Fastest Star in the Milky Way Could Reveal Black Hole Spin
Astronomers have found the fastest known star in our galaxy. Named S301, it circles Sagittarius A*, the giant black hole at the centre of the Milky Way, which has about four million times the mass of the Sun. The European Southern Observatory (ESO) announced the discovery on 19 August, and the study was published in Nature.
At its closest point, S301 travels at around 25,000 kilometres per second. That is more than 8% of the speed of light. It takes just 8.7 years to complete one orbit and passes about 1.78 billion kilometres from the black hole, roughly 12 times the distance between Earth and the Sun.
Finding it was hard work. In our sky, the star looks about two billion times fainter than Betelgeuse. The team used the GRAVITY instrument, now upgraded to GRAVITY+, on ESO's Very Large Telescope Interferometer at Paranal in Chile. It combines light from four 8-metre telescopes to see 15 times more detail than a single one. The researchers first spotted S301 in spring 2023 and traced its path back to 2017.
Why does it matter? S301 is the first known star that could let scientists directly measure how fast a black hole spins. Einstein's general theory of relativity predicts that a spinning black hole drags space and time around with it, which changes the orbits of nearby stars.
Lead author Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics in Germany, said the team hopes to measure the spin within the next 10 years. The star's next close pass comes in 2031.
The researchers think S301 was once part of a pair of stars that the black hole's pull split up. S301 was captured, while its partner was sent away fast enough to probably leave the galaxy.
