Free Astronomy Magazine July-August 2024

36 JULY-AUGUST 2024 ASTRO PUBLISHING T his collection of panels shows three artist’s impressions depicting the violent event that changed the fate of the stellar system HD 148937; a real astro- nomical image is shown in the last panel. Originally, the system had at least three stars (top left panel), two of them close together and another one much more distant, until one day the two inner stars clashed and merged (top right panel). This violent event created a new, larger and magnetic star, now in a pair with the more distant one (bottom left panel). The merger also released the ma- terials that created the spectacular nebula now surrounding the stars (bottom right panel). [ESO/L. Calçada, VPHAS+ team. Acknowledgement: CASU] in a violent manner, creating a mag- netic star and throwing out some material, which created the nebula. The more distant star formed a new orbit with the newly merged, now- magnetic star, creating the binary we see today at the centre of the nebula.” “The merger scenario was already in my head back in 2017 when I stud- ied nebula observations obtained with the European Space Agency’s Herschel Space Telescope,” adds co- author Laurent Mahy, currently a senior researcher at the Royal Ob- servatory of Belgium. “Finding an age discrepancy between the stars suggests that this scenario is the most plausible one and it was only possible to show it with the new ESO data.” This scenario also explains why one of the stars in the system is mag- netic and the other is not — an- other peculiar feature of HD 148937 spotted in the VLTI data. At the same time, it helps solve a long- standing mystery in astronomy: how massive stars get their magnetic fields. While magnetic fields are a common feature of low-mass stars like our Sun, more massive stars can- not sustain magnetic fields in the same way. Yet some massive stars are indeed magnetic. Astronomers had suspected for some time that massive stars could acquire magnetic fields when two stars merge. But this is the first time researchers find such direct evi- dence of this happening. In the case of HD 148937, the merger must have happened recently. “Magnet- ism in massive stars isn’t expected to last very long compared to the life- time of the star, so it seems we have observed this rare event very soon after it happened,” Frost adds. ESO’s Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert, will enable researchers to work out what happened in the system in more detail, and perhaps reveal even more surprises. !