by neufer » Tue May 19, 2015 4:05 pm
Chris Peterson wrote:
Globular clusters are not stable. Stars are occasionally ejected, and since there are only a finite number of stars, they eventually evaporate away. Stellar collisions are vanishingly rare, so there's no mechanism to create a black hole (although some globulars already have one, probably for the same reason most galaxies do... although that reason is not well understood).
http://en.wikipedia.org/wiki/Globular_cluster wrote:
<<The results of N-body simulations have shown that [globular cluster] stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 10
10 years.>>
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<<Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars formed at the beginning of the cluster.
Astronomers have searched for black holes within globular clusters since the 1970s. The resolution requirements for this task, however, are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000 M☉ intermediate-mass black hole has been suggested to exist based on HST observations in the globular cluster M15 and a 20,000 M☉ black hole in the Mayall II cluster in the Andromeda Galaxy. Both x-ray and radio emissions from Mayall II appear to be consistent with an intermediate-mass black hole. These are of particular interest because they are the first black holes discovered that were intermediate in mass between the conventional stellar-mass black hole and the supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies. Claims of intermediate mass black holes have been met with some skepticism. The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation. As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15 and Mayall II.>>
JeffKLass wrote:
The expression in the note "white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass" seems somewhat awkward and is probably the reverse of what is actually happening (most likely in ALL globular clusters), namely, that the heavier stars are attracting each other and slowly 'shuffling' themselves toward the 'attractor' at the center, thus leaving the lighter white dwarfs as a 'residue' at the outer edge.
[quote="Chris Peterson"]
Globular clusters are not stable. Stars are occasionally ejected, and since there are only a finite number of stars, they eventually evaporate away. Stellar collisions are vanishingly rare, so there's no mechanism to create a black hole (although some globulars already have one, probably for the same reason most galaxies do... although that reason is not well understood).[/quote][quote=" http://en.wikipedia.org/wiki/Globular_cluster"]
<<The results of N-body simulations have shown that [globular cluster] stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 10[sup]10[/sup] years.>>
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[float=right][img3="[b][color=#0000FF][size=125]Messier 53 has surprised astronomers[/size]
with its unusual number of blue stragglers.[/color][/b]"]http://upload.wikimedia.org/wikipedia/commons/thumb/2/23/Messier_53_HST.jpg/220px-Messier_53_HST.jpg[/img3][/float]<<Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars formed at the beginning of the cluster.
Astronomers have searched for black holes within globular clusters since the 1970s. The resolution requirements for this task, however, are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000 M☉ intermediate-mass black hole has been suggested to exist based on HST observations in the globular cluster M15 and a 20,000 M☉ black hole in the Mayall II cluster in the Andromeda Galaxy. Both x-ray and radio emissions from Mayall II appear to be consistent with an intermediate-mass black hole. These are of particular interest because they are the first black holes discovered that were intermediate in mass between the conventional stellar-mass black hole and the supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies. Claims of intermediate mass black holes have been met with some skepticism. The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation. As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15 and Mayall II.>>[/quote][quote="JeffKLass"]
The expression in the note "white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass" seems somewhat awkward and is probably the reverse of what is actually happening (most likely in ALL globular clusters), namely, that the heavier stars are attracting each other and slowly 'shuffling' themselves toward the 'attractor' at the center, thus leaving the lighter white dwarfs as a 'residue' at the outer edge.[/quote]