Einstein is right: Star races around black hole in a rosette orbit

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Even after more than 100 years, Albert Einstein’s most famous theory stands up to observations from one of the most extreme places in the universe.

The star S2 at the centre of the Milky Way orbits around the supermassive black hole Sagittarius A* in the form of a rosette rather than in the form of an ellipse as postulated by Isaac Newton. With this observation, researchers at the Max Planck Institute (MPE) for Extraterrestrial Physics have once again found proof for Albert Einstein’s theory of relativity and at the same time have confirmed that Sagittarius A* must be a black hole with four million times the mass of the Sun. Using instruments from the European Southern Observatory ESO, they have observed the star for more than two decades and are now presenting their observations in the journal Astronomy & Astrophysics.

Theory stops at the most extreme place

S2 belongs to a small group of stars that orbit the supermassive black hole 26,000 light years away from us very quickly and very closely. The conditions there are among the most extreme in the observable universe, and their observation allows precise checks to be made on the predictions that follow from the theory of relativity. S2, for example, during its 16-year orbit with about 120 astronomical units, is closer to the black hole than Voyager 1 is currently away from Earth. It reaches a speed of 3 percent of the speed of light. Two years ago, astronomers had already presented measurements of its orbit that could not be explained by Isaac Newton’s theory of gravity, but only by general relativity.

The MPE researchers are now supplementing these data with more precise results for the orbit of S2. Its closest point to Sagittarius A* changes with each orbit, so each orbit is rotated relative to the previous one. Over time, this creates the rosette shape postulated by Albert Einstein. The effect was first observed on Mercury, the planet closest to the Sun, the earliest confirmation of the theory. The fact that the predictions are so accurate even for the much more extreme orbit of S2 is once again a success. At the same time, the measurements of the so-called Schwarzschild precession allow more precise calculations to be made about the black hole, for example about any dark matter or other smaller black holes that may be present there, the researchers explain.

Waiting for better instruments

As the scientists around MPE director Reinhard Genzel write, a total of more than 330 measurements were made with different instruments. As early as 2018, they had also explained that the light from S2 is stretched to longer wavelengths due to the immense gravitational pull, making it redder overall, which they can observe. For even better measurements, they hope to use the Extremely Large Telescope (ELT) currently under construction in the future. With a bit of luck, they could use it to analyse stars so close to the black hole that they could determine its rotation: “This would be another completely different step in testing the theory of relativity,” says Andreas Eckart from the University of Cologne.

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I am The Washington Newsday correspondent. I cover general science and Nasa news. I have been in the Science Desk's Technology Beat since joining Washington Newsday in 2018. You can contact me at [email protected]

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