With the help of an artificial neural network, ESA’s Gaia satellite spotted six hypervelocity stars — stars whose velocity in the rest frame of our Milky Way Galaxy exceeds the Galaxy’s local escape velocity.

The Milky Way contains more than a hundred billion stars, all kept together by gravity.

Most are located in a flattened structure — the Galactic disc — with a bulge at its center, while the remaining stars are distributed in a wider spherical halo.

Stars are not motionless in the Galaxy but move around its center with a variety of velocities depending on their location — for example, the Sun orbits at about 220 km/s, while the average in the halo is of about 150 km/s.

Occasionally, a few stars exceed these already quite impressive velocities.

Some are accelerated by a close stellar encounter or the supernova explosion of a stellar companion, resulting in runaway stars with speeds up to a few hundred km/s above the average.

A new class of high-speed stars — hypervelocity stars — was discovered in 2005 by astronomer Dr. Warren Brown of the Harvard-Smithsonian Center for Astrophysics.

Swooping through the Galaxy at several hundred of km/s, these are the result of past interactions with the supermassive black hole that resides at the center of the Milky Way and, with a mass of 4 million solar masses, governs the orbits of stars in its vicinity.

So far, only 20 hypervelocity stars have been spotted.

“These stars are extremely important to study the overall structure of our Milky Way,” said Leiden University astronomer Dr. Elena Maria Rossi.

“These are stars that have traveled great distances through the Galaxy but can be traced back to its core — an area so dense and obscured by interstellar gas and dust that it is normally very difficult to observe — so they yield crucial information about the gravitational field of the Milky Way from the center to its outskirts.”

Dr. Rossi and her colleagues applied a new data mining algorithm based on machine learning techniques, an artificial neural network, to the Tycho-Gaia astrometric solution (TGAS) catalogue.

They found a total of 6 hypervelocity stars: TYC 2298-66-1, TYC 8422-875-1, TYC 2456-2178-1, TYC 2348-333-1, TYC 49-1326-1, and TYC 5890-971-1. All have low masses, similar to the mass of our Sun.

“We found that six stars can be traced back to the Galactic center, all with velocities above 360 km/s,” said Tommaso Marchetti, a PhD student at Leiden University.

One of the six stars, TYC 2298-66-1, seems to be speeding so fast, at 530 km/s, that it is no longer bound by the gravity of the Milky Way and will eventually leave.

The other five stars are somewhat slower (360-450 km/s) and are still bound to the Galaxy.

These slightly slower stars are perhaps even more fascinating, as astronomers are eager to learn what slowed them down — the invisible dark matter that is thought to pervade the Milky Way might also have played a role.

“This result showcases the great potential of Gaia opening up new avenues to investigate the structure and dynamics of our Galaxy,” said Dr. Anthony Brown, also from Leiden University.

The research is published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).

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T. Marchetti et al. 2017. An artificial neural network to discover hypervelocity stars: candidates in Gaia DR1/TGAS. Mon Not R Astron Soc 470 (2): 1388-1403; doi: 10.1093/mnras/stx1304