Astronomy Picture of the Day

Explanation: A recent informal poll found that astronomers don't yet have a good collective noun for a group of black holes. But they need one. The red circles in this Chandra Observatory X-ray image identify a group of a dozen black holes in binary star systems. With 5 to 30 times the mass of the Sun, they are swarming within about 3 light-years of the center of our galaxy where the supermassive black hole identified as Sagittarius A* (Sgr A*) resides. Yellow circles indicate X-ray sources that are likely less massive neutron stars or white dwarf stars in binary star systems. Alone black holes would be invisible, but in binary star systems they accrete material from their normal companion star and generate X-rays. At the distance of the galactic center Chandra can detect the only the brighter of these black hole binary systems as point-like sources of X-rays, hinting that hundreds of fainter black hole X-ray binaries should exist there, as yet undetected.