The Milky Way's Fastest Star Could Expose Our Black Hole's Spin

The Milky Way's fastest star, named S301, could provide unprecedented insight into the rotation of the galaxy's supermassive black hole, Sagittarius A* (Sgr A*). This unique star orbits Sgr A* so closely that its trajectory should be influenced by the black hole's spin. Discovered using the European Southern Observatory's Very Large Telescope Interferometer in Chile, S301 is the fastest star ever observed in the Milky Way, reaching speeds of up to 25,000 kilometers per second at its nearest point to Sgr A*.

This proximity, which occurs every 8.7 years, is the shortest orbital period ever measured around the galactic center, with the star coming within about the same distance as Saturn is from the sun to the black hole. S301's close approach could allow scientists to measure the spin of Sgr A* for the first time, which has been an elusive goal due to the difficulty in obtaining a direct measurement.

While black holes are generally believed to possess spin, characterized by three fundamental properties - mass, charge, and spin - directly measuring this spin has been challenging due to the lack of suitable probes. The study, published in Nature, suggests that S301 was once part of a binary star system that was disrupted by its proximity to the monstrous central black hole, with one star escaping the galaxy while the other was captured by Sgr A*'s gravitational pull.

While the researchers are optimistic about the potential to determine Sgr A*'s spin using S301, they acknowledge that more data and a combination of multiple datasets will be necessary to definitively confirm the spin.