How supernovae get a boost of brilliance

190516 supernova 1 scaled
Remnant of Kepler’s supernova, a Type Ia supernova, discovered by Johannes Kepler in 1604.

Some exploding stars get a boost of brilliance years after the initial blast – and astronomers have shown how.

A team from the US and Australia watched the afterglow of a supernova through the Hubble Space Telescope for nearly three years. Its light curve matched a prediction made by one of the researchers, Australian National University’s Ivo Seitenzahl, seven years ago – that an isotope of cobalt was responsible for the radiance top-up.

Type Ia supernovae, also known in astronomy as “standard candles”, comprise a bright, predictable class of exploding star which can be seen in far-away galaxies. They’re a staple of astronomy – Saul Perlmutter, Brian Schmidt and Adam Reiss won the 2011 Nobel prize for physics for using them to show the Universe is expanding and accelerating.

But how Type Ia supernovae form is still a bit of a mystery. There are two theories: delayed detonation and violent mergers.

Delayed detonation happens when a white dwarf star, sucking matter from another star, outgrows the “Chandrasekhar limit” (the maximum size a white dwarf can be stable, around 1.44 times the mass of the Sun) and explodes.

Violent mergers, as their name suggests, happen when two stars each smaller than the Chandrasekhar limit collide and detonate.

After the initial blast, carbon and oxygen atoms in the core fuse to become cobalt-56, which decays to iron-56. But modelling shows that isn’t enough to explain why some supernovae stay brighter, longer. 

In the Monthly Notices of the Royal Astronomical Society in 2009, Seitenzahl and colleagues suggested cobalt-57, which is like cobalt-56 but has an extra neutron, might do the trick. Where cobalt-56 has a half-life of 77 days (meaning after 77 days, half the atoms will have decayed into lighter elements), cobalt-57’s half-life is 272 days.

So Or Graur, from New York University, and colleagues tested this by watching as a supernova called SN 2012cg faded over 1,055 days.

The light curve matched that proposed by the cobalt-57 theory. X-rays spat out as cobalt-57 decayed into iron-57, as well as electrons, kept the supernova glowing after all the cobalt-56 radiation faded, they write.

Seitenzahl says he is “thrilled” his theory is supported after only seven years: “I was sceptical whether clues for the presence of cobalt-57 in Type Ia supernovae would be observed in my lifetime.”

The work was published in The Astrophysical Journal.

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