A star that is pirouetting around the supermassive black hole at the center of the Milky Way, our galaxy, has validated a key part of Albert Einstein’s theory of general relativity. The star, called S2, is like “a precision probe of the gravitational field around the closest massive black hole," according to a study published on Thursday in Astronomy & Astrophysics.

For decades, scientists have gazed across 26,000 light years to watch S2 orbiting Sagittarius A*, our galactic center, which is occupied by a black hole with an estimated mass of four million Suns.

S2 is a perfect natural laboratory to test out some of Einstein’s predictions about the environment near extremely massive objects, including a phenomenon called Schwarzschild precession. This precession occurs when an object is locked in a very close orbit with a much more massive object. Instead of tracing out the same elliptical orbit, as modeled in Newtonian physics, general relativity predicts that the smaller object will have an orbit shaped like a stenciled rosette, due to the curvature of spacetime around the massive object.

The precession of Mercury around the Sun. Image: Rainer Zenz

A century ago, Einstein figured out that Mercury’s orbit was precessing in this rosette pattern due to the Sun’s effect on space and time. Now, a team of scientists known as the GRAVITY collaboration has found the same pattern on a much more massive scale.

“We are repeating the same experiments from back then,” said study lead Frank Eisenhauer, a senior staff scientist at the Max-Planck Institute for Extraterrestrial Physics, in a call. “But now, we do it with a black hole.”

The team is named after the GRAVITY instrument on the European Southern Observatory's Very Large Telescope (VLT) in Chile. Since it first became operational in 2017, GRAVITY has combined light from VLT’s four telescopes to become a “super-telescope” that can sharply resolve S2’s orbit around Sagittarius A*.

Observations with GRAVITY and other advanced instruments have now produced the “first direct detection” of Schwarzschild precession around Sagittarius A*, according to the study. S2 orbits in the same rosette pattern around the galactic center as Mercury does around the Sun, corroborating Einstein’s theories yet again.

This is the GRAVITY collaboration’s second major confirmation of general relativity on the scale of black holes. The instrument came online just in time to watch S2 make its closest approach to the black hole in May 2018. The star passed within 20 billion kilometers of Sagittarius A*, a distance only four times larger than Neptune’s orbit around the Sun. As a result of the intense gravitational forces near the black hole, S2 accelerated to three percent the speed of light during the pass.

Observations from the event revealed that the black hole’s gravitational field stretched light from S2 into longer wavelengths, causing it to redden from our perspective. This effect, known as gravitational redshift, was another prediction of relativistic effects on time and space.

“The gravitational redshift probes the time part, and this orbit, how a star moves in space, probes the space part,” said Eisenhauer. “So far, it clearly holds well. Einstein is right.”

Despite how well it has passed tests so far, general relativity may eventually be pushed to its limits by future observations of stars near the galactic center. If there are fainter stars even closer to Sagittarius A* than S2, perhaps with orbits measured in months rather than years, a new generation of extremely large telescopes coming online this decade may be able to spot them.

These stars could be close enough to the black hole to display signs of the rotation of spacetime itself. “The star will not only make a rosette, but since the whole space is rotating, the star will rotate with the space,” Eisenhauer explained.

“We might then come into a situation where we actually see deviations from Einstein’s theory because conceptually we know that Einstein’s theory cannot correctly describe everything,” he added. “We know when scales get very small, it does not fit together with quantum physics.”