Astronomers – including scientists from UNSW Sydney – have spotted an ultrafast star, traveling at a blistering 6 million km/h, after being ejected by the supermassive black hole at the heart at the Milky Way five million years ago. This star is moving so fast, it will leave the galaxy and head into intergalactic space. 

The discovery of the star, known as S5-HVS1, was made by Sergey Koposov from Carnegie Mellon University 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 10 times faster than most stars in the Milky Way. 

Associate Professor Sarah Martell

Associate Professor Sarah Martell, Scientia Fellow at UNSW Science’s School of Physics, never expected to find a fast star when she started the study.

“When we started this project I never thought we would accidentally find a star escaping the Milky Way,” said co-author Associate Professor Sarah Martell, Scientia Fellow at UNSW Science’s School of Physics.

“It highlights the importance of the investment by UNSW and other Australian universities in the continuing operation of the Anglo-Australian Telescope and Siding Spring Observatory.”

Dr Jeffrey Simpson, Post-Doctoral Research Fellow at UNSW Science’s School of Physics, was also a co-author on the paper. 

“I was really surprised when we found a star going so fast. It only sank in when we got follow-up observations that confirmed the speed,” he said. 

“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 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 right into the centre of the Milky Way, where a 4 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 Centre,” said Sergey Koposov, the lead author of this work. “We think the black hole ejected the star with a speed of thousands of kilometres per second about 5 million years ago. This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.” 

Velocity of the fast star

The velocity of the star is so high that it will inevitably leave the Galaxy, said study co-author Douglas Boubert from the University of Oxford

Superfast stars can be ejected by black holes via the Hills Mechanism, proposed by astronomer Jack Hills 30 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 captured by the black hole, while S5-HVS1 was thrown out at extremely high speed. 

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

The discovery of S5-HVS1 was made with the 3.9m Anglo-Australian Telescope (AAT) near Coonabarabran, NSW, Australia, coupled with superb observations from the European Space Agency’s (ESA) Gaia satellite, that allowed the astronomers to reveal the full speed of the star and its journey from the centre 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 Monthly Notices of the Royal Astronomical Society, and the S5 collaboration unites astronomers from the United States, United Kingdom, Australia and Chile. 

“I am so excited this fast-moving star was discovered by S5,” says Kyler Kuehn, at 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!” 

Find out more on the S5 website: