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A swarm of black holes

ASA / CXC / STANFORD / ZHURAVLEVA, I. et al. The center of the Milky Way. Sagittarius A* is shown within the yellow squareASA / CXC / STANFORD / ZHURAVLEVA, I. et al.

Astrophysicists from the United States and Chile have discovered a dozen stellar-mass black holes (some larger than the Sun) in the center of the Milky Way, the galaxy that is home to our Solar System (Nature, April 5). The small group of celestial objects is orbiting Sagittarius A*, a black hole whose mass is four million times that of the sun. Black holes do not emit light, but the matter drawn into their gravitational pull releases X-rays as it spirals before being swallowed. The 12 objects were identified with help from the Chandra X-ray Observatory, a NASA satellite, which spent 115 days observing the center of the Milky Way. Sagittarius A* is surrounded by gas and dust in a region where many stars are formed, some with enough mass to gravitationally collapse, creating stellar-mass black holes. The discovery is crucial to understanding the interaction between these two types of black holes. For astrophysicist Rodrigo Nemmen, a professor at the University of São Paulo (USP) who specializes in black holes, identifying these objects is so difficult that it “is equivalent to finding a bacterium on the surface of the Moon.” The discovery confirms a theory proposed by astrophysicists Jordi Miralda-Escudé and Andrew Gould in 2000. At the time, the Ohio State University researchers suggested that there may be as many as 25,000 black holes at the center of the galaxy.