Using data gathered since 2023 by the GRAVITY instrument on the Very Large Telescope in Chile, astronomers have now mapped the orbit a star circling Sagittarius A*, the Milky Way’s central supermassive black hole, every 8.7 years that when it zips past on its closest approach is moving more than 15,500 miles per second, about 8% of the speed of light.

The graphic to the right is figure 2 of the research paper [pdf], and shows the orbit as presently tracked. The 2017 location comes from archival data found later by the science team.

A team led by the Max Planck Institute for Extraterrestrial Physics (MPE) describes the discovery of a star, dubbed S301. It is unremarkable—apart from its extreme proximity to our local supermassive black hole, which has 4.3 million times the mass of the Sun. If the black hole sat at the center of the Solar System, its event horizon—the threshold beyond which even light cannot escape—would be 17 times the diameter of the Sun. S301 has a very elongated orbit, but every 8.7 years it swings close to the black hole—only slightly farther out than Saturn in this imagined system—reaching a speed that would carry it across the Solar System in a few days.

…The team first spotted S301 in early 2023 during one of its monthly observations with GRAVITY, says MPE’s Stefan Gillessen, one of the lead authors of the Nature paper. Although S301 was 100 times fainter than S2 [the previous speed record-holder orbiting Sagittarius A*], subsequent observations showed it racing away from the black hole. By 2024 its path began to curve, and it slowed as it approached the most distant point in its orbit. “By the end of ’24, we had sort of a rough estimate of the orbit,” Gillessen says. “In late ’25, we were already pretty sure.” The team also searched through archival observations, spotting S301 in 2021 and 2017. With most of its 8.7-year orbit mapped out, “that already beautifully shows Schwarzschild precession,” Gillessen says.

The scientists believe the precession, or shift in S301’s orbit shown in the graphic, is a change imposed each orbit by the spin of Sagittarius A*, and will provide them a method for measuring precisely that spin in the coming years. That shift is similar to the precession in Mercury’s orbit which for decades baffled astronomers until Einstein’s theories of relativity explained it.