Our Milky Way is home to some bizarrely fast stars, which have somewhat defied explanation. Now, scientists have come up with an answer – they might have originated in an entirely different galaxy.
These runaway stars can be seen around our galaxy. About two dozen have been found so far, mostly in the constellations Leo and Sextans, moving at extreme speeds of up to 1,000 kilometers (620 miles) per second. The average star in our galaxy moves at about a quarter of that speed.
It was thought that these fast stars may have formed from binaries, two stars orbiting close to each other. They would sort of wind up, until there was enough gravitational energy for one of the stars to be flung outwards. This can result in a binary supernova scenario (BSS), where one star achieves extremely high velocities.
However, while this can account for rogue runaway stars, it cannot necessarily account for hypervelocity stars. Thus, researchers led by the University of Cambridge looked to one of our neighboring galaxies that’s moving much faster than us, the Large Magellanic Cloud (LMC), for an answer.
The results are published in the Monthly Notices of the Royal Astronomical Society, and will be presented today at the National Astronomy Meeting in Hull, UK.
“We've shown that at least 75 percent of the hypervelocity stars we already know of are very consistent with being stars that were ejected at high speeds from the LMC,” Douglas Boubert, the study’s lead author, told IFLScience. Two promising candidates are the “Outcast Star”, found 230,000 light-years away, and HE 0437-5439, found 200,000 light-years away.
Using the Sloan Digital Sky Survey and computer simulations, the researchers modeled how these stars could have escaped the LMC. This is the biggest and fastest galaxy orbiting us, and it could have given the stars the observed velocity.
The main evidence for these hypervelocity stars originating in the LMC comes from their position. Across the night sky, they seem to be concentrated in a line between the constellations of Leo and Sextans in the northern sky and the LMC in the south. An upcoming survey using ESA’s Gaia satellite, which will monitor the positions and motion of a billion stars in our galaxy, could find more stars in this region and cement the theory.
This dwarf galaxy, which orbits our own, is rotating at about 378 kilometers (235 miles) per second, much faster than our Milky Way at about 250 kilometers (155 miles) per second. Stars undergoing BSS in the LMC can thus get enough velocity to escape the dwarf galaxy, although they are dragged back somewhat by its gravitational pull, and make their way to our own. Eventually, these stars will leave our galaxy too.
The researchers estimate as many as 10,000 such stars may have made their way here. But interestingly, it’s not just stars that might have migrated over. They propose that up to a million runaway neutron stars and black holes may also be traveling through our galaxy.
It’s likely that the same process creating these speedy stars in the LMC is also occurring here; we’ve got some evidence for runaway stars originating in the Milky Way from BSS. But for the fastest of the hypervelocity stars, it just doesn’t seem likely they were created here.
And there’s one more interesting point from this study. Although Boubert notes it’s a bit of a cliché, these stars could be good locations for life to migrate from place to place. “Rocks orbiting these stars carrying microbes would be an excellent way to spread life between galaxies,” he said.