Black holes can't be seen, but they're detected by noting their effects on stars or gas around them. They're so dense that nothing, including light, escapes them. Only two classes of black holes are firmly established to exist: Stellar black holes typically weigh a few times the mass of the sun; supermassive black holes are loaded with millions or billions of solar masses.

Astronomers have long debated the existence of a class of middleweight black holes, which could be a missing link in the evolution of the universe's first stellar black holes to supermassive black holes that anchor most major galaxies.

Data gathered by instruments on the Hubble Space Telescope and the Gemini South telescope in Chile show that the sky's largest star cluster, Omega Centauri, might harbor an elusive intermediate-mass black hole in its center.

"This result shows that there is a continuous range of masses for black holes, from supermassive, to intermediate-mass, to small stellar mass types," said astronomer Eva Noyola of the Max-Planck Institute for Extraterrestrial Physics in Germany, and leader of the team that made the discovery, which was announced today.

Located 17,000 light-years away, Omega Centauri has a long history of perplexing astronomers. Nearly 2,000 years ago, scientists listed it as a single star. In 1677, Edmond Halley revised Omega's identity as a nebula, and it wasn't until the 1830s that English astronomer John Herschel recognized it as a globular cluster.

The entire cluster contains about 10 million stars tightly bound by gravity, making Omega Centauri among the biggest and most massive of some 200 globular clusters orbiting the Milky Way. These clusters tend to house mostly ancient stars and are thought to be remnants of the early universe.

By measuring the velocities of stars in the cluster's center, Noyola and her colleagues calculated Omega's total mass to be far higher than expected based on the number and type of stars seen.

The researchers suspect the missing mass comes from a black hole weighing 40,000 solar masses at the center of the cluster.

Other possible explanations for the missing mass include a collection of unseen burnt-out stars, such as white dwarfs or neutron stars, or a group of stars with elongated orbits that would make the stars closest to the center appear to speed up and result in an overestimation of mass. Noyola says these alternatives are unlikely, partially due to the fact that both scenarios would be very short-lived.

Enlarge NASA, ESA, and the Hubble Heritage Team (STScI/AURA) The core of the globular cluster Omega Centauri glitters with the combined light of 2 million stars. Researchers have detected a possible intermediate-mass black hole in the cluster's core.

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