This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers have discovered the fastest known star in our galaxy, the Milky Way, orbiting the black hole at its center. The star, named S301, was detected with the European Southern Observatory's Very Large Telescope Interferometer (ESO's VLTI) and reaches speeds of 25,000 km/s (15,500 miles per second) as it travels around the four-million-solar-mass black hole.
It comes closer to it than any other star observed before, so close that it feels the effects of the black hole's rotation. "Decades of carefully tracking stars orbiting our galaxy's central black hole, Sagittarius A*, have led to this breakthrough discovery of a very promising star. Because it orbits so close to Sagittarius A*, S301 opens a new window to the fundamental properties of spacetime in this extreme black-hole environment," says Nobel Prize winner Reinhard Genzel, director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and a founding member of the collaboration that made the new observations.
"What is special about this star is that it's orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the sun. That is unprecedented," says Felix Mang, a Ph.D. student at MPE and author of the study published in Nature. During its closest passage to the black hole, the star travels at around 25,000 kilometers per second—100,000 times faster than a commercial plane, or more than 8% of the speed of light—making it the record holder for the fastest star in the Milky Way.
S301 also comes closer to Sagittarius A* than any other star observed so far, approaching the black hole at around the distance of Saturn to the sun. Because S301 comes so close to Sagittarius A*, it is the first known star that could be used to directly measure the rotation of a black hole. Like most things in our universe, astronomers predict that Sagittarius A* spins.
According to Einstein's general theory of relativity, a spinning black hole drags spacetime along with it and twists it, which affects the orbits of surrounding stars. The effect is felt more strongly by objects orbiting fast-rotating black holes at close range. "With this star we hope to measure, within the next 10 years, the spin of the black hole," says Mang.
MPE researcher Stefan Gillessen, who also had a leading role in the new study, adds, "For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein's theory." Juan Osorno, an astronomer at LIRA Observatoire de Paris–PSL, France, who also had a key role in the study, adds, "Without this star, we would need to measure the motion of other stars for several more decades to get anywhere close to measuring the spin of the black hole." Finding S301, which appears 2 billion times fainter than Betelgeuse (the orange star in the constellation Orion) in the sky, was no easy feat. The team used the VLTI, a facility at ESO's Paranal Observatory in Chile, and its GRAVITY instrument, now known as GRAVITY+ following an infrastructure upgrade. The VLTI's superpower lies in its ability to combine the light from four 8-meter (26-foot) telescopes to create a 'virtual' telescope with 15 times the spatial resolution of a single 8-meter (26-foot) telescope.
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