Runaway star escaping Milky Way

Thanks to 21st-century technological developments, astronomy is undergoing a golden age, as astronomers are increasingly able to prove theoretical conjectures. The latest proof of this concept came when the McWilliams Center for Cosmology at Carnegie Mellon's Professor Sergey Koposov discovered a high-velocity star escaping the Milky Way for the first time.

The star, S5-HVS1, was discovered tangentially as a part of the Southern Stellar Stream Spectroscopic Survey (S5). S5-HVS1 was originally part of a binary star system that drifted too close to the black hole at our galactic center: Sagittarius A*.

“We know pretty much for sure that it was ejected from the galactic center, which is the first time ever that we have a confident association between a star and the galactic center,” Koposov said in an interview with The Tartan.

The observation of a high-velocity star emanating from a black hole appears to be a proof of concept of Hills Mechanism, stating that when a binary star system is disrupted by a supermassive black hole, one will orbit around the black hole while the other is flung outward at high-velocity: essentially a gravitational trebuchet.

“It’s a three-body mechanism similar to what people use to launch satellites to distant planets. They have gravitational encounters with Jupiter and the sun that cause it to move much faster, like the Voyager Missions [which used Saturn] ... When three bodies interact, you can produce chaotic outcomes where one object is ejected at high speed,” Koposov said.

But S5-HVS1 is only a small part of a huge puzzle when it comes to the gravimetric interactions that occur near a black hole. Of interest to Koposov is an accretion of stars in a disc around Sagittarius A* that is not parallel to the plane of our galaxy.

“It turns out that around the black hole there are young stars, and we don’t quite know why… It also turns out that some of the stars near the black hole form into a disc rotating in one plane, and we don’t know how that happened. And, surprisingly, [S5-HVS1] is actually flying in a direction that is within that disc of young stars. So, it could be a coincidence, but it could indicate that the process that ejected it is the same that led to the creation of this disc,” Koposov added.

Another mystery that S5-HVS1 may provide a clue toward is dark matter. Scientists have known for some time that the amount of observable matter in the galaxy does not correspond to the gravimetric interactions observed in the outermost regions of the spiral arms of the Milky Way; thus, there must be some extra matter.

“When you have one star and you know it was ejected from one point in the galaxy, if you see that the vector doesn’t quite point to the center, you know the trajectory was deflected by something, and it was likely deflected by the presence of dark matter in the galaxy. With one star, you cannot quite do this measurement, but with more stars, we might be able to model them together and say ‘oh, the Milky Way must have this amount of dark matter and it is distributed in this way,’” Koposov said.

S5-HVS1 is not a particularly old star. It is a blue star, which tends to live much shorter lifespans than yellow or red suns, a bit over twice the size of our sun. “It’s not older than maybe 100 million years old, and it was ejected five million years ago, and the stars in the galactic center are about five million years old which is surprisingly coincidental with the flight time of [S5-HVS1], which is another reason to believe that they are connected,” Koposov said.

Since this is the first observance of a high-velocity star being ejected by a black hole, there is no way to know the frequency of such events. However, Koposov estimates that it may happen to 10s or 100s of stars in each galaxy, so it is quite a rare occurrence considering that galaxies have hundreds of billions of stars.