The Milky Way could contain thousands of stars from another galaxy

We know there are stars moving fast enough to escape the Milky Way, so the same is probably true of other galaxies. Now, simulations suggest there could be almost 4000 stars from the Andromeda galaxy in the Milky Way today

Andromeda is our nearest galactic neighbour
Robert Gendler/ESA

The Milky Way might contain thousands of stars that were expelled from Andromeda, our closest galactic neighbour – and may even have brought planets with them.

Some stars in the Milky Way are moving so fast that they can escape our galaxy’s gravitational pull, which raises the question of whether similarly speedy stars could exist in, and potentially leave, other galaxies as well.

Now, Lukas Gülzow at the Karlsruhe Institute of Technology in Germany and his colleagues have calculated that the Milky Way could be home to anywhere from 12 to almost 4000 stars that originated in the Andromeda galaxy, which is around 2.5 million light years from Earth.

Gülzow and his team focused on stars that were once part of a pair that got too close to a supermassive black hole — one of the stars is absorbed while the other is shot out at high speed. They simulated 18 million stars produced in this way, from the black hole at Andromeda’s centre, and then tracked the trajectories according to the gravitational pull of both galaxies to see how many might reach the Milky Way.

“Only a small number, around 0.08 per cent of them, can make it to the Milky Way, and even less would be measurable,” says Gülzow.

Assuming that Andromeda produces as many high-velocity stars as the Milky Way, the team found that anywhere from 12 to 3909 stars may be in our cosmic neighbourhood today. If the number is towards the upper range of the researchers’ calculations, then this may be enough stars for the Gaia spacecraft, which tracks the location of around 1 per cent of stars in the Milky Way, to find them. These stars would have a telltale direction and speed pointing exactly away from Andromeda, says Gülzow.

Such stars could also bring their planets along for the ride, says Gülzow, assuming the worlds survive being blasted by a black hole. But given the lack of planetary material in between galaxies and the speed at which the stars would be travelling, they are unlikely to develop any new planets during the migration, he says.

“It’s an interesting idea, and there very likely are indeed hypervelocity stars in the Milky Way coming from Andromeda and from other galaxies,” says Kareem El-Badry at Harvard University.

Reference:

arXivDOI: 10.48550/arXiv.2306.08143