ESO · Ruimte

Fastest Star in Milky Way Orbits Supermassive Black Hole Closely, Revealing Spin Effects

Astronomers have identified S301, the fastest known star in the Milky Way, orbiting Sagittarius A*, the supermassive black hole at the galaxy's center. Detected using the European Southern Observatory's Very Large Telescope Interferometer, S301 travels at 25,000 km/s and approaches the black hole closer than any previously observed star, allowing for direct measurement of the black hole's spin.

S301 orbits Sagittarius A* on a tight 8.7-year cycle, reaching a distance of approximately 12 times Earth's distance from the Sun at its closest approach. This proximity means the star experiences the effects of the black hole's rotation, a phenomenon predicted by Einstein's general theory of relativity where a spinning black hole drags and twists spacetime.

The star's speed during its closest passage is over 8% of the speed of light, making it the fastest star recorded in the Milky Way. Its close orbit, about the distance of Saturn to the Sun, is unprecedented and offers a unique opportunity to measure the black hole's spin within the next decade, providing a crucial test of Einstein's theory.

Detecting S301, which is extremely faint, was achieved using the VLTI's GRAVITY instrument, which combines light from four 8-meter telescopes to achieve high spatial resolution. The team believes S301 was likely part of a binary pair torn apart by Sagittarius A*'s tidal forces, with S301 being trapped by the black hole's gravity while its companion was ejected.

Future observations with GRAVITY+ and ESO's Extremely Large Telescope are planned to precisely track S301's trajectory over the next decade. Observing at least two full orbits will allow astronomers to directly determine Sagittarius A*'s spin for the first time.

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