Science and Technology

A star dancing around a supermassive black hole proves Einstein was right

Professor Vítor Cardoso is one of the researchers involved in the observations made by the Very Large Telescope (VLT), which prove Einstein's Theory of General Relativity.

For decades, a group of researchers – including a Técnico professor – have set their eyes on the galaxy, and ended up discovering a star known as S2 orbiting the supermassive black hole, exactly as it was predicted by Albert Einstein in his general theory of relativity. Professor Vítor Cardoso (Técnico – Department of Physics /CENTRA researcher) is involved in this observation made with the Very Large Telescope (VLT). Professor António Amorim (Faculty of Sciences – Universidade de Lisboa), and professor Paulo Garcia, (Faculty of Engineering – University of Porto), also represented CENTRA in GRAVITY project.

The observations made with ESO’s Very Large Telescope (VLT) revealed, for the first time, that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein’s general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton’s theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years.

The results of the investigation, which involved making accurate measurements of the star’s orbit for about 30 years, were published in the journal Astronomy & Astrophysics this Thursday, April 16. The ESO’s VLT, located in the Atacama Desert in Chile, enabled French, German and Portuguese scientists to see that the star’s orbit is shaped like a rosette. “This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun”, stresses Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and the architect of the 30-year-long 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 (one hundred and twenty 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.

“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”, as published in the journal Astronomy & Astrophysics. 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”, explain the researchers.

The Portuguese team participated in more than 330 measurements of S2’s position, which takes 16 Earth years to complete one orbit around Sagittarius A, and in the “design and construction” of a VLT telescope instrument that allows to obtain most detailed observations of material orbiting close to a black hole”, as stressed by professor António Amorim

According to professor Vítor Cardoso, “this discovery shows that the general relativity theory remains valid in the vicinity of a million-solar-mass black hole”. GRAVITY project will allow to study gravity near a black hole’s horizon.