Fastest observed Milky Way star offers new probe of central black hole

Astronomers have identified S301, a faint star that becomes the fastest ever observed in the Milky Way as it sweeps around Sagittarius A*, the supermassive black hole at the galaxy’s centre. Near its closest approach, it reaches about 25,000 kilometres per second, roughly 8 per cent of the speed of light. It is also the closest known star to the black hole, passing at approximately the distance between Saturn and the Sun.

Researchers found S301 using four European Southern Observatory telescopes at the Very Large Telescope site in Chile. Infrared observations allowed them to see through dust and gas obscuring the galactic centre. After noticing a faint, fast-moving point of light, the team tracked its position and examined earlier data to reconstruct its highly elongated, 8.7-year orbit. The findings were published in Nature.

The star’s proximity matters more scientifically than its speed. Its motion records how the black hole, which has about four million times the Sun’s mass, distorts space and time. Continued observations, including its next close approach in 2031, could let astronomers measure Sagittarius A*’s rotation within the next decade. Such a measurement would test how a spinning black hole drags nearby space-time and could offer clues to how the black hole and the Milky Way developed.