Free Astronomy Magazine March-April 2025

MARCH-APRIL 2025 T his image indicates the location of the newly discovered binary star D9, which is orbiting Sagittarius A*, the supermassive black hole at the centre of our galaxy. It is the first star pair ever found near a supermassive black hole. The cut-out shows the binary system as detected by the SINFONI spectrograph on ESO’s Very Large Telescope. While the two stars cannot be discerned separately in this image, the binary nature of D9 was revealed by the spectra captured by SINFONI over several years. These spectra showed that the light emitted by hy- drogen gas around D9 oscillates periodically towards red and blue wavelengths as the two stars orbit each other. [ESO/F. Peißker et al., S. Guisard] suggests that it could be a very young stellar system that must have formed in the vicinity of the super- massive black hole,” explains co-au- thor Michal Zaja č ek, a researcher at Masaryk University, Czechia, and the University of Cologne. The newly discovered binary was found in a dense cluster of stars and other objects or- biting Sagittarius A*, called the S cluster. Most enigmatic in this cluster are the G ob- jects, which behave like stars but look like clouds of gas and dust. It was during their observa- tions of these mysterious objects that the team found a surprising pattern in D9. The data obtained with the VLT’s ERIS instrument, com- bined with archival data from the SINFONI instru- ment, revealed recurring variations in the velocity of the star, indicating D9 was actually two stars orbiting each other. “I thought that my analysis was wrong,” Peißker says. “But the spectroscopic pattern covered about 15 years, and it was clear this detec- tion is indeed the first binary ob- served in the S cluster.” The results shed new light on what the mysterious G objects could be. The team proposes that they might actually be a combination of binary stars that have not yet merged and the leftover material from already merged stars. The precise nature of many of the objects orbiting Sagittarius A*, as well as how they could have formed so close to the supermassive black hole, remain a mystery. But soon, the GRAVITY+ upgrade to the VLT Interferometer and the METIS in- strument on ESO’s Extremely Large Telescope (ELT), under con- struction in Chile, could change this. Both facilities will allow the team to carry out even more detailed ob- servations of the Galactic centre, revealing the nature of known objects and un- doubtedly uncovering more binary stars and young sys- tems. “Our discovery lets us speculate about the pres- ence of planets, since these are often formed around young stars. It seems plausi- ble that the detection of planets in the Galactic cen- tre is just a matter of time,” concludes Peißker. 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 in- strument on ESO’s Very Large Tele- scope. The instru- ment provides a spectrum for every single pixel; over the years, the emis- sion around D9 was found to oscillate periodically towards red and blue wavelengths which revealed that D9 is actu- ally two stars orbiting each other. [ESO/F. Peißker et al.] !