The brightest supernova ever seen may be the first known
example of a rare type of stellar explosion.

The supernova, spotted in 2016 in a galaxy about 4.6 billion
light-years away, radiated about 5 sexdecillion (5 followed by 51 zeros) ergs
of energy. That’s about twice the amount of radiation emitted by the previous
record-holder, and hundreds of times more energetic than normal supernovas. At
its brightest, this supernova was as bright as all the stars in the Milky Way
put together.

Such a bright blast could have been a
pulsational pair-instability supernova
— thought to occur when an extremely
massive supernova collides with a shell of material cast off by the star before
it exploded, researchers report online April 13 in Nature Astronomy.

“There’s no single, well-established case of such a
supernova,” says Philipp Podsiadlowski, an astrophysicist at the University of
Oxford not involved in the work. “This could be one.” Computer simulations of
the event may help confirm the nature of the star’s demise.

After the supernova, dubbed SN2016aps, was identified in
observations from the Pan-STARRS survey, astronomer Matt Nicholl and colleagues
monitored its fading light for about two years. The amount of stellar debris
left over from the supernova indicates that this star was at least 50 to 100
times as massive as the sun, whereas the stars behind ordinary supernovas are
around 10 solar masses.

The telescope observations also revealed a surprising amount
of hydrogen in the wreckage. More massive stars generally lose their hydrogen
faster than smaller stars. “So, for stars in this 100-solar-mass regime, you
expect that all the hydrogen is long gone well before it explodes,” says
Nicholl, of the University of Birmingham in England. This finding suggests that
two smaller stars still containing hydrogen merged into a supersized star that
underwent a pulsational pair-instability supernova.

This exotic type of supernova is predicted to happen only to
stellar juggernauts. Inside extremely massive stars, “the temperature in the
core can get so high that photons, which are what keeps the star up and
supports it from collapsing under its own gravity, get converted into pairs of
particles — electrons and positrons,” Nicholl says. When these photons, or
particles of light, disappear, “you lose some of the pressure in the core, and
it starts to contract. This can lead to thermonuclear runaway, like an atom
bomb going off.”

That explosive reaction can release enough energy to blow
off the outer layers of the star into an enormous shell. When the star
ultimately goes supernova, the explosion collides with the shell to release huge
amounts of radiation. Nicholl’s team speculates that the stellar remnant forged
during this type of supernova might be an intensely magnetic neutron star
called a magnetar (SN:
11/8/17
), which could pump energy into an explosion to make it as bright as
the one seen in 2016.

This general scenario seems plausible to Stan Woosley, an
astrophysicist at the University of California, Santa Cruz, not involved in the
work. But the size of the star that underwent this explosion leads him to think
that the 2016 supernova may have forged a black hole, instead of making a magnetar.