A Newly Identified Star Races Around the Milky Way’s Central Black Hole

A star racing around the center of the Milky Way may give astronomers their first direct way to measure the rotation of Sagittarius A*, the galaxy’s supermassive black hole. The star, named S301, was identified near Sagittarius A*, which lies about 27,000 light-years from Earth and is estimated to have roughly 4.3 million times the Sun’s mass. Astronomers have long suspected that the black hole spins, but that rotation has not been confirmed through a direct measurement. Researchers detected S301 in 2023 using the European Southern Observatory’s Very Large Telescope Interferometer in Chile. They combined those observations with follow-up measurements and archival data collected as far back as 2017. The resulting analysis showed that S301 follows an elongated orbit, with Sagittarius A* positioned near one end. The star completes its circuit in 8.7 years. At its closest approach, it passes within about 12 times the distance between Earth and the Sun, then swings away again along its oval path. S301 reaches a speed of approximately 56 million miles per hour—about 8 percent of the speed of light. According to the Smithsonian Magazine report on the research, that makes it the fastest star yet recorded in the Milky Way. Its orbit is especially valuable because S301 is the only known star close enough to Sagittarius A* for the black hole’s theorized spin to significantly affect its path. Under Einstein’s general theory of relativity, a rotating black hole should twist the surrounding space-time. That effect could push the star onto a slightly different orbit after each close pass. By tracking those changes over the next decade, researchers may be able to determine how quickly Sagittarius A* rotates. The measurement could also provide a test of general relativity and offer information about the Milky Way’s history. The researchers believe S301 may once have belonged to a two-star system. Sagittarius A*’s gravity could have torn the pair apart, retaining S301 while ejecting its former companion beyond the galaxy. That origin remains a research interpretation rather than a confirmed observation.