A team of astronomers has just detected the fastest star in the galaxy. Its anomalous speed has a more unsettling explanation: It orbits Sagittarius A*, the supermassive black hole at the center of the Milky Way, the mass of which is equivalent to four million times that of the sun.
S301, as astronomers have named the star, reaches 25,000 kilometers per second (km/s) at its maximum speed and takes 8.7 years to complete one orbit around Sagittarius A*. (By comparison, the sun orbits the center of the galaxy at about 230 km/s.) At its peak speed, S301 moves more than 100 times faster than our star, and reaches 8 percent of the speed of light.
The key to S301’s speed lies in how close it passes to the supermassive black hole. According to the European Southern Observatory (ESO), at its closest approach, the distance between the two is comparable to that between Saturn and the sun.
The enormous gravitational pull of Sagittarius A* accelerates the star as it approaches the black hole. S301, however, does not fall directly into it, but instead follows an extremely elongated orbit around it.
The shape of its orbit also explains why its speed varies so much. At its closest approach, it moves faster, while when it is farther away, it moves more slowly. Something similar happens in the solar system with some comets, such as Halley’s Comet.
S301 is the star with the highest-known orbital velocity in the Milky Way, and also the one that comes closest to Sagittarius A*. It likely did not form where we see it today, and may have lost a companion along the way.
“S301’s orbital properties, and the fact that stars cannot form so close to a massive black hole, indicate that the star was likely part of a binary pair that was torn apart by the tidal forces of Sagittarius A*. In the process, S301 became trapped by the black hole’s gravity while its companion star was kicked out with high velocity, most likely enough to leave the galaxy altogether,” explains the ESO.
Astronomers believe that within 10 years they will be able to determine the rotation of Sagittarius A*. Mass and rotation, or spin, are two of the fundamental properties that scientists use to describe a black hole astrophysically. So far, there are signs that Sagittarius A* is spinning, but it is not yet possible to determine precisely how much or in which direction it is spinning.
According to the theory of general relativity, a rotating black hole drags the spacetime around it along with it. That effect should slightly alter the orbit of S301. If astronomers are able to measure these small deviations over the next few years, they will be able to determine the spin of Sagittarius A*.
Astronomers first detected S301 in 2023 using GRAVITY, an instrument on ESO’s VLTI in Chile. Since then, they have tracked its movement and, after identifying it, have been able to trace it backward through observations dating back to 2017. By combining this data, they reconstructed its orbit with greater precision and determined that the star passed its closest approach to Sagittarius A* in early 2023. It will approach the black hole again in 2031.
S301 is not the only known star orbiting close to Sagittarius A*. For decades, stars such as S2 have allowed astronomers to determine the black hole’s mass and test predictions of general relativity. The difference is that S301 passes so close to Sagittarius A* that it could allow them to directly measure its spin over the next decade—something that would require several more decades of observations with other stars.
