Free Astronomy Magazine May-June 2020

7 MAY-JUNE 2020 SPACE CHRONICLES Extraterrestrial Physics (MPE) in Garching, Germany and the archi- tect of the 30-year-long pro- gramme 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 to- wards the supermassive black hole to a closest distance less than 20 bil- lion 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. “After following the star in its orbit for over two and a half decades, our exquisite meas- urements 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 pub- lished today in the journal Astron- omy & 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 T his simulation shows the orbits of stars very close to the super- massive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. This is a perfect laboratory to test gravitational physics and specifically Einstein’s general theory of relativity. [ESO/L. Calçada /spaceengine.org ]