HEAPOW: Avoid the Holes (2018 Aug 13)
Models of galaxies like the Milky Way predict that there should be an extraordinarily large population of very massive stars within a very small region of space near the center of the galaxy. In the Milky Way, for example, stars will inexorably sink to the center, and as the stars in this region increase over time, gravity becomes stronger, drawing even more stars to this region. As these stars evolve and die, they will leave behind an extraordinarily large number of black holes. These black holes can collide and merge, generating gravitational waves as they build up ever larger black holes with ever more forceful gravity. While we have seen lots of stars near the center of the Milky Way with masses of dozens of times the mass of the Sun, it's very difficult to determine how many stellar-mass black holes lie near the Galactic center. X-rays are a good way to find compact objects like black holes that may be bound to normal stars, since matter from the normal star can be drawn to the compact object, heating up and producing tell-tale X-rays. A recent study of X-ray emission near the center of the Milky Way using data from the Chandra X-ray Observatory has, for the first time, provided some lower limit on the number of black holes in this region. The image above is a Chandra X-ray image of the center of the Milky Way. Analysis of this image revealed a large number of individual X-ray sources, marked by circles, and further study of the properties of the X-ray sources allowed astronomers to identify the subset (marked in red) of probable black hole systems. (The big blue circle marks the location of Sgr A*, the Milky Way's very own supermassive black hole.) About a dozen black hole candidates have been found, but this is just the tip of a very black iceberg: extrapolating from these identified sources, and given how hard it is to find these black hole systems, astronomers suggest that there may be well over ten thousand black holes in this tiny region of the Galaxy. So, if you ever go wandering around the Galactic center, choose your path carefully!
CXC: Black Hole Bounty Captured in the Milky Way Center
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