hypervelocity star
May 8, 2014

What Can A Hypervelocity Star Tell Us About The Milky Way’s Black Hole?

John P. Millis, Ph.D. for redOrbit.com - Your Universe Online

Most stars move around the galaxy in well-defined orbits, pulled under the influence of the Milky Way’s gravity. It is the predictable motion of the stars around the Milky Way’s supermassive black hole core that gives the galaxy its spiral structure. There are, of course, deviations from the norm, but on average the stars move at a leisurely pace of roughly 500,000 miles per hour.

But this is not always the case. In recent years, astronomers have found some 20 so-called hypervelocity stars. These rebels streak across the galaxy at speeds approaching one million miles per hour or more.

Researchers believe that these objects are the remnants of binary systems that strayed too close to our galactic center; the intense gravitational well that resides there, slingshotting the erstwhile star across the Milky Way. While, at the same time, the companion star is absorbed into the black hole.

A team of scientists, led by Zheng Zheng at the University of Utah, found what they believe is the closest and one of the brightest such stars to date. Using the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), located at the Xinglong Observing Station of the National Astronomical Observatories in China, the team noticed that a new star – some nine times the mass of our Sun – was moving through the galaxy at a speed of about 1.4 million miles per hour relative to our Solar System.

"If you're looking at a herd of cows, and one starts going 60 mph, that's telling you something important," says Ben Bromley, a University of Utah physics and astronomy professor who was not involved with Zheng's study. "You may not know at first what that is. But for hypervelocity stars, one of their mysteries is where they come from – and the massive black hole in our galaxy is implicated."

Zheng believes that this new star is part of a grouping of other hypervelocity stars located above the disk of our Milky Way galaxy, having originated near the galactic center. But what excites astronomers is the possibility that these stars can illuminate the mystery of dark matter.

Measurements of our galaxy’s rotation suggest that it exists in a halo dominated by dark matter – a form of matter that doesn’t interact electromagnetically, but still maintains a gravitational interaction with other forms of matter.

Because it is difficult to measure directly, the exact nature and distribution of the dark matter halo is not well defined. However, by studying the speed and trajectory of the hypervelocity stars traveling through it, astronomers may be able to better characterize the dark matter.

"The hypervelocity star tells us a lot about our galaxy – especially its center and the dark matter halo," says Zheng Zheng, lead author of the study published recently in Astrophysical Journal Letters by a team of U.S. and Chinese astronomers. "We can't see the dark matter halo, but its gravity acts on the star. We gain insight from the star's trajectory and velocity, which are affected by gravity from different parts of our galaxy."