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Old 08-09-2011, 04:35 PM
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madbadgalaxyman (Robert)
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Join Date: Mar 2011
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Messier 11 - a Young globular star cluster?

esky,
That was an interesting thought about M11.......the idea that it may be thought of as a globular star cluster rather than an open star cluster.

It turns out that its total mass is about 10,000 times the mass of the sun, which does put it within the range of masses that is found for the globular clusters of the Milky Way and the nearby galaxies.

The distinction between open star clusters and globular star clusters becomes quite minimal above a certain value of the total cluster mass, as the cluster will adopt the characteristic Symmetry and the characteristic Radial Surface Brightness profile (of diminishing surface brightness with radius) that is typical of a globular cluster.(this is called a King profile)

Many other objects containing ~10,000 or more solar masses in the form of stars will also adopt a globular-cluster-like morphology; such as the nuclear star clusters that exist at the very centres of many galaxies.

See the discussion about "High mass open clusters" vs "low mass globulars" (is there a difference?) in the post "Terzan Trio" in the Deep Space forum.

Young Clusters that are very similar to globular clusters (globulars are generally thought to be very old) do continue to form in some galaxies. For instance in the Milky Way, we have the objects NGC 3603 and Westerlund 1. In the Tarantula nebula, we have the cluster R136a .
These objects are sometimes called Super Star Clusters, or massive compact young clusters.

Cheers, madbadgalaxyman

P.S. I never see wild ducks, mythological animals, or any other fanciful outlines in the sky. Generally, all I see are geometric shapes!

The wikipedia entry is pretty confused about the distinction between R136a and R136.....many wikipedia articles make mistakes because a person of limited astronomical background is trying to abstract complex information from the scientific literature.
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