Astronomers have detected the fastest known star in the Milky Way, hurtling around the supermassive black hole at the heart of the galaxy. The star, called S301, travels at about 25,000 kilometers (15,500 miles) per second, 100,000 times faster than a commercial airliner. It is also the closest known star to the Milky Way’s central black hole, Sagittarius A*, approaching it at approximately the distance of Saturn from the Sun. “What is special about this star is that it is orbiting Sagittarius A* in a very narrow orbit, taking only 8.7 years to complete, and it is approaching the black hole at only 12 times the distance of the Earth from the Sun. This is unprecedented,” said Felix Mang, a doctoral student at the Max Institute. Planck of Extraterrestrial Physics (MPE) in Garching, Germany, and author of the study published on the findings. The star’s extreme proximity to Sagittarius A* means it could open a new window into the fundamental properties of the black hole and the extreme environment around it. Like many objects in the night sky, black holes are predicted to spin. According to Einstein’s general theory of relativity, a rotating black hole drags space-time with it, warping the orbits of surrounding stars. This effect is only significant for objects orbiting rapidly rotating black holes at close range. S301 appears to have the perfect combination of a very eccentric (unbalanced) and very narrow orbit. “For the first time, we could very directly measure the spin of a massive black hole, which would be a key test of Einstein’s theory,” said Dr. Stefan Gillessen, senior scientist at MPE and co-author of the study, which is published in the journal Nature. ESO’s VLTI shows several stars orbiting Sagittarius A*. S301 passes closer to Sagittarius A* than the others; The solid curve shows the part of its orbit already completed, while the dashed curve shows the path it will follow. Photograph: ESO/Gravity/Reuters It had previously been assumed that it would be necessary to measure the motion of multiple stars over several decades to estimate the spin of Sagittarius A*. Scientists were able to track S301’s orbit back to 2017, and by tracking it to its next closest pass in 2031, they hope to have enough data. “With this star we hope to measure, within the next 10 years, the spin of the black hole,” Mang said. The observations were made using the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) at the Paranal Observatory in Chile, which combines light from four 8-metre telescopes. Although the star is unusually close to the central black hole, it is not expected to cross its event horizon, the point beyond which objects can never escape. “Black holes are perceived as these cosmic vacuum cleaners that gobble up everything they can handle,” Mang said. “But it’s just a massive object. It’s really not much different from something that orbits the Sun and will stay in this orbit.” Dr Ziri Younsi, an astrophysicist at University College London, who was not involved in the observations, said: “This is incredibly important as a discovery. It’s very exciting because this thing is getting incredibly close to Sagittarius A* and traveling, at its peak, at about 8% of the speed of light, which is crazy.” Younsi said using the orbit to provide a measurement of the black hole’s spin would be a turning point in understanding the evolution of the Milky Way.
