Pair of Black Holes in Star Cluster Surprises Scientists

Black holes might seem too monstrous to keep company, but surprising new findings suggest they can live in groups within clusters of stars inside our Milky Way galaxy, researchers say.

The presence of multiple black holes within these clusters might drastically alter the way way these major components of galaxies evolve, scientists added.

"Before this work, there were zero black holes known in Milky Way globular clusters, so even finding one would have been exciting," said lead study author Jay Strader, an astronomer at Michigan State University in East Lansing.

Black holes are the densest objects in the universe, with the largest ones, found at the centers of galaxies, containing millions to billions times more mass than the sun. Stellar-mass size black holes are born from the explosive deaths of stars known as supernovas.

Hundreds of black holes, each with the mass of a star, probably form in globular clusters, spherical collections of hundreds of thousands of stars that orbit the center of the galaxy. However, past research suggested these clusters would never house multiple black holes at any one time. Since black holes are so massive, they tend to fall toward the center of globular clusters, similar to how denser materials made their way to Earth's center during its formation. At the hearts of clusters, these black holes would gravitationally tug at each other and tend to kick all, or perhaps all but one, out of the clusters.

Based on radio emissions, however, scientists have apparently discovered a pair of black holes within the large globular cluster M22, located about 10,600 light-years away in the constellation Sagittarius, near the Milky Way's bulge. M22 is one of the brightest globular clusters in the night sky, and holds nearly a million stars.

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