There’s a neutron star in Andromeda that is behaving like a fairytale villain. It used to be a quickly rotating pulsar, however over time it has slowed down, spinning on its axis every few seconds. But it’s having another chance at youth. It has now encountered another star and is stealing its material, which means it has begun accelerating again.

Astronomers suggest that the pulsar, known as XB091D, started this rejuvenation process in the last million years and that we are seeing the beginning of the pulsar's attempt to return to youth. It currently spins on its axis every 1.2 seconds, but it was probably spinning a hundred times each second when it formed.

The research, published in the Astrophysical Journal, used data from the European observatory XMM-Newton. This is only the second pulsar found outside the Milky Way and its companion. It’s also the slowest X-ray pulsar discovered in a globular cluster, a spherical collection of a large number of stars.

“In our Galaxy, no such slow X-ray pulsars are observed in hundred and fifty known globular clusters, because their cores are not big and dense enough to form close binary stars at sufficiently high rate,” lead author Ivan Zolotukhin, from the Lomonosov Moscow State University, said in a statement. “This indicates that the B091D cluster core, with an extremely dense composition of stars in the XB091D is much larger than that of the usual cluster.”

The high density of the cluster has allowed the pulsar to capture the companion star, and the researchers think that the cluster is the remains of a bygone dwarf galaxy that has been cannibalized by Andromeda.

“We are dealing with a large and rather rare object – with a dense remnant of a small galaxy that the Andromeda galaxy once devoured," Zolotukhin added. "The density of the stars here, in a region that is about 2.5 light-years across, is about ten million times higher than in the vicinity of the Sun.”

Pulsars are just one of the possible ends of massive stars. After they go supernova, if they don’t have enough mass, they collapse into a dense compact neutron star, just a few tens of kilometers across. In their collapse, they retain the angular momentum and magnetism of the original star, spinning at an incredible rate and emitting regular pulses, like cosmic lighthouses.

^{Infographic of XB091D. Artwork by A. Zolotov}