13 May 2015
A few weeks ago, around a galaxy far, far away, a new type of mysterious “dark” star cluster was spotted.
Our Galaxy, the Milky Way, is surrounded by about 150 groups of stars, which are called globular clusters. Each one contains enough material (mass) to make tens or hundreds of thousands of stars.
A galaxy called Centaurus A (sen-TOR-us Ay) is surrounded by thousands of globular clusters. Each of the circles in this picture shows the location of a cluster. Many of them are normal. The brighter ones have more stars, and that makes them more massive.
But a small number looked really strange. They look like they hold more than just stars. Something in these clusters is dark, hidden and has lots of mass. But what?
The team of astronomers looked at different possibilities. Maybe the dark clusters contain black holes, which we can’t see but have high mass? Or maybe the clusters could be full of dark matter?
Dark matter is a bizarre material that doesn’t give off any light — it’s totally invisible. This strange stuff isn't normally found in globular clusters but perhaps, for some unknown reason, these ones do have some? This would explain the odd findings. But until we know for sure, these clusters remain a mystery.
These globular clusters aren't the only dark object in Centaurus A, at the centre of this galaxy lies a black hole that is 55 million times more massive than our Sun!