Three years after its explosion, a supernova continues to shine brighter than anyone had expected.
Observations made with the Hubble Space Telescope suggest the powerful explosion produced an abundance of a heavy form of cobalt that gives the heat from nuclear decay an extra energy boost.
The work could help astronomers pinpoint the parents of type Ia supernovae - a type of stellar explosion used to measure distances to faraway galaxies - and reveal what happens during such explosions.
This image, captured by the Hubble Space Telescope, shows galaxy NGC 4424, 50 million light years away from Earth. The close-ups show a type Ia supernova that exploded three years ago. In the upper left, the difference in the brightness of the supernova is shown about a year apart
Current research suggests type Ia supernova explosions originate from binary star systems - two stars orbiting one another - in which at least one star is a white dwarf.
A white dwarf contains the dense remains of a star that was a few times more massive than our sun.
The explosion is the result of a thermonuclear chain reaction, which produces a large amount of heavy elements.
The light that researchers see when a type Ia supernova explodes comes from the radioactive decay of an isotope of nickel (56Ni) into an isotope of cobalt (56Co) and then into a stable isotope of iron (56Fe).
Although peak brightness is reached relatively quickly, and most researchers stop watching supernovae after about 100 days past the beginning of the explosion, the light radiates for years.
Previous studies predicted that about 500 days after an explosion, researchers should see a sharp drop-off in the brightness of these supernovae, an idea called the 'infrared catastrophe.'
However, no such drop-offs have been observed, so Ivo Seitenzahl, a researcher at the Australian National University and the ARC Centre of Excellence for All-sky Astrophysics predicted in 2009 that it must be due to the radioactive decay of 57Co.
This is a heavier isotope of cobalt with a longer half-life than 56Co, and it is expected to provide an extra energy source that would kick in around two to three years after the explosion.
The researchers tested the prediction directly by using the Hubble Space Telescope to observe the type Ia supernova SN 2012cg.
It exploded more than three years ago in the galaxy NGC 4424, which is about 50 million light years away.
'Type Ia supernovae became very important to physics, as a whole, a couple of decades ago when they were used to show that the expansion of the universe is accelerating,' said lead author Or Graur, a research associate in the American Museum of Natural History's Department of Astrophysics.
Current research suggests that type Ia supernova (shown) explosions originate from binary star systems - two stars orbiting one another - in which at least one star is a white dwarf. Previous studies predicted that 500 days after an explosion, researchers should see a sharp drop-off in the brightness of supernovae
'Yet we still do not know exactly what type of star system explodes as a type Ia supernova or how the explosion takes place.
'We saw the supernova's brightness evolve just as Ivo predicted,' Graur said.
'Interestingly, though, we found that the amount of 57Co needed to produce the observed brightness was about twice the amount expected.'
'When we made our prediction in 2009, I was skeptical whether clues for the presence of 57Co in type Ia supernovae would be observed in my lifetime,' Seitenzahl added.
'I am absolutely thrilled that now, only seven years later, we are already constraining explosion scenarios based on our measurements.'
The findings are published in The Astrophysical Journal.
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