Supernova: Star dies after double explosion

Sydney. Astronomers refer to the explosion of a star as a supernova. In some supernovae, the explosion occurs not just once, but twice, as an international research team has now demonstrated for the first time. A gas cloud 160,000 light-years away in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, is the remnant of such a double stellar explosion, as the scientists report in the journal "Nature Astronomy."

Most supernovae herald the death of a large, massive star. When such stars exhaust their supply of nuclear fuel, they become unbalanced. As their cores collapse, becoming a neutron star or even a black hole, the resulting shock wave catapults their outer layers into space, causing the dying star to glow brightly.

But there are also completely different stellar explosions, referred to by astronomers as "Type Ia." These supernovae occur when a white dwarf star forms a close binary system with another, larger star. Gas from the larger star can then flow down onto the white dwarf. If the accumulated gas reaches a critical amount, a thermonuclear explosion—a Type Ia supernova—occurs, destroying the white dwarf.

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"The explosions of white dwarfs play a crucial role in astronomy," explains Priyam Das of the University of New South Wales in Australia, who led the research team. They serve as a cosmic benchmark for astronomers: The brightness of the explosion determines its distance, which astronomers can then use to measure the expansion of the universe. "But despite their importance, the long-standing mystery surrounding the exact mechanism that triggers such an explosion remains unsolved," emphasizes Das.

Theoretical considerations suggest an alternative to the scenario described. In this scenario, the gas coming from the second star initially forms a shell around the white dwarf. This shell can become unstable and ignite thermonuclearly. The shock wave from this first explosion then hits the white dwarf, compressing it, and thus triggering a second detonation. However, so far, there has been no visible evidence of such double explosions.

That's now changed. Using a special add-on instrument, the Multi Unit Spectroscopic Explorer (MUSE), on the Very Large Telescope of the European Southern Observatory (ESO) in Chile, Das and his colleagues took a close look at the gas cloud 0509-67.5. Astronomers have long known that this strikingly ring-shaped object is the remnant of a star that exploded about 400 years ago. But did it explode once or twice?

MUSE generates spectra, breaking down the gas's radiation into its wavelengths. This allows researchers to determine what the gas is made of and how it moves. It turns out that the cloud contains a lot of calcium, which was created during the thermonuclear explosion. This calcium is concentrated in two shells of the cloud – proof to the astronomers that the calcium was formed not in a single explosion, but in two consecutive ones.

"This concrete evidence of a double detonation not only contributes to solving a long-standing mystery, but also provides a visual spectacle," Das enthuses. The gas cloud created by the supernova possesses a "beautifully layered structure." And for him as an astronomer, it is "incredibly illuminating to uncover the inner workings of such a spectacular cosmic explosion."

RND/dpa