Fast stars
© Pluto/Alamy Stock Photo
Fast stars whipped to high speed by a black hole or supernova
At least two intergalactic interlopers are hurtling through our galaxy at more than 700 kilometres per second. These stars from outside the Milky Way are among almost 30 runaways that have been spotted in a treasure trove of data from the European Space Agency's Gaia satellite mission.

The Gaia satellite has been charting the stars for years in an effort to make the largest 3D map of our galaxy. On 25 April, Gaia released its second batch of data on 1.7 billion stars. For a subset of 7 million, Gaia measured how fast they are moving away from or towards Earth.

Of these, Tommaso Marchetti and colleagues at Leiden University in the Netherlands looked for hypervelocity stars, those travelling at speeds greater than 450 kilometres per second. They found 165 candidates.

The team calculated that 28 have a greater than 50 per cent chance of escaping our galaxy's gravitational pull. "They are basically flying away forever from the Milky Way," says Marchetti.

A handful of these unbound stars have paths that are consistent with having been ejected from the galactic centre, where the gravity of the massive black hole that sits there may have ripped apart a binary star system, sending one of the pair soaring away.

"Some were white dwarfs that were ejected when their companion stars went supernova"

But half have a more cosmic origin: reconstructions of speeds and orbits suggest they originated outside our galaxy. The most intriguing are two extremely fast stars with velocities in excess of 700 kilometres per second (

"They have done a really good job of identifying all the possible candidate hypervelocity stars in this subset of stars," says Douglas Boubert at the University of Cambridge. "They have one object which is travelling towards us. Its radial velocity is minus 600 kilometres per second. That is pretty much a certain discovery."

Boubert is part of another team that has analysed the Gaia data for runaway stars, this time white dwarfs that were ejected from a binary system when one went supernova.

This occurs when one dwarf slowly slurps matter from the other, exploding when it reaches 1.4 times the mass of our sun, and flinging its companion into space at more than 1000 kilometres per second.

The researchers, led by Ken Shen at the University of California at Berkeley, found seven possible high-speed white dwarfs in the Gaia data ( They then used ground-based telescopes to study the spectrum of light from these stars and showed that three are very likely to have originated in this way.

This article appeared in print under the headline "Runaway stars to escape the galaxy"