Free Astronomy Magazine September-October 2018

32 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES our galaxy — makes it the perfect place to explore gravitational phys- ics, and particularly to test Einstein’s general theory of relativity. New infrared observations from the exquisitely sensitive GRAVITY, SIN- FONI and NACO instruments on ESO’s Very Large Telescope (VLT) have now allowed astronomers to follow one of these stars, called S2, as it passed very close to the black hole during May 2018. At the closest point this star was at a distance of less than 20 billion kilometres from the black hole and moving at a speed in excess of 25 million kilome- tres per hour — almost three per- cent of the speed of light. The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are incon- sistent with Newtonian predictions and in excellent agreement with the predictions of general relativity. These extremely precise measure- ments were made by an interna- tional team led by Reinhard Genzel of the Max Planck Institute for Extra- terrestrial Physics (MPE) in Garching, Germany, in conjunction with collab- orators around the world, at the Paris Observatory–PSL, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Por- tuguese CENTRA – Centro de As- trofisica e Gravitação and ESO. The observations are the culmination of a 26-year series of ever-more-precise observations of the centre of the Milky Way using ESO instruments. “This is the second time that we First successful test of Einstein’s general relativity near a SMBH by ESO O bscured by thick clouds of ab- sorbing dust, the closest su- permassive black hole to the Earth lies 26,000 light-years away at the centre of the Milky Way. This gravitational monster, which has a mass four million times that of the Sun, is surrounded by a small group of stars orbiting around it at high speed. This extreme environment — the strongest gravitational field in T his animation shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein’s general thery of relativity. In this graphic the colour effect, speed and size of the objects have been ex- aggerated for clarity. [ESO/M. Kornmesser]