The closest galaxy we can see other than our own Milky Way may be inside the Milky Way. Its outer precincts have been stripped away, leaving only its core – a tight ball of 10 million stars. And a rare type of black hole appears to lurk in its middle.

Omega Centauri rolls low across the south during the night. The view is better from the southern half of the country. To the eye alone, it looks like a fuzzy star.

Omega Centauri is classified as a globular cluster – a family of very old stars. It’s the biggest one in the galaxy. But it probably wasn’t born in the Milky Way. Instead, it began as a separate galaxy. But it was reeled in by the Milky Way’s gravity, which also pulled away most of its stars. Only the stars in the galaxy’s core stuck together.

An intermediate-mass black hole appears to inhabit the center of the cluster. Such beasties are rare. Most black holes are either no more than about a hundred times the mass of the Sun, or a few hundred thousand times the Sun’s mass or more.

A study a couple of decades ago reported a possible black hole in Omega Centauri weighing 40,000 times the Sun’s mass. Later work suggested that number was too high. The most recent estimate was compiled from 20 years of observations by Hubble Space Telescope. It puts the black hole at about 8,000 times the Sun’s mass – a rare black hole in the remnant of a dead galaxy.

More about mid-sized black holes tomorrow.

Script by Damond Benningfield