At Dawn of the Human Species –Milky Way’s Central Black Hole Slingshotted a Star at a Blistering 6 Million KPH


Supermassive Black Hole


“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,” said NASA Hubble Fellow Ting Li with the Carnegie Observatories and Princeton University, about the discovery of the ultrafast star, S5-HVS1 in 2019, traveling at a blistering 6 million km/h. The doomed star was slingshotted by the supermassive black hole at the heart of the Milky Way five million years ago at the dawn of the human species.

The discovery of the orphaned star was made by Carnegie Mellon University Assistant Professor of Physics Sergey Koposov as part of the Southern Stellar Stream Spectroscopic Survey (S5). Located in the constellation of Grus – the Crane – S5-HVS1 was found to be moving ten times faster than most stars in the Milky Way.

 Will Inevitably Leave the Galaxy

“The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return,” said Douglas Boubert from the University of Oxford, a co-author on the study.

Astronomers have wondered about hyper-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 thousand light years away. With this information, astronomers could track its journey back into the center of the Milky Way (image above), where our four million solar mass black hole, known as Sagittarius A*, lurks.

Strange Stars at Milky Way’s Supermassive Black Hole

The Hills Mechanism

“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,” said Koposov, the lead author of this work and member of Carnegie Mellon’s McWilliams Center for Cosmology. “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.”

Thousands of Black Holes Surround Mlky Way’s Supermassive Sagittarius A*

Originally a ‘Doomed’ Binary

Superfast stars can be ejected by black holes via the Hills Mechanism, proposed by astronomer Jack Hills 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 companion star was tidally captured by the black hole. The combination of gravitational accelerations between the two stars and the supermassive black hole created a slingshot effect, jettisoning S5-HVS1outward at 0.6% the speed of light. 

“This is the first clear demonstration of the Hills Mechanism in action,” said Ting Li, leader of the S5 Collaboration. The Southern Stellar Stream Spectroscopic Survey (S⁵) is a spectroscopic survey of stars in the stellar streams of the Southern sky, with the aim of mapping their kinematics and chemistry. The streams represent material tidally striped from dwarf galaxies and globular clusters, and are essential dynamical probes of the dark matter distribution of the Milky Way.

“Dangerous Physics” –Milky-Way Nest of Massive Stars Poised for Most Powerful Explosion Since Big Bang”

The discovery of S5-HVS1 was made with the 3.9-meter Anglo-Australian Telescope (AAT) near Coonabarabran, NSW, Australia, coupled with superb observations from the European Space Agency’s Gaia satellite, that 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,” said 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.”

These results were published in the Monthly Notices of the Royal Astronomical Society, and the S5 collaboration unites astronomers from the United States, United Kingdom, Australia and Chile.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Carnegie Mellon University

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