Look up at the northern night sky, and you will be able to pick out the distinctive W-shaped constellation of Cassiopeia. If each of the two ‘V’s that make up this shape are seen as a downwards-pointing arrow then the right-hand one will guide you to a barely perceptible fuzzy smudge. This is the Andromeda galaxy, one of the furthest objects from Earth that is visible with the unaided eye. Yet there is one naked-eye object that’s even further afield, although you will need incredibly dark skies to see it: the Triangulum galaxy (M33), named after the constellation that borders Andromeda.
It was when studying this distant city of stars over a decade ago that astronomers made a remarkable discovery – something that at first didn’t make sense and that they are still scrabbling to fully understand. They’ve dubbed it Object X. “It has gained a certain level of notoriety,” says Roberta Humphreys from the University of Minnesota, who has worked on deciphering the mysteries surrounding Object X.
Look at M33 in visible light – the same light that our eyes see in – and astronomers can barely make out Object X at all. Perhaps that’s not surprising, given that the galaxy is some 3 million light-years from Earth. However, switching to mid infrared observations makes a big difference. Suddenly, Object X was the brightest thing in the entire galaxy, outshining tens of billions of stars.
The team that made the discovery, led by Rubab Khan from Ohio State University, started sleuthing.
