Globular Cluster Dynamics

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Jeremy</i>
<br />How are these structures able to maintain their spherical shape over billions of years without collapsing inward?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Short answer: angular momentum. Longer answer: as far as each individual star is concerned, it is as if there were a large mass in the center, and the star was orbiting it. Orbits that are highly elongated are exchange kinetic and potential energy, but are potentially as viable as near-circular orbits.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">I am not aware of the most recent theories regarding this problem but I have heard explanations that involved stars looping around inward and out kind of like a swarm of bees.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

That is correct. Individual stars have high radial velocities, showing that they resemble a swarm of bees.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Does the most recent evidence bear any of this out or do the clusters remain a mystery?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

No mystery. We can measure both radial velocities and proper motions of individual stars in some globular clusters, so the dynamics are well-mapped out. Recent evidence suggests that "dark matter" or the MOND effect sets in for the outermost stars in these clusters.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Do the clusters rotate? What the does the MM have to say about their gravitational condition?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

I can't recall seeing any detection of a net rotation. MM says the same thing as standard dynamics, except for the explanation of the "dark matter" effect. -|Tom|-

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I can't recall seeing any detection of a net rotation. MM says the same thing as standard dynamics, except for the explanation of the "dark matter" effect. -|Tom|-
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This is very interesting, I hadn't heard about MOND effects showing up in these structures. Does the MOND effect show up at the distance you have predicted for the gravity limit or do we have enough data points along the radius to determine the velocities towards the center as well? In the galactic evolution scheme of things do the globular clusters have the same origin as the standard model or does MM explain their origin in a different fashion?

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Jeremy</i>
<br />Does the MOND effect show up at the distance you have predicted for the gravity limit or do we have enough data points along the radius to determine the velocities towards the center as well?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">It shows up at distances of under 100 pc, which is smaller than predicted. I'm not sure yet what that means to the physical interpretation, but have been concerned for some time that the MM predictions are reasonably complete only for the graviton medium, but not for the elysium medium. That part needs work.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">In the galactic evolution scheme of things do the globular clusters have the same origin as the standard model or does MM explain their origin in a different fashion?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">MM thinks of globulars as by-products of galaxy evolution as large assemblies of stars spiral away from the galactic halo region, rather than as building blocks for galaxy formation. -|Tom|-

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">MM thinks of globulars as by-products of galaxy evolution as large assemblies of stars spiral away from the galactic halo region, rather than as building blocks for galaxy formation. -|Tom|-
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So, to use a crude analogy, the clusters are like droplets thrown off a spinning lawn sprinkler? I thought the globulars were spherically distributed about the galaxy, how would they get into such a distribution if they were thrown off material? Shouldn't they be in the plane of galactic rotation?

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by tvanflandern</i>
It shows up at distances of under 100 pc, which is smaller than predicted. I'm not sure yet what that means to the physical interpretation, but have been concerned for some time that the MM predictions are reasonably complete only for the graviton medium, but not for the elysium medium. That part needs work.
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Dr. Van Flandern,

I've been wondering about some things that seem related to this issue.

Your "rule of thumb" for the mean free path of gravitons is derived primarily (? exclusively ?) from observations of the real behavior of spiral galaxies, IIRC. And the MFP is the main determinant of the length of the gravitational shadow.

But wouldn't the diameter of a gravitating mass also influence the length of its shadow? This sould be true for both "solid" objects (like planets and stars) and extended objects like galactic cores and galaxies.

Intuitively it seems that, for any given value of the graviton MFP, the shadow of a proton would be shorter than the shadow of a planet, which in turn would be shorter than the shadow of a star, etc.

Spiral galaxies have cores that are larger and more dense than globular cluster cores. Any shodow cast by the latter should be shorter and fainter than a shadow cast by the former. ??

If these speculations are valid then I would expect that the behavior of smaller and/or less dense objects would be noticeably different than larger and/or more dense objects.

Regards,
LB

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Jeremy</i>
<br />So, to use a crude analogy, the clusters are like droplets thrown off a spinning lawn sprinkler? I thought the globulars were spherically distributed about the galaxy, how would they get into such a distribution if they were thrown off material? Shouldn't they be in the plane of galactic rotation?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">No, the inverse square law breaks down at a distance of 1-2 kpc, a little less than the size of a typical galaxy halo. So a 3-D lawn sprinkler is a good analogy for globulars.

Our Sun orbits the galactic center -- not because it feels any force from that distance, but because it feels more force from the greater number of stars within 1-2 kpc closer to the center than us than it feels from the lesser number of stars within 1-2 kpc farther from the center than us. This is also why galaxy disks are flat, whereas globular clusters are mostly spherical. If gravity extended to infinite scales, we would expect to see spherical galaxies too, which we do not.

See also my answer to Larry for more about why this is. -|Tom|-