Double detonation: new image shows remains of star destroyed by pair of explosions

For the first time, an international team of astronomers have obtained visual evidence that a star met its end by detonating twice. By studying the centuries-old remains of a supernova with the European Southern Observatory’s Very Large Telescope (ESO’s VLT), they have found patterns that confirm its star suffered a pair of explosive blasts. This discovery shows some of the most important explosions in the Universe in a new light. The work has now been published in Nature Astronomy.

Most supernovae are the explosive deaths of massive stars, but one important variety comes from an unassuming source. White dwarfs, the small, inactive cores left over after stars like our Sun burn out their nuclear fuel, can produce what astronomers call a Type Ia supernova.

“The explosions of white dwarfs play a crucial role in astronomy,” says Priyam Das, a PhD student at the University of New South Wales Canberra, Australia, and a short-time visiting scientist at the Heidelberg Institute for Theoretical Studies (HITS), who led the study on the supernovaSNR 0509-67.5. Much of our knowledge of how the Universe expands rests on Type Ia supernovae, and they are also the primary source of iron on our planet, including the iron in our blood. “Yet, despite their importance, the long-standing puzzle of the exact mechanism triggering their explosion remains unsolved,” he adds.

A new explanation: double detonation

All models that explain Type Ia supernovae 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 theory behind these detonations, the white dwarf accumulates matter from its companion until it reaches a critical mass, at which point it undergoes a single explosion. However, recent studies have hinted that at least some Type Ia supernovae could be better explained by a double 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 supernovae explode through a ‘double-detonation’ mechanism instead. In this alternative model, the white dwarf forms a blanket of stolen helium around itself, which can become 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 undergoing a double detonation. Recently, astronomers have predicted that this process would create a distinctive pattern or fingerprint in the supernova’s still-glowing remains, visible long after the initial explosion. Research suggests that remnants of such a supernova would contain two separate shells of calcium.  

A key to our understanding of the Universe

Astronomers have now found this fingerprint in a supernova’s remains. Ivo Seitenzahl, who led the observations and was at HITS when the study was conducted, says these results show “a clear indication that white dwarfs can explode well before they reach the famous Chandrasekhar mass limit, and that the ‘double-detonation’ mechanism does indeed occur in nature.” The team were able to detect these calcium layers (in blue in the image above) in the supernova remnant SNR 0509-67.5 by observing it with the Multi Unit Spectroscopic Explorer (MUSE) on ESO’s VLT. This provides definite evidence that a Type Ia supernova can occur before its parent white dwarf reaches a critical mass.

More information and images: ESO press release https://www.eso.org/public/news/eso2511/

This research was presented in Nature Astronomy titled “Calcium in a supernova remnant shows the fingerprint of a sub-Chandrasekhar mass explosion”. https://www.nature.com/articles/s41550-025-02589-5