“Hypervelocity stars” escaping the Milky Way could represent entirely new class

NASA/Julie Turner

The gravitational pull from the black hole at the center of our galaxy does a pretty good job of corralling all of the stars in the Milky Way, but it appears there are some rogue “hypervelocity” stars that are capable of escaping that pull and may even represent an entirely new class. The research was led by Lauren Palladino of Vanderbilt University and was published in the Astrophysical Journal. It was also presented this week at the 223rd Meeting of the American Astronomical Society.

High-velocity stars have been seen before, but not like this. The traditional high-velocity star is a pair of large blue binary stars that originate near the black hole. As they reach incredible speeds at the center of the galaxy, one is whipped out at incredible speeds. They have vast elliptical orbits, which allow them to escape the galaxy. There are only eighteen stars of this type that have been documented, which shows how rare it is for a star to escape the black hole’s incredible gravitational pull. 

These newly discovered stars are only about the size of our sun and do not come from the center of the galaxy. Over the course of their massive survey which covered about a quarter of the sky, the team detected 20 stars that did not fit the bill for traditional high-velocity stars. They are so unique, they could possibly represent an entirely new class. Of course, that means that this discovery has raised plenty of questions. Primarily, how could these small stars have gotten such speed? The stars could be moving over a million kilometers per hour and there isn’t a known mechanism that would cause them to act like that if they did not originate in the center and get the boost from the black hole.

Some suspect that these stars actually originated outside of our galaxy and are merely passing through. However, the chemical composition is the same as those formed in the halo of the galaxy as it was forming, so it cannot be ruled out that they did indeed originate inside of the Milky Way. 

Of course, stars of this nature must be monitored over the course of several decades to ensure that their speeds and orbits have been correctly estimated. The team ran statistical tests with good results. While it is possible that some of these stars could have been incorrectly categorized, most of them are likely a never-before-seen class of hypervelocity star.


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