Star seen ‘dancing’ around supermassive black hole proves Einstein was right

16 Apr 2020

Artist’s impression of S2’s orbit around the supermassive black hole. Image: ESO/L Calçada

A star was seen orbiting a supermassive black hole in the way that Einstein’s general theory of relativity had predicted.

The European Southern Observatory’s (ESO) Very Large Telescope has revealed the first sighting of a star orbiting a supermassive black hole in a rather unique way at the centre of the Milky Way. This breakthrough, almost 30 years in the making, was predicted by Albert Einstein’s general theory of relativity.

Writing in Astronomy and Astrophysics, a team of researchers described the star’s orbit as having a rosette shape, unlike the ellipse shape as predicted by Isaac Newton’s theory of gravity.

Located 26,000 light years from the sun, Sagittarius A* and its surrounding dense cluster of stars is described by ESO as being a “unique laboratory for testing physics” in an otherwise unexplored and extreme regime of gravity.

One of these stars, named S2, sweeps towards the supermassive black hole and gets within 20bn km of it on the star’s closest approach. Equivalent to 120 times the distance from Earth to the sun, this makes it one of the closest stars ever found in orbit around the black hole.

Simulation of stars orbiting the supermassive black holes.

This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. Image: ESO/L Calçada/

4m times the mass of our sun

When S2 is at its closest approach to the black hole, it is hurtling through space at 3pc of the speed of light, completing an orbit once every 16 years. Using these latest figures, the astronomers can predict that Sagittarius A* must have a mass 4m times that of our sun.

The orbit of S2 makes it a significant outlier, as typically stars and planets will move closer to and further away from the object they are rotating around. However, S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole, change with each turn – resulting in the rosette shape.

This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.

Over 27 years of observations of the S2 star, ESO’s instruments made more than 330 measurements. Now, with the organisation’s upcoming Extremely Large Telescope, the team believes it will be possible to see even fainter stars orbiting the supermassive black hole.

“If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” said Andreas Eckart from Cologne University, one of the lead scientists of the project. “That would be again a completely different level of testing relativity.”

Colm Gorey was a senior journalist with Silicon Republic