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Star Clusters

Medium Black Holes

M82 Medium Black Hole Evidence indicates that the bright and peculiar galaxy M82 had collided with its neighboring galaxy M81. Inspection of the image in Figure 06-08 and other Hubble Space Telescope images now indicate massive young globular star clusters were formed during the encounter. Stars in these clusters that are 600 million years old are just now exhausting their central hydrogen fuel, indicating that M82 brightening occurred just that long ago. M82 is located about 12 million light years away and visible

Figure 06-08 M82
[view large image]

Figure 06-09 Medium Black- hole [view large image]

with binoculars towards the constellation of Ursa Major. The star-field shown in the figure spans about 10,000 light years.

Recent observation in 2004 suggests that these star clusters may harbour blackholes of intermediate-mass in the 100 - 1000 solar-mass range. This kind of medium blackhole with mass in between the supermassive blackhole at the center of galaxy and the stellar blackhole with a few solar mass is expected but has never been detected until observations from NASA's Chandra space observatory captured the X-ray image as shown in Figure 06-09. Analysis shows that one cluster, MGG 11, coincides spatially with the brightest ULX (ultraluminous X-ray source) discovered so far, the luminosity of which corresponds to an intermediate-mass blackhole of 300 - 900 solar masses. Numerical simulations indicate that collisions between stars could have created a "runaway" star that ultimately became an intermediate-mass blackhole. On the other hand, another star cluster MGG 9 in the same image fails to develop a blackhole because the larger cluster radius leads to a timescale of five times longer than for MGG 11.

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