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Surprising new class of “hypervelocity stars” discovered escaping the galaxy

by | Jan. 9, 2014, 11:44 AM | Want more research news? Subscribe to our weekly newsletter »

Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. The general directions from which the stars have come are shown by the colored bands. (Graphic design by Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory.)

An international team of astronomers has discovered a surprising new class of “hypervelocity stars” – solitary stars moving fast enough to escape the gravitational grasp of the Milky Way galaxy.

The discovery of this new set of “hypervelocity” stars was described at the annual meeting of the American Astronomical Society this week in Washington, D.C., and is published in the Jan. 1 issue of the Astrophysical Journal.

“These new hypervelocity stars are very different from the ones that have been discovered previously,” said Vanderbilt University graduate student Lauren Palladino, lead author on the study. “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.”

The discovery came as Palladino, working under the supervision of Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt, was mapping the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey, a massive census of the stars and galaxies in a region covering nearly one quarter of the sky.

“It’s very hard to kick a star out of the galaxy,” said Holley-Bockelmann. “The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy. None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism.”

Kelly and Lauren

Lauren Palladino, right, and Kelly Holley-Bockelmann. (John Russell / Vanderbilt)

Astrophysicists calculate that a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity. They also estimate that the Milky Way’s central black hole has a mass equivalent to four million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities. The typical scenario involves a binary pair of stars that get caught in the black hole’s grip. As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity. So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.

Now Palladino and her colleagues have discovered an additional 20 sun-sized stars that they characterize as possible hypervelocity stars. “One caveat concerns the known errors in measuring stellar motions,” she said. “To get the speed of a star, you have to measure the position really accurately over decades. If the position is measured badly a few times over that long time interval, it can seem to move a lot faster than it really does. We did several statistical tests to increase the accuracy of our estimates. So we think that, although some of our candidates may be flukes, the majority are real.”

The astronomers are following up with additional observations.

The new rogues appear to have the same composition as normal disk stars, so the astronomers do not think that their birthplace was in the galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the galaxy.

The big question is: what boosted these stars up to such extreme velocities? We are working on that now,” said Holley-Bockelmann.

Katharine Schlesinger from the Australian National University, Carlos Allende Prieto from the Universidad de La Laguna in Spain, Timothy Beers from the National Optical Astronomy Observatory in Tucson, Young Sun Lee from New Mexico State University and Donald Schneider from Pennsylvania State University also contributed to the discovery.

The research was supported by funds from the Graduate Assistance in Areas of National Need program, National Science Foundation grants AST 0847696, AST 0607482, Physics Frontier Center grants PHY 0216783, the Aspen Center for Physics, the Alfred P. Sloan Foundation and the U.S. Department of Energy Office of Science.

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  • PhytoPhotonics

    My guess would be that they are not Milky Way stars at all, but rather extra-galactic. From the images shown, it appears that they are simply passing through. They could very well have been ejected from another galaxy by being sucked towards a core black hole, but missing the event horizon by just enough to be sling-shot out. A check to determine whether they are accompanied by planets would be helpful, if possible. The lack thereof would lend some credence to my postulation; considering that such a close encounter with a black hole might strip away any preexisting companions.

    It is rather frightening, though, to realize that asteroids are not the only wayward celestial bodies that might threaten our existence.

    • James Ellis

      You did read this part of the article right? “have the same composition as normal disk stars, so the astronomers do not think that their birthplace was in the galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the galaxy.” which would seem to argue against your theory.

  • Wesley Parish

    The first thing that comes to mind is an open cluster. The question then becomes, how can a star stay inside an open cluster for long enough to build up such a velocity? (Unless it wandered in and was thrown every which way before gaining enough velocity to escape the cluster then the galaxy.)

    Just my 0.02c worth – don’t spend it all at once!

  • Jared Bryan

    Great discovery! Good job

  • Franklin_Delano_Roosevelt

    Of course they do not come from the galactic core any that did would be long gone. They are passing through our galaxy.

  • kuliphex

    I’d agree with PhytoPhotonics. Anyone who has seen dynamic computer models of galactic evolution has seen that during the first encounter, the outer stars of one smaller galaxy or cluster often get thrown out into whipped tails by the core of a larger galaxy. Some of these faster stars are still not fast enough to escape the system, and a portion of the created tail gets nailed again by the core of the larger galaxy upon the second collision, jacking up their speeds once more. This “damped bouncing” process continues until the two systems are merged. As galaxies grow by such cannibalism, in order to become dense and compact, they HAVE to eject other stars in the process to conserve total energy. One possible explanation for the observation described in the article is that the Milky way has only recently cannibalized a small galaxy or very large cluster which contained 2nd or 3rd generation sun-like main sequence stars, and the hyperfast components have not yet finished escaping the Milky way.

  • Bruce Salem

    Doppler shifted spectral lines don’t get radial velocity of these candidates?

  • Thommy Berlin

    An interesting article, and nice work. One wonders if these stars had Sagittarian origins…

  • Richard J Osborne

    I think that the images are our own solar system from the the past. I think we have been circling the galaxy for a while now.