A powerful and extremely rare nuclear explosion, called “hyperburst”, observed for the first time in a neutron star

Astronomers think they have found evidence for an extremely powerful nuclear explosion in space – one so rare that we are unlikely to ever see its like again.

The explosion seems to have taken place inside a strange neutron star located 140,000 light-years from Earth called MAXI J0556–332. Neutron stars are the remnant cores of larger stars that have exploded in supernovas, leaving city-sized objects that contain up to twice the mass of our sun. MAXI J0556–332 was discovered in 2011, paired with another larger star. It has confused astronomers ever since, as it was twice the normal temperature of neutron stars when it was first spotted, though it has since cooled.

Now we might know why. Dany Page at the National Autonomous University of Mexico and his colleagues think a huge and unstable thermonuclear explosion may have taken place inside the neutron star. The event, which they dub a “hyperburst”, was so deep inside the star it was undetectable. It would have caused significant heating, explaining the gargantuan spike in temperature.

“Finally we have a physical explanation why it is so hot,” says Page. “Everything makes sense.”

Neutron stars in pairs like this can gain material from their companion star, sucking huge amounts of gas onto their surfaces. This process makes the neutron star very hot, but can also result in detectable bursts near the surface when hydrogen and helium burn, which can happen as frequently as every few minutes. More powerful bursts, known as superbursts, occur every few years as heavier carbon is burned about 100 metres below the surface of the neutron star, releasing 100 times more energy than a regular burst.

The team’s modelling suggests hyperbursts would be 100 times stronger still and occur 500 metres below the surface, deep inside the ocean of thick plasma that encompasses neutron stars. They would result from the burning of oxygen, which would build up to eventually “generate more energy than can be leaked away”, says Page, with temperatures approaching 400 million °C. The result would be an explosion that released more energy in a matter of milliseconds than our sun does in 100,000 years – but it would be undetectable from outside the neutron star because of its depth.

Pushing enough matter into the neutron star to drive this explosion, however, would take a long time. “You have to wait for maybe 1000 years,” says Page. The neutron star must also have stopped gaining material from its companion in order for the temperature of the explosion to be noticeable, something only seen in a handful of binary neutron stars. This rare combination of circumstances means this might be the only hyperburst we ever witness. “We’re lucky to have one,” says Page.

Anna Watts at the University of Amsterdam says it is a “really interesting idea”, noting that previous attempts to explain this neutron star’s unusual temperature relied on a “hand wavy” idea called shallow heating. This suggested there was some sort of heating process taking place in the crust of the neutron star, but the science was uncertain. “A hyperburst would certainly solve the energy problem,” says Jean in ‘t Zand at the Netherlands Institute for Space Research.

There might be an unusual way to test the idea: by never observing a hyperburst again. This would suggest Page’s idea for their rarity is correct. “You just really hope they don’t find another one now,” says Watts, a somewhat strange wish for a potentially new astronomical discovery.


Dany Page, Jeroen Homan, et al, A "Hyperburst" in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion arxiv.org/abs/2202.03962

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