Schwarzchild Prescession — The cosmic dance of Sagittarius A* and S2

The beautiful rosette shape traced out by S2’s orbit around SgrA* is a result of an effect known as Schwarzschild precession and the team’s findings represent the first time that this prediction, which emerges from the field equations of general relativity, has been measured around a supermassive black hole.

“After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” says Stefan Gillessen, MPE, who led the study.

Despite the common image of a circular orbit, most stars and planets have an elliptical orbit around their parent body. This means that during the course of that orbit they move closer in and further away from the body that they revolve around.

(ESO/GRAVITY Collaboration/ Robert Lea)

S2’s highly elliptical orbit shifts so that the closest point of its approach is at a different location every passage around the black hole. This has the effect of rotating each orbit the star makes around SgrA* — thus tracing out a rosette pattern and confirming the presence of the Schwarzschild precession. Using Einstein’s theory of general relativity theoretical physicists have been able to calculate exactly how the orbit should process and the measurements detailed in the team's study match these predictions precisely.

Observations made with ESO’s Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the centre of the Milky Way moves just as predicted by Einstein’s theory of general relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton’s theory of gravity. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole. This artist’s impression illustrates the precession of the star’s orbit, with the effect exaggerated for easier visualisation. (ESO/L. Calçada)

In addition to confirming Schwarzschild precession around a black hole and thus adding further experimental verification to Einstein’s most revolutionary theory, the team’s study — the culmination of 27 years of observations of S2 — help confirm that SgrA* is indeed a supermassive black hole. The research could also potentially open the door to a more detailed study of the environment around such monstrous cosmic events.

“Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*,” says Guy Perrin and Karine Perraut, lead scientists of the project.

“This is of great interest for understanding the formation and evolution of supermassive black holes.”

The data used by the team came, in the most part, from observations made over the last 27 years by the VLT array — located in Chile’s Atacama desert — arguably the world’s most advanced optical instrument which consists of four massive telescopes working in unison. As S2 takes 16 years to complete a single orbit of SgrA* it was vital that the team’s 330 measurements were taken over a period of three decades in order to correct trace the precession of its orbit.

Despite the culmination of this long study, the team say they aren’t quite done with S2 and the cluster in which it sits just yet. They believe that the upcoming Extremely Large Telescope (ELT) —scheduled to begin observations in 2025 — will allow them to observe other stars in the dense cluster around the centre of the Milky Way. And some of these fainter stars, not currently individually discernable, could pass even closer to SgrA* than S2 does.

“If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” concludes Andreas Eckart from Cologne University, also a lead scientist on the project.