Everything about black holes is extreme. That includes the range of their masses – from a few times the mass of the Sun, to a few billion. But there’s not much in the middle – between about a hundred and a hundred thousand times the Sun’s mass.

Those middleweights are known as intermediate-mass black holes. And they’re extremely rare – astronomers have cataloged no more than a few dozen of them. But some recent work has solidified the cases for two of them.

One of the black holes is in I-R-S 13 – a star cluster that’s quite close to the supermassive black hole at the center of the galaxy. The cluster is more tightly packed than any other cluster in the Milky Way.

Astronomers recently looked at stars at the cluster’s heart. The stars appear to be orbiting something small but heavy. The most likely culprit is a black hole about 30,000 times the mass of the Sun.

The other black hole is in the heart of Omega Centauri – the biggest star cluster in the galaxy. Earlier studies had suggested the cluster has a black hole up to 40,000 times the Sun’s mass.

Hubble Space Telescope revealed that several stars near the center of Omega Centauri move especially fast – so fast that they would be flung out of the cluster if they weren’t held in place by something especially heavy. That “anchor” appears to be a black hole more than 8,000 times the mass of the Sun – a rare intermediate-mass black hole.

Script by Damond Benningfield