Free Astronomy Magazine September-October 2018

33 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES T his artist’s impression 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 theory of relativity. In this graphic the colour effect and size of the objects have been exaggerated for clarity. [ESO/M. Kornmesser] have observed the close passage of S2 around the black hole in our galactic centre. But this time, be- cause of much improved instrumen- tation, we were able to observe the star with unprecedented resolu- tion,” explains Genzel. “We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe gen- eral relativistic effects.” The new measurements clearly re- veal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein’s theory of general rela- tivity. This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the mo- tion of a star around a supermassive black hole. The team used SINFONI to measure the velocity of S2 to- wards and away from Earth and the GRAVITY instrument in the VLT In- terferometer (VLTI) to make extraor- dinarily precise measurements of the changing position of S2 in order to define the shape of its orbit. GRAV- ITY creates such sharp images that it can reveal the motion of the star from night to night as it passes close to the black hole — 26,000 light- years from Earth. “Our first observations of S2 with GRAVITY, about two years ago, al- ready showed that we would have the ideal black hole laboratory,” adds Frank Eisenhauer (MPE), Princi- pal Investigator of GRAVITY and the SINFONI spectrograph. “During the close passage, we could even detect the faint glow around the black hole on most of the images, which al- lowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational red- shift in the spectrum of S2.” More than one hundred years after he published his paper setting out the equations of general relativity,