Free Astronomy Magazine July-August 2024

34 venated the more massive star. An- other piece of the puzzle is the neb- ula surrounding the stars, known as NGC 6164/6165. It is 7500 years old, hundreds of times younger than both stars. The nebula also shows very high amounts of nitrogen, car- bon and oxygen. This is surprising as these elements are normally ex- pected deep inside a star, not out- side; it is as if some violent event had set them free. To unravel the mystery, the team as- sembled nine years’ worth of data from the PIONIER and GRAVITY in- struments, both on ESO’s Very Large Telescope Interferometer (VLTI), lo- cated in Chile’s Atacama Desert. They also used archival data from the FEROS instrument at ESO’s La Silla Observatory. “We think this system had at least three stars originally; two of them had to be close together at one point in the orbit whilst another star was much more distant,” ex- plains Hugues Sana, a professor at KU Leuven in Belgium and the prin- cipal investigator of the observa- tions. “The two inner stars merge d by ESO − Bárbara Ferreira Clash of stars solves stellar mystery “W hen doing back- ground reading, I was struck by how special this system seemed,” says Abigail Frost, an astronomer at ESO in Chile and lead author of the study published in Science . The sys- tem, HD 148937, is located about 3800 light-years away from Earth in the direction of the Norma constel- lation. It is made up of two stars much more massive than the Sun and surrounded by a beautiful neb- ula, a cloud of gas and dust. “A neb- ula surrounding two massive stars is a rarity, and it really made us feel like something cool had to have happened in this system. When looking at the data, the coolness only increased.” “After a detailed analysis, we could determine that the more massive star appears much younger than its companion, which doesn’t make any sense since they should have formed at the same time!” Frost says. The age difference — one star appears to be at least 1.5 million years younger than the other — suggests something must have reju-