Messier 31, the Andromeda galaxy (Image: Jason Ware). By watching a gravitational microlensing event, UK astronomers may have detected a brown dwarf in the Andromeda galaxy (M31) for the first time.

Microlensing events occur when light travelling towards Earth is deflected by the gravity of an intervening mass. If a dark object passes across the observer’s line of sight, the light varies in a characteristic pattern analogous to the effect of a glass lens.

The brightness of the Andromeda source over 120 days. The time-symmetric rise and fall confirms it as a microlensing event. (Image: Wyn Evans, Oxford University). The astronomers, who include Dr. Wyn Evans from Oxford University have now seen a microlensing event towards Andromeda. The event was seen as the brightening and fading of a single pixel on successive images, as the galaxy is too distant for most of its individual stars to be resolved. The lens responsible may be a brown dwarf in the halo of otherwise unseen (‘dark’) matter thought to surround M31 and other galaxies.

If the detection is genuine then a brown dwarf will have been found at a distance of 2 million light years, one thousand times further away than the most distant known in our own galaxy.