How bright can a supernova shine?

Ultraluminous supernovae, which blaze hundreds of times brighter than a standard exploding star, do have their limits. Belinda Smith reports.

210416 supernova h.jpg?ixlib=rails 2.1
Type Ia supernovae, such as SN 2014J, have a brightness upper limit of five trillion Suns.
NASA / CXC / SAO / R.Margutti et al

Exploding stars can blaze with the fire of a trillion Suns, but do have a limit to their brightness, according to a pair of astrophysicists from the University of California, Santa Cruz.

Tuguldar Sukhbold and Stanford Woosley calculated the brightest possible supernova is around five trillion times the luminosity of the Sun, but must be powered by a dense spinning star called a magnetar to hit that peak. They published their calculations in The Astrophysical Journal Letters.

In January, an international team reported the most luminous supernova yet: a cosmic explosion 3.8 billion light-years away that flashed 200 times brighter than a normal supernova, or 20 times more brilliant than the 100 billion stars in the Milky Way galaxy combined.

The fuel driving the ultraluminous supernova, dubbed ASSASN-15lh, is still a mystery. So is it as bright as supernovae can get?

Sukhbold and Woosley's calculations show it's definitely up there. They arrived at their five-trillion-Suns limit by examining different supernova brightness constraints.

A supernova blast happens in the death throes of a star collapsing on its core. It sends out a cataclysmic blast of shells of dust and metals.

So Sukhbold and Woosley tweaked four parameters: the time it takes for a star's core to collapse, radioactivity in the core, if inner shells of exploding material catch up to and collide with outer shells, and if the collapsing star forms a magnetar.

Magnetars are incredibly dense, spinning stars with a super-strong magnetic field. The theory is as its spin slows, the magnetar dumps energy into the ejected supernova material and giving it a boost of brightness.

They found a prompt core collapse decreases luminosity, as does radioactivity. Calculating the effect of colliding shells was a bit tricky – given not much in known about the masses of supernova ejecta shells, the pair write, "constraints were less accurate".

Described in the paper as a "special case", magnetar-powered supernovae were the brightest. And given ASSASN-15lh far outshone their calculated brightest possible non-magnetar supernova, the pair suggests it could have only been powered by a magnetar – "barring new physics" or a new type of object, that is.

Further reading:
Is this giant supernova a new type of cosmic object?
Supernova shockwave caught in visible light for the first time

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