How do stars orbit in a globular cluster?

BoJangles · Aug 21, 2008

How do stars orbit in a globular cluster, randomly, or on some sort of defined plane? Do they exhibit a flat rotation curve? <div class="Discussion_UserSignature"> -------------- Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog. With that out of the way, I have several stupid questions... *** A few months blogging can save a few hours in research *** </div>

derekmcd · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>How do stars orbit in a globular cluster, randomly, or on some sort of defined plane? Do they exhibit a flat rotation curve? Posted by Manwh0re</DIV>Being that they form in spherical clusters, I doubt a defined plane would be exhibited.  I'm guessing with the stars being so tighly packed, the perturbations would make the orbits quite random.   As for flat rotation curve, I doubt it.  I believe that is more reserved for larger formation surrounded by mass amounts of dark matter (i.e. galaxies).I can't give you a "for sure" answer though...  <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

vogon13 · Aug 21, 2008

Individual stars in a globular cluster act almost like atoms in a gas in some ways.   <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

MeteorWayne · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Individual stars in a globular cluster act almost like atoms in a gas in some ways.   Posted by vogon13</DIV> Except that they orbit around the center of mass of the whole cluster, rather than in random directions. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

vogon13 · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Except that they orbit around the center of mass of the whole cluster, rather than in random directions. Posted by MeteorWayne</DIV> I did not want to use the word 'stochastic' this early in the day . . .     <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

BoJangles · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Except that they orbit around the center of mass of the whole cluster, rather than in random directions. Posted by MeteorWayne</DIV> why should they orbit around the centre of mass of the whole cluster when the main gravitational affect would be from their neighbours?  <div class="Discussion_UserSignature"> -------------- Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog. With that out of the way, I have several stupid questions... *** A few months blogging can save a few hours in research *** </div>

MeteorWayne · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>why should they orbit around the centre of mass of the whole cluster when the main gravitational affect would be from their neighbours?  Posted by Manwh0re</DIV> Orbiting objects orbit around a point called the barycenter. It is the combined center of mass for the whole system. For example, the moon does not orbit the earth, rather it orbits the center of mass of the earth-moon system, which while inside the earth is not at the center. In fact the earth orbits about that point as well, so appears to wobble in it's path.The center of mass of the solar system is not at the center of the sun, in fact when the larger planets are all on one side of the sun, that center of mass is outside of the sun's surface so the sun itself orbits around that point.In a globular cluster, it's infinately more complex since there are so many objects, but all the stars orbit around the center of mass of the whole cluster. Each individual star sees the mass of the whole cluster as a point about which it orbit. Of course, since all the stars are moving, that center of mass is constantly moving as well, that's why I said it's infinately more complicated. However, tracking the motion of individual stars in clusters easily show that this is correct.The wiki page on barycenter is good, but a pretty tough read if you're not into all the math.MW <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

BoJangles · Aug 21, 2008

hrm thanks for the answer, i do understand the barycentre and the complicities of the calculations involved as i have written my own Newtonian gravity simluations in code.i suppose i was wondering why they even all orbit around a barycentre, when the main source of gravity felt by each star would be greater with its neighbour, than the barycentre. i suppose that's got more to do with the evolution of the galaxy than its current situationI.E with a galaxy 10's of thousands of light years across, it wouldn't even feel the affect from a barycentre, the most gravitation affect would be felt by its neighbours not the centre of the galaxy or even the centre of mass of the whole galaxy. i think im confusing the question here, pleas disregard my ignorance.   <div class="Discussion_UserSignature"> -------------- Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog. With that out of the way, I have several stupid questions... *** A few months blogging can save a few hours in research *** </div>

derekmcd · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>hrm thanks for the answer, i do understand the barycentre and the complicities of the calculations involved as i have written my own Newtonian gravity simluations in code.i suppose i was wondering why they even all orbit around a barycentre, when the main source of gravity felt by each star would be greater with its neighbour, than the barycentre. i suppose that's got more to do with the evolution of the galaxy than its current situationI.E with a galaxy 10's of thousands of light years across, it wouldn't even feel the affect from a barycentre, the most gravitation affect would be felt by its neighbours not the centre of the galaxy or even the centre of mass of the whole galaxy. i think im confusing the question here, pleas disregard my ignorance.   Posted by Manwh0re</DIV>I think what they are trying to say is that no matter how erratic the orbits might be, there is still going to be a conservation of angular momentum for the entire cluster.  No doubt that there are multiple star systems orbiting about each other in random ways, but those systems are still orbiting the center.Europa orbits Jupiter as it is the overwhelming force in the neighborhood, but Europa also still orbits the sun if you track it path through space.  <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

MeteorWayne · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>hrm thanks for the answer, i do understand the barycentre and the complicities of the calculations involved as i have written my own Newtonian gravity simluations in code.i suppose i was wondering why they even all orbit around a barycentre, when the main source of gravity felt by each star would be greater with its neighbour, than the barycentre. i suppose that's got more to do with the evolution of the galaxy than its current situationI.E with a galaxy 10's of thousands of light years across, it wouldn't even feel the affect from a barycentre, the most gravitation affect would be felt by its neighbours not the centre of the galaxy or even the centre of mass of the whole galaxy. i think im confusing the question here, pleas disregard my ignorance.   Posted by Manwh0re</DIV>I think part of the question is that your question asked about globular clutsers. These are realatively small and compact objects scattered withing the larger galaxy. If fact there is some speculation they they may be the cores of small galaxies that the Milky Way has devoured, stripping away the outer gas and stars. As such, you are right, they feel their neighbors (the cluster) much more strongly than they feel the rest of the galaxy; that is what keeps them together. And the combined force from all those cluster stars is what creates the local barycenter within the cluster.An analgous situation might be near the core of the Milky Way, where stars whip around in extreme orbits. In that case the black hole provides much of the mass, but even if there was no black hole the local density of the stars in that area would provide a local source for the stars to continue to orbit about.. Stars there don't feel much of an effect from the rest of the galaxy, just as stars in a cluster are somewhat isolated.Hope that clears it up a bit.MW <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

UFmbutler · Aug 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>If fact there is some speculation they they may be the cores of small galaxies that the Milky Way has devoured, stripping away the outer gas and stars. Posted by MeteorWayne</DIV>It should be noted that the evidence right now doesn't entirely support this idea, since the composition of most globulars indicates they are not similar at all to dwarf ellipticals, which are, in general, the class of galaxies that would be consumed by the Milky Way.  I suppose it's possible, but in every other collision we know of with the Milky Way, the stars become stripped and trapped inside the disk of the galaxy.  There is some Sagittarius dwarf I believe that we can see in the disk that has been "consumed".  <div class="Discussion_UserSignature"> </div>

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How do stars orbit in a globular cluster?

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