Astronomers in Chile using one of the world’s largest telescopes have found a star “dancing” around a black hole in the Milky Way just as Albert Einstein might have predicted more than a century ago. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton’s theory of gravity.

Einstein’s General Theory of Relativity, published in 1915, is a foundation of modern physics. It has long helped scientists understand the forces of gravity.

But, the announcement from the European Southern Observatory (ESO), an inter-governmental group of European astronomers that operates in Chile, proves the theory applies even to a star some 26,000 light years from the Sun.

Observing S2 for over two decades

“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. 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 four million times the mass of the Sun,” says Reinhard Genzel, director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and the architect of the programme that led to this result.

Located 26,000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. 

One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometres (120 times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. 

“After following the star in its orbit for over two decades, our exquisite measurements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” says Stefan Gillessen of the MPE, who led the analysis of the measurements published in the journal Astronomy & Astrophysics.

Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. 

General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.

Very Large Telescope

The Very Large Telescope array (VLT) is the flagship facility for European visible light astronomy. It is the world’s most advanced optical instrument, consisting of four Unit Telescopes with main mirrors 8.2 metres in diameter and four movable 1.8-metre Auxiliary Telescopes. 

The four telescopes are named Antu, Kueyen, Melipal and Yepun.

The telescopes can work together to form the giant VLT Interferometer (VLTI) allowing astronomers to see details up to 25 times finer than with the individual telescopes. 

The light beams are combined in the VLTI using a complex system of mirrors in underground tunnels where the light paths must be kept equal to distances of less than one-thousandth of a millimetre over a hundred metres. 

With this kind of precision the VLTI can reconstruct images with an angular resolution of milliarcseconds, equivalent to distinguishing the two headlights of a car at the distance of the Moon.

The 8.2-metre Unit Telescopes can also be used individually to obtain images of celestial objects in one-hour exposures that are four billion times fainter than those visible to the naked eye.

European Southern Observatory 

The European Southern Observatory (ESO) is an inter-governmental astronomy organisation established in 1962. It is supported by 16 countries. ESO headquarters is situated in Garching, Germany.

ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. 

ESO operates facilities at three unique world-class observing sites in the Atacama Desert region of Chile: La Silla, Paranal and Chajnantor.

The Atacama Desert is one of the darkest and driest places in the world. The region’s low humidity and smooth airflow create unrivaled visibility for the high-tech telescopes that scientists use to shed light on the formation of the universe and the possibility of extraterrestrial life.

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