl.gov/abs/astro-ph/0601450


    I couldn't really say in general. It depends on the tidal radius of the cluster and the velocities of the stars in question.


    Did you include softening? How long did it take to evaporate? How massive were the stars? Globular clusters do exhibit core collapse behavior after long times, but I'm hesitant to say straight off that this was the reason.
     
  12. Feb 27, 2006 #11

    Chronos

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    I'm inclined to see globular clusters as reflecting the distribution of dark matter clumps in the early universe. Gravitational lensing studies suggest a large amount of missing matter associated with these structures. It also appears they are about the right size with respect to the smallest dark matter clumps believed to form. It makes sense to me that globular clusters are as old, or older than the galaxies they are associated with. If dark matter clumped in conformance with the bottom up merger scenario, globular cluster should be among the oldest structures in the universe.
     
    Last edited: Feb 27, 2006
  13. Feb 28, 2006 #12

    tony873004

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    What is softening? Guess that means I didn't use it :)

    It never completely evaporated, even over timescales exceeding the life of the universe.
     
  14. Feb 28, 2006 #13

    SpaceTiger

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    Some related discussion here. Basically, softening involves changing the law of gravity when two objects are very near one another. The reason that this needs to be done is that N-Body simulations are approximating extended (or sometimes, composite) objects with point particles. When two point particles get really close to one another, they scatter very strongly and it can sometimes cause one of the particles to be ejected with an unphysical velocity. This isn't an issue for many solar system applications because the bodies don't actually get that close to one another, but in globular clusters, where the stellar densities are very high and the velocities mostly random, you definitely need to include it.

    That's one possible reason for the halo of unbound objects on the outskirts of your cluster -- they could be scattered from unphysical close encounters. I couldn't say for sure without knowing the parameters of your simulation.
     
  15. Feb 28, 2006 #14

    tony873004

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    Thanks ST, I didn't know what that term meant, but I am familiar with concept.

    I tried once putting in some code that recognized when the time step was too fast and temporarily reduced it until the close encounter was over, but it introduced other errors so I abandoned that idea.

    There's definitely a lot of stars that get ejected at unrealistic speeds (maybe close to C). A much slower time step would fix this, but I wanted to simulate the age of the universe in a couple of days. I get bored with my simulations when they run longer than a few days.

    The particles ejected at unrealistic speeds don't comprise the halo. They're way faster, and thus much farther than the halo stars. Zooming out to see them reduces the cluster, halo and all, to a single pixel. Since my time step is slow enough to simulate real ejections, I just figure that any object ejected as super speed would have also been ejected, but at a realistic speed, with a slower time step. Or perhaps they represented stars that would have collided, but I think the odds of this are much lower. This means that my halo should actually be denser than it is.

    I know any results I get are going to be unrealistic since I'm using too few objects, and too large a time step, but I just wanted to see if I could spot any trends that might indicate how things work on a larger scale. I've found that if one object is significantly heavier than all other objects (>= 10x the mass of the 2nd heaviest object) that the system evaporates much slower, hence my black hole question.

    But the simulation I described above didn't have a heavy member. Everything was 1 solar mass.
     
  16. Mar 12, 2006 #15
    I have been quite interested in the origins of globular clusters for some time and have searched several times for theories on this and find very little information and the discussion here suggests that this is the case.

    A lot of globular clusters are very old from their main sequence structures and I suggest the some of them could in fact harbour the relics of the very first stars.

    Here is a suggestion how they might have been formed

    The nature of gravitational collapse is that it happens faster in the middle so if a very large cloud of material was slowly contracting it is quite likely that the centre would collapse first to form the first star probably a very large one that would run quickly through its lifecycle and go supernova while the cloud is collapsing. during the life of this star it would introduce much more turbulence into the collapsing cloud which would fragment and collapse into many smaller stars these would continue to collapse towards each other and share their angular momentum but because this all happened before "viscocity" could create a disk the structure would continue to be approximately spherical.

    One important difference in the propeties of this early cloud is that it would be much more transparent that clouds with a higher metallicy because ther would be very little particualte matter because it's nearly all hydrogen helium and duterium therefore most of the stellar radiation would pass through the cloud and not halt the collapse.
     
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