This discovery could shed new light on how supernovae work.
This image of SNR 0509-75.5 was taken with ESO’s Very Large Telescope’s Multi-Unit Spectroscopic Explorer. This instrument lets astronomers map the distribution of chemical elements. Here, calcium is shown as blue. The two concentric shells visible prove the original star exploded twice.
Credit: ESO/P. Das et al. Background stars (Hubble): K. Noll et al.
In a stunning first-ever image, astronomers have seen a star that died by exploding twice. They pointed the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at supernova SNR 0509-67.5, located within the Large Magellanic Cloud, a satellite galaxy of our Milky Way in the far-southern constellation of Dorado the Swordfish. When they examined the image, it showed patterns that confirm its star suffered a pair of explosive blasts.
Most supernovae are the explosive deaths of massive stars. Type Ia supernova, however, originate with white dwarfs, the small, inactive cores left over after stars like our Sun exhaust their nuclear fuel. The explosions of these stars are the primary source of the element iron on Earth.
“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, who led the study on SNR 0509-67.5 published today in Nature Astronomy. Much of our knowledge of how the universe expands rests on Type Ia supernovae. “Yet, despite their importance, the long-standing puzzle of the exact mechanism triggering their explosion remains unsolved,” he adds.
Type Ia supernovae start as a white dwarf being one component in a binary star system. If it orbits close enough to its companion star, the dwarf can steal material from its partner. The white dwarf continues to gather material until it reaches a critical mass, at which point it explodes. That’s what everyone thought. However, recent studies show that some Type Ia supernovae could be better explained by a double explosion.
The new image of SNR 0509-67.5 proves that at least some Type Ia supernovae explode twice. In this model, the white dwarf forms a layer around itself of helium that it took from its companion. If that layer becomes unstable and ignites, the explosion generates a shockwave that travels around the white dwarf and inward. That triggers a second detonation in the star’s core: the supernova.
Until now, there was no visual evidence of a white dwarf undergoing two explosions. Recently, astronomers have predicted that this process would create a supernova remnant with two separate shells rich in calcium. SNR 0509-67.5 has these two shells. In the image, they appear as blue.
Ivo Seitenzahl, who led the observations and was at Germany’s Heidelberg Institute for Theoretical Studies 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 detected the calcium layers by observing the supernova remnant with the Multi Unit Spectroscopic Explorer on ESO’s VLT.
Type Ia supernovae behave in consistent ways, and their predictable brightness helps astronomers to measure distances in space. Using them as a cosmic measuring tape, astronomers found that universe was expanding, a discovery that won the Physics Nobel Prize in 2011. Studying how they explode helps us to understand why they have such a predictable brightness.
Das added, “This tangible evidence of a double-detonation not only contributes towards solving a long-standing mystery, but also offers a visual spectacle,” he says, describing the “beautifully layered structure” that a supernova creates. For him, “revealing the inner workings of such a spectacular cosmic explosion is incredibly rewarding.”
