Astronomers using ESO’s Very Large Telescope (VLT) have imaged SNR 0509-67.5, a very young (300-350 years old) remnant of Type Ia supernova, and spotted patterns that confirm its star suffered a pair of explosive blasts.
This image, taken with the Multi-Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope (VLT), shows the supernova remnant SNR 0509-67.5 — the expanding remains of a star that exploded hundreds of years ago in a double-detonation. Calcium is shown in blue, and it is arranged in two concentric shells. These two layers indicate that the star exploded with a double-detonation. Image credit: ESO / Das et al. / Noll et al.
“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,” said Priyam Das, a Ph.D. student at the University of New South Wales Canberra.
“Much of our knowledge of how the Universe expands rests on these 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.”
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 Type Ia supernovae, 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.
The new VLT image of SNR 0509-67.5 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.
Das and colleagues found this fingerprint in a supernova’s remains.
“The 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,” said Dr. Ivo Seitenzahl, an astronomer at Heidelberg Institute for Theoretical Studies.
The astronomers were able to detect these calcium layers in SNR 0509-67.5 by observing it with the Multi Unit Spectroscopic Explorer (MUSE) on VLT.
This provides strong evidence that a Type Ia supernova can occur before its parent white dwarf reaches a critical mass.
“This tangible evidence of a double-detonation not only contributes towards solving a long-standing mystery, but also offers a visual spectacle,” Das said.
“Revealing the inner workings of such a spectacular cosmic explosion is incredibly rewarding.”
The team’s results appear today in the journal Nature Astronomy.
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P. Das et al. Calcium in a supernova remnant as a fingerprint of a sub-Chandrasekhar-mass explosion. Nat Astron, published online July 2, 2025; doi: 10.1038/s41550-025-02589-5
