Getting the Stellar Boot
Hyper-Velocity Star Wanders in the Wrong Neighborhood
An artist's conception of a star getting the boot from the Milky Way Galaxy. Courtesy: STScI
What happens when a little group of stars traveling together through the galaxy find themselves in the wrong neighborhood? If it’s the hyper-velocity (meaning super-fast, moving at 2.5 million kilometers per hour, or three times the Sun’s velocity through space) hot blue, supermassive star HE 0437-5439, then the scenario is one worthy of a TV detective show.
It’s one that astronomers have pieced together, like CSI technicians at a cosmic crime scene. The only evidence they have is the star itself. It’s hot, massive, and blue. It’s at least 100 million years old, since that’s how long it would take for such a fast-moving object to travel out along the trajectory it has been following from the center of the Milky Way. By all rights, a 100-million-year-old star this massive should have burned out by now. Instead, there it is, speeding away from something that happened to it.
The most likely explanation for the star’s blue color and extreme speed is that it was part of a triple-star system traveling happily along through the galaxy. At some point it wandered in the wrong place. That started a gravitational billiard-ball game with the galaxy’s monster black hole. And, where three stars were first involved, one got sucked into the black hole and the other two merged to create the hot, massive star that then got the gravitational kick out of the galaxy.
This concept for imparting an escape velocity on stars was first proposed in 1988. The theory suggests that the Milky Way’s black hole should eject a star about once every 100,000 years. And so, here is HE 0437-5439, a supermassive blue star that should be dead by now, but instead it’s blasting away from the Milky Way, having once been the leftover pair of stars given the boot by a gravitational assist from the unlucky third member of the triplet as it was devoured by the black hole.
So, as they say on one of the CSI episodes on TV: what’s the evidence? How can we know what happened hundred million years ago and to such a massive star? Based on the speed and position of HE 0437-5439, the star would have to be at least 100 million years old to have journeyed from the Milky Way’s core. Yet its mass — nine times that of our Sun — and blue color mean that it should have burned out after only 20 million years — far shorter than the transit time it took to get to its current location.
To solve the puzzle of a massive star traveling so fast away from the core of the galaxy required a number of observations to chart the star’s path. It was first discovered by the Hamburg/European Southern Observatory survey in 2005. HE 0437-5439 was then observed by the Hubble Space Telescope, which charted the star’s position in 2009. Astronomers then compared that position with positions calculated in the images taken in 2006. They could figure out its trajectory, which took it very near the core of our galaxy. The rest of the story is based on deducing the mass of the star and then using stellar evolution to come up with a plausible scenario of how such a massive star could end up where it is, traveling so fast.
There are more of these high-speed stragglers to study, and astronomers may likely find that some of them also wandered where they shouldn’t have, suffering a similar fate to the progenitors of HE 0437-5439. Stay tuned!
