Free Astronomy Magazine September-October 2025

9 ASTRO PUBLISHING SEPTEMBER-OCTOBER 2025 All models that explain Type Ia su- pernovae begin with a white dwarf in a pair of stars. If it orbits close enough to the other star in this pair, the dwarf can steal material from its partner. In the most established the- ory behind Type Ia supernovae, the white dwarf accumulates matter from its companion until it reaches a critical mass, at which point it un- dergoes a single explosion. How- ever, recent studies have hinted that at least some Type Ia supernovae could be better explained by a dou- ble explosion triggered before the star reached this critical mass. Now, astronomers have captured a new image that proves their hunch was right: at least some Type Ia su- pernovae explode through a ‘dou- ble-detonation’ mechanism instead. In this alternative model, the white dwarf forms a blanket of stolen he- lium around itself, which can be- come unstable and ignite. This first explosion generates a shockwave that travels around the white dwarf and inwards, triggering a second detonation in the core of the star — ultimately creating the supernova. Until now, there had been no clear, visual evidence of a white dwarf un- dergoing a double detonation. Re- cently, astronomers have predicted that this process would create a dis- tinctive pattern or fingerprint in the supernova’s still-glowing remains, visible long after the initial explo- sion. Research suggests that rem- nants of such a supernova would contain two separate shells of cal- cium. Astronomers have now found this fingerprint in a supernova’s remains. Ivo Seitenzahl, who led the observa- tions and was at Germany’s Heidel- berg Institute for Theoretical Studies when the study was conducted, says these results show “a clear indica- tion that white dwarfs can explode well before they reach the famous Chandrasekhar mass limit, and that T his image, taken with ESO’s Very Large Telescope (VLT), shows the supernova remnant SNR 0509-67.5. These are the expanding remains of a star that ex- ploded hundreds of years ago in a double-detonation – the first photographic evidence that stars can die with two blasts. The data were captured with the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the VLT. MUSE allows as- tronomers to map the distribution of different chemical elements, displayed here in different colours. Calcium is shown in blue, and it is arranged in two concentric shells. These two layers indicate that the now-dead star exploded with a double- detonation. Hydrogen (H alpha) is shown in orange. [ESO/P. Das et al. Background stars (Hubble): K. Noll et al.]