A group of astronomers has discovered the fastest known star in the Milky Way, traveling at 25,000 kilometers per second as it orbits the supermassive black hole at the center of our galaxy.
The star, named S301 and detected at the Paranal Observatory in northern Chile, could allow scientists to determine the black hole’s rotation within a decade and test Albert Einstein’s theory of space-time, according to the researchers behind the discovery, which was published in the journal Nature.
“Decades of carefully tracking stars orbiting our galaxy’s central black hole, Sagittarius A*, have led to the groundbreaking discovery of this promising star,” Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and a founding member of the collaboration that conducted the observations, said in a statement from the European Southern Observatory.
“Because it orbits so close to Sagittarius A*, S301 opens a new window for studying the fundamental properties of space-time in this extreme black hole environment,” explained Genzel, who shared the 2020 Nobel Prize in Physics for the discovery of the black hole at the center of the Milky Way.
To discover S301, which appears two billion times fainter in the sky than Betelgeuse, the orange star in the constellation Orion, the team used the Very Large Telescope Interferometer at ESO’s Paranal Observatory in Chile, along with its GRAVITY+ instrument.
The interferometer combines light from four eight-meter telescopes to create a “virtual telescope” with spatial resolution 15 times greater than that of a single eight-meter telescope. Scientists first observed S301 in the spring of 2023 and continued tracking it to determine its orbit.
Twelve Times the Distance Between Earth and the Sun
“What makes this star special is that it orbits Sagittarius A* in a very tight orbit, taking only 8.7 years to complete it, and comes within just 12 times the distance between Earth and the Sun of the black hole. This is unprecedented,” said Felix Mang, a doctoral student at the Max Planck Institute and co-author of the study.
At its closest point to the black hole, the star travels at approximately 25,000 kilometers per second. That is 100,000 times faster than a passenger plane and about 8% of the speed of light, making it the fastest known star in the Milky Way. S301’s orbital characteristics, together with the fact that stars cannot form so close to a supermassive black hole, suggest that it once belonged to a binary star system that was torn apart by the forces of Sagittarius A*.
During that process, S301 became trapped by the black hole’s gravity, while the other star was ejected at high speed and most likely left the galaxy. Astronomers will continue observing S301, which will make its next closest approach to the black hole in 2031.
Observing at least two of its complete orbits could allow scientists to determine its path accurately enough to measure the rotation of Sagittarius A*. Like most objects in the universe, astrophysicists believe Sagittarius A* rotates on its axis.
According to Einstein’s theory of relativity, a rotating black hole drags space-time with it and causes it to warp, changing the orbits of nearby stars. Over the next 10 years, the team hopes to obtain what would be the first direct measurement of a black hole’s rotation.
“Without this star, we would have to measure the movement of other stars for several more decades before coming close to measuring the black hole’s spin,” said Juan Osorno, an astronomer at the LIRA laboratory of the Paris Observatory–PSL in France and a co-author of the study.