Impossibly fast-moving stars are revealing the Milky Way’s mysterious heart — SkimNews
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- S301 is the fastest-known star orbiting Sagittarius A*, travelling at more than 8% the speed of light, yet it shouldn't exist because black holes normally shred the gas clouds needed for star formation
- S0-2, tracked since 2002 by Reinhard Genzel's team, reaches 7650 km/s (~3% of light speed) at closest approach and passes within just 17 light hours of Sagittarius A*, giving astronomers the data to weigh the black hole
- Reinhard Genzel and Andrea Ghez shared the 2020 Nobel Prize after decades of work using S0-2's orbit and 2018 interferometry observations to confirm Sagittarius A* as a supermassive black hole
- The S stars are surprisingly young — they belong to a stellar family with only a ~50 million-year lifespan — yet they orbit deep inside the galactic centre where there shouldn't have been enough gas and dust for them to form
- Xiaochen Zheng at the Beijing Planetarium and colleagues proposed a unified model in which a hidden massive companion (100 to 1000 times the Sun's mass, possibly a smaller black hole) explains all three stellar populations and the puzzling "zone of avoidance"
- Sagittarius A*'s spin remains unmeasured because there is no matter or merger signal to read, and tracking full orbits of close-in stars like S0-2 (a 16-year circuit) is the only practical route to test general relativity in such extreme gravity
- The galactic centre sits 26,500 light years from Earth, and progress was only possible after adaptive optics in the 1990s and interferometry in 2018 cut through atmospheric blurring, dust shrouds and the region's blinding star density
Why it matters: Sagittarius A*'s spin — one of only two defining properties of any black hole — is still unmeasured, and these close-orbit stars are currently the only practical probe. Completing S0-2's 16-year orbit with interferometry could finally pin down that number and test general relativity under the most extreme gravity in the galaxy.
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