Within the confines of the Triangulum Galaxy lies a mysterious object, but have scientists finally worked out why it behaves so strangely?

Reported by Colin Stuart

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. They looked back at old photographic plates of M33 taken in visible light from both 1949 and 1991. Object X wasn’t there, so it hasn’t been visible to ordinary telescopes knocking on the door for a century. Whatever the reason behind this, it is not short-lived – at least not on a human timescale.

So what is going on? Further observations offer some important clues. The meagre visible light it emits varies over timescales of weeks to months. The infrared light it kicks out varies a little, too. This is exactly how you would expect a star to behave, but if it is a star then why wasn’t it showing up easily using ordinary telescopes?