The supermassive black hole at the center of our home galaxy, the Milky Way, has a lot of company — thousands of smaller black holes. NASA’s Chandra X-ray Observatory has found evidence to support the theory which says as many as 20,000 stellar-mass black holes could have drifted toward the galactic center and collected around Sagittarius A*, the Milky Way’s supermassive black hole also known as Sgr A*.
Stellar-mass black holes usually contain between five and 30 times the mass of the sun, while supermassive black holes are hundreds of thousands to billions of times more massive than the sun. Black holes are invisible, since their extreme gravity does not allow even light to escape, and are therefore observed using other methods.
One such method works for black holes which are locked in close orbit with another star. In these “X-ray binary” systems, the black hole — or neutron star — pulls gas from its companion star, and that gas falls onto the accretion disk around the black hole. That causes the gas to heat up to millions of degrees, producing X-rays that can be observed before they disappear into the black hole. Chandra captured some of these X-ray binaries within three light-years of Sgr A*.
Chuck Hailey of Columbia University in New York led a team of researchers who used Chandra data to search for X-ray binaries containing black holes near Sgr A*, analyzing X-ray spectra within about 12 light-years of the supermassive black hole. After shortlisting and eliminating candidates — based on data from known X-ray binaries and neutron stars — the team surmised there was a strong possibility of at least one dozen black holes in the vicinity of Sgr A* (shown in the image as red circles).
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