Superfast star launched from black hole at Milky Way’s center

"The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return."

Jocelyn Duffy-Carnegie Mellon • futurity
Nov. 18, 2019 4 minSource

A shooting star shape is covered in holiday lights, which aren't on, against a blue sky

Astronomers have spotted a high velocity star, traveling at a blistering six million kilometers per hour (3,728,227 miles per hour), that the supermassive black hole at the heart at the Milky Way ejected five million years ago.

The researchers saw the star, known as S5-HVS1 and located in the constellation of Grus—the Crane, was moving 10 times faster than most stars in the Milky Way.

“The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return,” says coauthor Douglas Boubert from the University of Oxford.

Astronomers have wondered about high velocity stars since their discovery only two decades ago. S5-HVS1 is unprecedented due to its high speed and close passage to the Earth, “only” 29,000 light years away. With this information, astronomers could track its journey back into the center of the Milky Way, where a four million solar mass black hole, known as Sagittarius A*, lurks.

“This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never had an unambiguous association of such a fast star with the galactic center,” says lead author Sergey Koposov, an assistant professor of physics and member of the McWilliams Center for Cosmology at Carnegie Mellon University.

“We think the black hole ejected the star with a speed of thousands of kilometers per second about five million years ago. This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.”

Black holes can eject superfast stars via the Hills Mechanism, which astronomer Jack Hills proposed thirty years ago. Originally, S5-HSV1 lived with a companion in a binary system, but they strayed too close to Sagittarius A*. In the gravitational tussle, the black hole captured the companion star, while it threw out S5-HVS1 at extremely high speed.

“This is the first clear demonstration of the Hills Mechanism in action,” says Ting Li from Carnegie Observatories and Princeton University, and leader of the S5 Collaboration. “Seeing this star is really amazing as we know it must have formed in the galactic center, a place very different to our local environment. It is a visitor from a strange land.”

The galaxy appears cloudy, with a red line showing the star's trajectory from the center of the galaxy
The location of the star on the sky and the direction of its motion. The star is flying away from the Galactic center, from which it was ejected 5 million years ago. (Credit: Sergey Koposov)

Researchers discovered S5-HVS1 with the 3.9-meter Anglo-Australian Telescope (AAT) near Coonabarabran, New South Wales, Australia, coupled with superb observations from the European Space Agency’s Gaia satellite, which allowed the astronomers to reveal the full speed of the star and its journey from the center of the Milky Way.

“The observations would not be possible without the unique capabilities of the 2dF instrument on the AAT,” says Daniel Zucker, an astronomer at Macquarie University in Sydney, Australia, and a member of the S5 executive committee. “It’s been conducting cutting-edge research for over two decades and still is the best facility in the world for our project.”

“I am so excited this fast-moving star was discovered by S5,” says Kyler Kuehn of Lowell Observatory and a member of the S5 executive committee. “While the main science goal of S5 is to probe the stellar streams—disrupting dwarf galaxies and globular clusters—we dedicated spare resources of the instrument to searching for interesting targets in the Milky Way, and voila, we found something amazing for ‘free.’ With our future observations, hopefully we will find even more!”

The results appear in the Monthly Notices of the Royal Astronomical Society.

Source: Carnegie Mellon University

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