Every so often, the Milky Way expels a star. The ejected star is typically expelled from the chaotic area at the center of the galaxy, where our Super Massive Black Hole (SMBH) lives. But at least one of them has been expelled from the relatively calm galactic disk, a discovery that has rethought astronomers the whole phenomenon of expulsion of the stars.
"This discovery radically changes our perspective on the origin of fast-moving stars."
Monica Valluri, Professor of Research, Department of Astronomy at the College of Literature of U-M, Science and Arts.
The star in question is a rapidly moving star, or that is also called a hypervelocity star. The stars of hypervelocity are quite rare in our galaxy. The first was discovered in 2005 and so far the researchers have discovered less than 30. They travel at more than 1
To understand what is happening, take a look at the overall structure of the Milky Way.
The galactic bulge is in the center, and deep within that bulge is SMBH of our galaxy, Sagittarius A * (Star to Sag.). All around is the galactic disk, built on the spiral arms of the galaxy. Of less importance in this study are the stars and globular clusters.
When a star is driven out of the galaxy, it is usually a star of a binary pair. Scientists think that as a binary couple gets too close to the SMBH and its crushing gravity, the hole captures one of the stars. The other star is thrown into space in a "gravitational slingshot". The black hole must be a super huge one, because only they have enough gravity to accelerate these fleeing stars at such high speeds.
But researchers from the University of Michigan have identified a hypervelocity star that appears to have been ejected from the stellar disc rather than the galactic bulge.
Monica Valluri and Kohei Hattori followed a hypervelocity star called
LAMOST-HVS1, a hypervelocity star that is closer to the Sun than any other. They used one of the Magellan telescopes to measure the speed and position of the star. Then they joined other colleagues and joined their data with the data of the Gaia mission of ESA to trace the trajectory of hypervelocity to its origin. They were surprised when the origin of the star was not the bulge, but the galactic disk.
"This discovery radically changes our perspective on the origin of fast-moving stars," said Monica Valluri, researcher at the Astronomy Department at the U-M College of Literature, Science and the Arts. "The fact that the trajectory of this massive, fast-moving star originates in the disk rather than the galactic center indicates that the very extreme environments needed to expel fast-moving stars may arise in places other than supermassive black holes."  "We must consider other possibilities for the origin of the star."
Kohei Hattori, postdoctoral researcher, University of Michigan.
"We thought this star came from the Galactic center. But if you look at its trajectory, it is clear that it is not connected to the galactic center," said Hattori. "We have to consider other possibilities for the origin of the star."
What would these possibilities be?
The authors are not sure at this point. One possibility is a meeting of a different kind. The fleeing star may have had an encounter with a whole group of other massive stars, and been expelled by a complex interaction of gravity.
This type of meeting has created stars on the run in the past. But nothing that travels as fast as LAMOST-HVS1. The stellar fugitives have been timed at 40-100 km / s (25-62 miles / second), but no one has come close to the 500 km / second that this star covers.
Un' other possibility more exotic is black hole. There may be other intermediate black holes in the galactic disk with enough gravity to throw the star into space. But this is little more than a hypothesis.
If it is a star cluster that expelled LAMOST-HVS1, then no one has been seen yet. The hypervelocity star came from the Norma's spiral arm, an area not associated with any massive mass of known stars. However, that area is well obscured by dust. There may be a cluster there with enough mass to expel the star.
If astronomers could find a massive cluster there, then it could show that all hypervelocity stars have been expelled from meetings with huge clusters, and the SMBH has nothing to do with it. Or bring me here, the huge star cluster could have an intermediate black hole at its center, powerful enough to eject the star.
For now, however, the origin of LAMOST-HVS1 remains uncertain.