Free Astronomy Magazine May-June 2020

9 MAY-JUNE 2020 SPACE CHRONICLES T his zoom video sequence starts with a broad view of the Milky Way. We then dive into the dusty central region to take a much closer look. There lurks a 4-million solar mass black hole, surrounded by a swarm of stars orbiting rapidly. We first see the stars in motion, thanks to 26 years of data from ESO’s telescopes. We then see an even closer view of one of the stars, known as S2, passing very close to the black hole in May 2018. The final part shows a simula- tion of the motions of the stars. [ESO/GRAVITY Collaboration] T his schematic illustration shows S2’s orbit around Sagitarius A*, the super- massive black hole at the centre of the Milky Way. The precession movement is exaggerated for easier viewing. [ESO/L. Calçada] This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO’s VLT, located in the Atacama Desert in Chile. The number of data points marking the star’s position and ve- locity attests to the thoroughness and accuracy of the new research: the team made over 330 measure- ments in total, using the GRAVITY, SINFONI and NACO instruments. Be- cause S2 takes years to orbit the su- permassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement. The research was conducted by an international team led by Frank Eisenhauer of the MPE with collab- orators from France, Portugal, Ger- many and ESO. The team make up the GRAVITY collaboration, named after the instrument they devel- oped for the VLT Interferometer, which combines the light of all four 8-metre VLT telescopes into a super- telescope (with a resolution equiv- alent to that of a telescope 130 me- tres in diameter). The same team reported in 2018 another effect predicted by General Relativity: they saw the light received from S2 being stretched to longer wave- lengths as the star passed close to Sagittarius A*. “Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity,” says Paulo Gar- cia, a researcher at Portugal’s Cen- tre for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project. With ESO’s upcoming Extremely Large Telescope, the team believes that they would be able to see much fainter stars orbiting even closer to 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,” says An- dreas Eckart from Cologne Univer- sity, another of the lead scientists of the project. This would mean as- tronomers would be able to meas- ure the two quantities, spin and mass, that characterise Sagittarius A* and define space and time around it. “That would be again a com- pletely different level of testing rel- ativity,” says Eckart. !