A recently spotted star at the center of the Milky Way is moving at a breathtaking speed and could provide information about the rotation of the black hole in our Galaxy.
Named S301, this star orbits Sagittarius A*, the black hole located at the center of the Milky Way. It completes an orbit in just 8.7 years. Its trajectory is highly elongated, so its distance from the black hole changes considerably over the course of its orbit.
Sequence of images obtained with GRAVITY showing the star S301 orbiting Sagittarius A*. S301's elliptical trajectory is indicated in each view.
Credit: ESO/GRAVITY collaboration
It is during its closest approach that S301 accelerates to around 25,000 km/s, or more than 8% of the speed of light. It then comes within approximately 11.5 times the distance between Earth and the Sun. Despite this proximity, the star remains far enough away not to be destroyed.
To understand the significance of this discovery, we need to focus on Sagittarius A*. This black hole has a mass approximately 4.3 million times that of the Sun. Its gravity is so strong that astronomers can test certain predictions of general relativity by precisely tracking the stars around it.
Another star, S2, is already used for this type of measurement. Its orbit lasts about 16 years, and observations have made it possible to detect several effects predicted by Einstein's theory. However, S301 passes much closer to the black hole. Its motion should therefore be more strongly affected by the black hole's rotation.
Specifically, a rotating black hole changes how space and time behave around it. This change should gradually shift S301's orbit. The effect is small, but current instruments can measure the star's position with great precision.
Illustration of the Lense–Thirring effect: the rotation of a black hole drags space-time around it and can alter the orbit of a nearby star.
Credit: ESO/M. Kornmesser
Researchers discovered S301 using GRAVITY, an instrument that combines light from several telescopes of the Very Large Telescope in Chile. The star was detected in 2023, and the team then traced it in observations dating back to 2017. These data made it possible to reconstruct its orbit.
Its highly elongated path also provides a clue about its past. S301 may once have belonged to a binary star system. As the pair passed near Sagittarius A*, they would have been separated by the black hole's gravity: one star would have been captured, while the other was ejected.
Astronomers estimate that a direct measurement of Sagittarius A*'s rotation could become possible over the next decade. S301 will make its closest approach to the black hole again around 2031. New observations, particularly with GRAVITY+ and future instruments, will need to track the tiny changes in its trajectory.