What mystery source is ejecting stars from the Milky Way?

Four different 'hypervelocity' stars whiz by the sun on their way out of the galaxy.

A mysterious source is kicking stars out of our galaxy and puzzling astronomers.

The supermassive black hole (SMBH) at the centre of the Milky Way, which goes by the name Sagittarius A*, has been known to fling a star or two (or 18) in its day. Astronomers have found evidence for 18 different stars that are now very quickly on their way out of the galaxy, headed straight out from the galactic core. The most likely case is that each of these were binary stars that circled one of the other 28 or so stars that are currently whizzing around Sagittarius A*. The combined gravity of the black hole and their companion likely acted like a slingshot to launch these stars towards intergalactic space.

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Recently, though, astronomers have uncovered 20 more stars that are hastily making an exit from the Milky Way, travelling from directions other than from the galactic core. These stars are moving in excess of two million kilometres per hour (by comparison, our solar system is moving at around 72,000 km/h). What gave them the boot is a mystery.

"These new hypervelocity stars are very different from the ones that have been discovered previously," study lead author Lauren Palladino, who is a graduate student at Vanderbilt University, said in a statement.

"The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small — about the size of the sun — and the surprising part is that none of them appear to come from the galactic core."

This second view shows the stars motion in relation to the galactic disk.

These stars were discovered by Palladino during her work with the Sloan Digital Sky Survey, while specifically tracking sun-like stars as they orbit around the galactic core. According to Palladino, there's a chance that a few of these stars could be just a case of statistical fluke — some bad tracking data gathered at some point during the survey could skew the speed and make it look like the stars are travelling much faster than they really are. However, after running tests to check the data, she and her colleagues believe most of the measurements are legitimate.

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What could have given these stars such a powerful kick that they could reach the galaxy's escape velocity of over two million kilometres per hour?

The researchers considered several possibilities and were able to rule out some of the big ones. They definitely don't come from our SMBH. Also, it wasn't the SMBH at the centre of the Andromeda Galaxy shooting them through our galaxy. The directions just don't work out, but it's possible they came from other galaxies. That would mean they made a very long journey, indeed.

There are definitely some more local possibilities, though. They may have been kicked out of star clusters or the globular clusters that orbit our galaxy, since stars there can be very tightly packed together. Some more interesting ideas include an unlucky encounter with a binary black hole or possibly even that they were blown away from a companion star that went supernova (something they can test for by looking for 'residue' of the explosion in the star's spectrum).

It's going to take more observations for the scientists to know for sure, though.

(Images courtesy: Julie Turner/Vanderbilt University, NASA, ESO)

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