Astronomers exploring the remnants of an exploded star have uncovered visual evidence that the star exploded not once, but twice.
Supernova remnant SNR 0509-67.5 is all that's left of a star that exploded centuries ago, and astronomers have been studying it with the European Southern Observatory’s Very Large Telescope.
They found patterns that confirm the star underwent two explosive blasts.
The discovery shows some of the most important explosions in the Universe in a new light, the team say.
Type Ia supernovae: key to measuring the cosmos
Most supernovae are the explosive deaths of massive stars, but one type of supernova occurs when white dwarfs burn out their fuel.
These are known as Type Ia supernovae, and they're an important tool for measuring distance in the Universe.
White dwarfs are the small, inactive cores left over after stars like our Sun reach the end of their lives.
They always explode with the same luminosity, so astronomers can compare how bright they appear with how bright we know they really are, and use that to calculate how far away they are.
Type Ia supernova were key, for example, in establishing what we know about the expansion of the Universe.
"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 in Nature Astronomy.
Much of our knowledge of how the Universe expands depends on Type Ia supernovae, and they are 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."
Uncovering the secrets of Type Ia supernovae
The accepted model as to how type Ia supernovae occur begins with a white dwarf in a pair of stars.
If the white dwarf orbits close enough to the other star, the dwarf can steal material from its partner.
The white dwarf accumulates matter from its companion until it reaches a critical mass, at which point it explodes.
Recent studies suggest some Type Ia supernovae are better explained by a double explosion, triggered before the star reaches the critical mass.
The image here, say astronomers, shows this is correct: some Type Ia supernovae explode twice.
The white dwarf forms a blanket of stolen helium around itself, which can become unstable and ignite.
This first explosion produces a shockwave that travels around the white dwarf, then inwards, causing a second explosion in the star's core and creating the supernova.
Seeing for the first time
The team say this image is the first ever visual evidence of a white dwarf's double detonation.
Astronomers predicted that the process would create a pattern or fingerprint in the supernova remnant.
Studies suggest these remnants would contain two separate shells of calcium. Astronomers have found this fingerprint in the supernova’s remains.
Ivo Seitenzahl, who led the observations, says 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."
They detected the calcium layers (blue in the image) in supernova remnant SNR 0509-67.5 by observing it with the Multi Unit Spectroscopic Explorer (MUSE) on ESO’s VLT.
"This tangible evidence of a double-detonation not only contributes towards solving a long-standing mystery, but also offers a visual spectacle,” says Das.
"Revealing the inner workings of such a spectacular cosmic explosion is incredibly rewarding."
Read the full paper (PDF)
