Relativistic effects for stars that orbit the supermassive black hole at the center of the Milky Way

Artist's concept of the orbits of 3 stars near the supermassive black hole at the center of the Milky Way (Image ESO/M. Parsa/L. Calçada)
Artist’s concept of the orbits of 3 stars near the supermassive black hole at the center of the Milky Way (Image ESO/M. Parsa/L. Calçada)

An article published in “The Astrophysical Journal” describes an analysis of the motions of stars that orbit the supermassive black hole at the center of the Milky Way, particularly the one known as S2. A team of astronomers applied a new analytical technique to observations conducted in the past with ESO’s Very Large Telescope (VLT) in Chile and other telescopes concluding that those orbits are influenced by the effects of Albert Einstein’s theory of general relativity.

The supermassive black hole at the center of the Milky Way, Sagittarius A* or simply Sgr A*, has a mass estimated at around 4 million solar masses. Around it there are some stars that have a very high speed within the powerful gravitational field of Sgr A*, characteristics which according to the first author of the article Marzieh Parsa make that area a sort of space laboratory where it’s possible to conduct verifications of the theory of general relativity.

Data collected over the course of 20 years by the VLT made it possible to reconstruct with precision the orbit of the star S2, which has a mass about 15 bigger the Sun. Precision is critical to determine if it’s influenced by relativistic effects because they’re almost imperceptible. The orbital period of S2 is about 15.6 Earth’s years at a speed that reaches about 2% of light’s.

The comparison between the calculations of the orbit of the star S2 according to the classical Isaac Newton’s physics and those according to Albert Einstein’s relativistic physics indicated the presence of relativistic effects. The calculations also allowed to calculate more accurately the mass of Sgr A* at approximately 4.2 million solar masses and its distance from Earth at nearly 27,000 light years.

This study is certainly interesting but represents one of the many verifications of the theory of relativity offered in almost a century. The curvature of stars’ light by the Sun was photographed in 1919 by the astronomer Arthur Eddington during a solar eclipse. However, the study of the area around Sagittarius A* is part of a larger program of observations of the center of the Milky Way that concerns both the supermassive black hole and the stars around it.

For this purpose, ESO installed the GRAVITY instrument on the VLT and the first successful observation was announced in June 2016. It will allow to measure the orbits of S2 and the other stars that orbit Sagittarius A* with a greater precision allowing to study a really extreme environment and if there’s a place where deviations from the physical laws we know can be discovered that’s exactly near a supermassive black hole.

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