The dark core of Omega Centauri

The largest, brightest star cluster in the night sky appears in the constellation Centarus. And astronomers are closing in on solving its centuries-long identity crisis. What Ptolemy once pegged as a single star and Sir John Herschel saw as a globular cluster (he was getting warmer) now appears to be the leftover core of a dwarf galaxy that the Milky Way gobbled up.

Globular clusters are groups of up to 1 million stars that are born so close together that their combined gravity binds them in a group, in contrast to open clusters such as the Pleiades, where the stars all move away from each other. But Omega Centauri, as the cluster in question is known, was odd, compared with other globulars: It rotates faster, has a flatter shape, and contains far more mass. Its stars display traits that imply two bursts of star formation – one when it formed, another when it became the Milky Way's snack.

An international team of astronomers, led by Eva Noyola at the Max Planck Institute for Extraterrestrial Physics in Germany, measured the motion of the cluster's stars. The innermost stars were moving unusually fast, given the overall mass of the stars in the cluster. Something really massive must lie at the cluster's heart, they reasoned. The most likely explanation: A rare, intermediate-class black hole – far more hefty than black holes that form from collapsed stars, but far less massive than the black holes found at the cores of large galaxies. If the hidden actor is a black hole, the team figures it tips the scales at 40,000 times the mass of the sun. The results have been accepted for publication in the Astrophysical Journal.