The Milky Way’s Black Hole Ejected a Star Towards Intergalactic Space at 6 Million km/h

During a time that humankind’s ancestors were learning to walk upright, use their hands, and create the first primitive tools, cosmic events left a mark in our galaxy that our developed society would later see, millions of years later.

Five million years after this dramatic ejection, a team of researchers led by Sergey Koposov of Carnegie Mellon University’s McWilliams Center for Cosmology discovered the star, known as S5-HVS1, in the Crane-shaped constellation Grus. The star was spotted traveling relatively close to Earth (29,000 light-years away) at unprecedented, blistering speeds — roughly ten times faster than most stars in our galaxy.

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

“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,” Koposov said in the statement. 

Observations from the Anglo-Australian Telescope (AAT), a 12.8-foot (3.9-meter) telescope, and the European Space Agency’s Gaia satellite led to the discovery of the star. The discovery was made as part of the Southern Stellar Stream Spectroscopic Survey (S5) by astronomers from Chile, the United States, the United Kingdom, and Australia.

Now that the star has been discovered, researchers may be able to trace it back to Sagittarius A*, the black hole at the heart of the Milky Way. It also serves as an incredible example of the Hills Mechanism, proposed by astronomer Jack Hills 30 years ago, in which stars are ejected at high speeds from the centers of galaxies following an interaction between a binary-star system and the black hole at the center of the galaxy.

The Milky Way’s Black Hole Ejected a Star Towards Intergalactic Space at 6 Million km/h
The location and direction of the star S5-HVS1 in the night sky. The star is rocketing away from the center of our galaxy. (Image credit: Sergey Koposov)

“This is the first clear demonstration of the Hills Mechanism in action,” Ting Li, a fellow  at the Carnegie Observatories and Princeton University who led the S5 collaboration, said in the statement. “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.”

“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!” Kyler Kuehn, deputy director of technology at the Lowell Observatory who is part of the S5 executive committee, added in the statement.

This discovery was published in a study in the journal the Monthly Notices of the Royal Astronomical Society.

READ MORE: Black hole captured eating a Star and spit some of it back out for the first time ever

The Milky Way’s Black Hole Ejected a Star Towards Intergalactic Space at 6 Million km/h

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