Image of the binary star D9 close to Sagittarius A* (annotated)

D9 is the first star pair ever found near Sagittarius A*, the supermassive black hole at the centre of the Milky Way. This image shows an emission line of hydrogen mapped by the SINFONI instrument on ESO’s Very Large Telescope. The instrument provides a spectrum for every single pixel; over the years, the emission around D9 was found to oscillate periodically towards red and blue wavelengths which revealed that D9 is actually two stars orbiting each other.