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PJM's Astronomy Gallery
IMAGE OF THE MONTH
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M15 - A Globular Cluster with a Black (Hole) Heart ?
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Are we looking at a black hole ?
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M15 ( NGC 7078 ) in Pegasus. 200 mm f/4 Schmidt-Newtonian image with light pollution filter on 19 October 2000. 2 minutes exposure.
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Data: Messier No. 15 (NGC 7078)
Location: Pegasus (R.A. 21:30:00 Dec. 12:10 )
Distance: 33600 light years
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Magnitude: 6.2
Size: 12 arc min.
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Information on M15 and other useful background to this news has been sourced at SEDS.
M15 is an interesting Messier object from a number of properties. Discovered by Jean-Dominique Maraldi in 1746 it was not resolved into its component stars, and hence not recognised as a star cluster, until 1783 by W Herschel. It's central stars have been resolved more recently using the Hubble Space Telescope. In absolute magnitude M15 is some 360,000 times brighter than the Sun with it's brightest stars some 1000 times brighter than our Sun. It also contains a planetary nebula (Pease 1), which was identified in 1927 using the Mt. Wilson telescope in U.S.A., and is one of only four known in Milky Way globular clusters. At magnitude 6.2 it can be difficult to find with the naked eye but binoculars or a small telescope will identify it as a nebulous object. It's true splendour is revealed through astrophotography.
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One of the most interesting features of M15, however, is that it is the most dense globular cluster in the Milky Way. This property had raised the question whether it was due to mutual attraction between it's inner core stars, or something more sinister lurking at it's centre, such as a black hole.
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Roeland van der Marel and Joris Gerssen of the Space Telescope Science Institute, as reported by A MacRobert in Sky and Telescope ( Vol. 104 (December), 18, 2002 ), examined this thesis through the measurement of the radial velocities of the central stars in M15 using the H.S.T. The results have been interpreted as stars orbiting a mass of some 4000 Sun's i.e. an "intermediate" mass black hole. The black hole has a mass greater than a collapsed star but less than that inside a galaxy. Moreover, the mass size in relation to the globular mass is interesting, because it follows a linear relationship, albeit at a much lower mass, found in galaxies between the mass of the central black hole and the mass of the galaxy core.
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It now remains to determine if black holes can be found in other globular clusters. One has already been found in a globular cluster in the Andromeda Galaxy. The finding raises questions about how the old stars found in globular clusters formed at the beginning of the universe. Alan MacRobert comments that most globular clusters were probably born before galaxies and were incorporated into them later. Hence, did mid-size holes in globulars become the giant centres of some of today's galaxies?
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A more recent explanation offered for the large central mass has been the presence of thousands of dim neutron stars. A black hole is still the favoured explanation, however...February 2003
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References and Further Reading
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