Astronomers Discover the Fastest Star Ever Recorded Orbiting the Milky Way’s Central Supermassive Black Hole
A team of astronomers has just identified the fastest-moving star ever documented in our galaxy, and its unexpected extreme velocity points to a fascinating explanation: it circles Sagittarius A*, the supermassive black hole sitting at the heart of the Milky Way, which holds a mass equal to 4 million times that of our Sun.
Dubbed S301 by the research team, the star hits a top speed of 25,000 kilometers per second (km/s) and completes one full orbit around Sagittarius A* every 8.7 years. For context, our Sun orbits the galactic center at just around 230 km/s. At its peak velocity, S301 moves more than 100 times faster than our host star, clocking in at 8% of the speed of light.
S301’s incredible speed boils down to just how close it ventures to the supermassive black hole. Per the European Southern Observatory (ESO), at its nearest pass to Sagittarius A, the gap between the star and the black hole is roughly the same as the distance between Saturn and the Sun. As S301 draws near, Sagittarius A’s overwhelming gravitational pull accelerates it to extreme speeds. Unlike stars that fall straight into the black hole, S301 follows a highly elongated elliptical orbit around Sagittarius A*—a shape that explains why its velocity shifts so dramatically over each lap. It zips at top speed when closest to the black hole, and slows considerably as it moves farther away, a pattern identical to how comets like Halley’s Comet behave in our own solar system.
S301 holds two galactic records: it is the fastest-orbiting star ever confirmed in the Milky Way, and it passes closer to Sagittarius A* than any other known star. Researchers say it almost certainly did not form in its current location, and was likely once part of a binary star system.
“S301’s orbital characteristics, combined with the fact that stars cannot form so close to a massive black hole, indicate that the star was originally one half of a binary pair that was ripped apart by Sagittarius A*’s tidal forces. During that disruption, S301 became trapped by the black hole’s gravity, while its former companion was flung out at such high speed it most likely escaped the galaxy entirely,” ESO explains.
Astronomers project that within the next 10 years, they will finally be able to measure the rotation rate of Sagittarius A. Mass and spin (rotation rate) are the two core properties astrophysicists use to describe a black hole. Right now, existing data suggests Sagittarius A is spinning, but researchers have not been able to pin down its exact rotation speed or direction.
Per the theory of general relativity, a rotating black hole drags the surrounding spacetime along with it, an effect that would subtly alter S301’s orbit over time. If astronomers can measure these tiny orbital deviations in the coming years, they will be able to calculate Sagittarius A*’s spin with precision.
Astronomers first spotted S301 in 2023 using GRAVITY, an instrument mounted on ESO’s Very Large Telescope Interferometer (VLTI) in Chile. After identifying the new star, the team tracked its movement and traced its orbit backward using earlier observations dating all the way back to 2017. Combining all this data allowed them to map S301’s path with unprecedented accuracy, confirming the star made its closest pass of Sagittarius A* in early 2023. It will make its next nearest approach to the black hole in 2031.
S301 is not the only known star that orbits close to Sagittarius A. For decades, stars like S2 have allowed astronomers to measure the black hole’s mass and test predictions of general relativity. The key difference is that S301’s extremely close pass to Sagittarius A means researchers can directly measure the black hole’s spin in the next decade—a measurement that would take multiple decades of observation to collect using other nearby stars.
This article is adapted from an original piece first published in WIRED en Español, translated and revised from the original Spanish text.
