Enigmatic Star Orbiting Giant Black Hole Imaged for First Time

Using ESO’s Very Large Telescope (VLT), researchers have demonstrated for the first time that a star, revolving the giant black hole in the middle of the Milky Way, travels just as projected by Albert Einstein’s general theory of relativity.

Instead of looking like an ellipse as projected by Newton’s theory of gravity, the star’s orbit is shaped similarly to a rosette. For a long time, researchers have been looking for this result that was eventually achieved by the progressive accurate measurements made over a period of almost three decades. This has allowed the researchers to reveal the secrets of the behemoth that is lurking at the core of the Milky Way.

Einstein’s General Relativity predicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess forwards in the plane of motion. This famous effect—first seen in the orbit of the planet Mercury around the Sun—was the first evidence in favour of General Relativity.

Reinhard Genzel, Director, Max Planck Institute for Extraterrestrial Physics

Genzel is also the architect of the 30-year program that led to this outcome.

Genzel continued, “One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the centre of the Milky Way. This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun.”

The Sagittarius A* is situated 26 000 light-years from the Sun. The thick cluster of stars surrounding the Sagittarius A* serves as a special laboratory for validating physics in an otherwise extreme and unchartered gravity regime.

S2 is one of these stars that races towards the giant black hole at the nearest distance of less than 20 billion km (that is, 120 times more than the distance between the Earth and the Sun). This makes it one of the nearest stars to be ever discovered that is orbiting around the huge giant. When approaching closest to the giant black hole, the S2 star moves rapidly through space at nearly 3% of the speed of light, finishing a single orbit once in 16 years.

“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*,” stated Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics, who headed the analyses of the measurements that were recently published in the Astronomy & Astrophysics journal.

A majority of the planets and stars are known to have a non-circular orbit and consequently move either nearer to or further away from the object they are orbiting around. The S2’s orbit precesses, which means the site of its nearest point to the giant black hole alters with every turn to such an extent that the following orbit is also rotated with respect to the earlier one, generating the shape of a rosette.

General Relativity gives an accurate prediction of the extent the S2’s orbit changes, and the newest measurements achieved from this study precisely match with the theory. Known as Schwarzschild precession, this effect had never been quantified before, especially for a star around a giant black hole. Furthermore, the ESO’s VLT study helps the researchers to study more details about the vicinity of the giant black hole in the middle of the Milky Way.

“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*. This is of great interest for understanding the formation and evolution of supermassive black holes,” stated Guy Perrin and Karine Perraut, the French lead researchers of the study.

This outcome is the conclusion of 27 years of visualizations of the S2 star utilizing, for the best part of this time, a range of instruments at ESO’s VLT, situated in the Atacama Desert in Chile. Also, the number of data points highlighting the velocity and position of the S2 star demonstrates the precision and thoroughness of the latest study—using the NACO, SINFONI, and GRAVITY instruments, the researchers completed a total of more than 330 measurements.

The S2 star can take years to revolve around the giant black hole, and hence it was important to track this star for approximately 30 years, to reveal the complexities of its orbital movement.

The study was carried out by an international research team headed by Frank Eisenhauer from the Max Planck Institute for Extraterrestrial Physics with the support of other researchers from ESO, Germany, Portugal, and France.

The researchers make up the GRAVITY association, called after the instrument developed by them for the VLT Interferometer. The VLT Interferometer integrates the light of all four 8 m VLT telescopes into a super-telescope, with a resolution that corresponds with that of a 130 m diameter telescope).

In 2018, the same set of researchers had reported yet another effect that was also projected by General Relativity: the team noted that the light received from the S2 star is being extended to longer wavelengths as it moved near the Sagittarius A*.

Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity.

Paulo Garcia, Researcher, Centre for Astrophysics and Gravitation, University of Porto

Garcia is also one of the lead researchers of the GRAVITY project.

With ESO’s forthcoming Extremely Large Telescope, the researchers believe that they would be able to observe relatively dimmer stars that are revolving even closer to the giant black hole.

If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole.

Andreas Eckart, Project Lead Scientist, Cologne University

This implies that astronomers would be able to quantify mass and spin—the two quantities—that define Sagittarius A* and also characterize the time and space around it. “That would be again a completely different level of testing relativity,” concluded Eckart.