The supermassive black hole at the core of the Milky Way adds to support for general relativity.

After following a star’s motion around a black hole for nearly three decades, astronomers have found that the star’s orbit matches a key prediction of Albert Einstein’s general theory of relativity.

Over the course of 27 years, Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and his collaborators used the European Southern Observatory’s increasingly precise instruments to track a star called S2, which makes a 16-year orbit around the Milky Way’s central black hole. They found that S2’s highly elongated trajectory does not retrace the same ellipse in each orbit, as Newton’s law of gravitation would suggest.

Instead, S2’s path undergoes a gradual shift, or precession, tracing a ‘flower’ pattern (pictured below) in the sky. The researchers’ findings also rule out the presence of multiple large masses, such as several large black holes, at the centre of the Milky Way.

The star named S2 (white dot, artist’s impression) traces a daisy-petal pattern around the black hole (black dot) at the centre of the Milky Way.Credit: L. Calçada/ESO

The team’s earlier studies of S2 showed that its light became redder as it sank deeper into the black hole’s gravitational well, and bluer as it orbited back out — another important effect of general relativity called gravitational redshift.