LAMOST: Nearest bright 'hypervelocity star' found: Speeding at 1 million mph

This is an astrophysicist-artist's conception of a hypervelocity star speeding away from the visible part of a spiral galaxy like our Milky Way and into the invisible halo of mysterious "dark matter" that surrounds the galaxy's visible portions. 

University of Utah researcher Zheng Zheng and colleagues in the US and China discovered the closest bright hypervelocity star yet found. 

Credit: Ben Bromley, University of Utah.

A University of Utah-led team discovered a "hypervelocity star" that is the closest, second-brightest and among the largest of 20 found so far.

Speeding at more than 1 million mph, the star may provide clues about the supermassive black hole at the center of our Milky Way and the halo of mysterious "dark matter" surrounding the galaxy, astronomers say.

Zheng Zheng

"The hypervelocity star tells us a lot about our galaxy, especially its center and the dark matter halo," says Zheng Zheng, an assistant professor of physics and astronomy and 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," Zheng says.

"We gain insight from the star's trajectory and velocity, which are affected by gravity from different parts of our galaxy."

In the past decade, astronomers have found about 20 of these odd stars.

Hypervelocity star appear to be remaining pairs of binary stars that once orbited each other and got too close to the supermassive black hole at the galaxy's center.

Intense gravity from the black hole – which has the mass of 4 million stars like our sun – captures one star so it orbits the hole closely, and slingshots the other on a trajectory headed beyond the galaxy.

Zheng and his colleagues discovered the new hypervelocity star while conducting other research into stars with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, (LAMOST), also called the Guo Shou Jing Telescope is located at the Xinglong Observing Station of the National Astronomical Observatories of China, about 110 miles northeast of Beijing.

LAMOST boasts a 13.1-foot-wide aperture and houses 4,000 optical fibers, which capture "spectra" or light-wavelength readings from as many as 4,000 stars at once. A star's spectrum reveals information about its velocity, temperature, luminosity and size.

LAMOST's main purpose is to study the distribution of stars in the Milky Way, and thus the galaxy's structure.

The new hypervelocity star, named LAMOST-HVS1, stood out because its speed is almost three times the usual star's 500,000-mph pace through space: 1.4 million mph relative to our solar system.

Its speed is about 1.1 million mph relative to the speed of the center of the Milky Way.

Despite being the closest hypervelocity star, it nonetheless is 249 quadrillion miles from Earth. (In U.S. usage, a quadrillion is 1,000,000,000,000,000 miles or 10 to the 15th power, or 1 million billion).

"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."

Background perspective

Our solar system is roughly 26,000 light years or 153 quadrillion miles from the centre of the galaxy, more than halfway out from the centre of the visible disk.

By comparison, the new hypervelocity star is about 62,000 light years or 364 quadrillion miles from the galactic centre, beyond as well as above the galaxy's visible disk.

It is about 42,400 light years from Earth, or about 249 quadrillion miles away.

As far as that is, the star has a magnitude of about 13, or 630 times fainter than stars that barely can be seen with the naked eye, it nevertheless "is the nearest, second-brightest, and one of the three most massive hypervelocity stars discovered so far," Zheng says.

It is nine times more massive than our sun, which makes it very similar to another hypervelocity star known as HE 0437-5439, discovered in 2005, and both are smaller than HD 271791, which was discovered in 2008 and is 11 times more massive than the sun.

As seen from Earth, only HD 271791 is brighter than LAMOST-HVS1, Zheng says.

The newly discovered hypervelocity star also outshines our own sun: It is four times hotter and about 3,400 times brighter (if viewed from the same distance), but compared with our 4.6-billion-year-old sun, the newly discovered LAMOST–HVS1 is a youngster born only 32 million years ago, based on its speed and position, Zheng says.

New class of 'hypervelocity stars' discovered escaping the Milky Way

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. Image courtesy Julie Turner, Vanderbilt University. 

The top view of the galaxy comes from the National Aeronautics and Space Administration and the side view comes from 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.

Lauren Palladino

"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."

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.