The massive explosion from the merger of two neutron stars has an unexpectedly perfect shape. The Quilonova explosion produced by the two stars was a completely symmetrical, nearly perfect sphere, according to a new analysis following a historic neutron star collision observed in 2017. And astronomers simply don’t know why. This contradicts all previous assumptions and kilonova models.

“No one expected the explosion to be like this. It doesn’t make sense for it to be spherical like a ball,” says astrophysicist Darach Watson of the Niels Bohr Institute in Denmark.

“But our calculations clearly show this to be the case. This probably means that important physics is missing from the kilonova theories and simulations we’ve been studying for the past 25 years.”

We rarely see neutron star collisions. This 2017 eruption, named GW170817, was not only the first eruption recorded, it remains unrivaled in detail. We learned many things about the universe from him. For example, these collisions are the source of bursts of gamma rays, the most energetic light in the universe. Cylon explosions are also factories for the production of heavy elements such as gold and platinum.

But we still don’t know much about them. Fortunately, so much data has been collected from GW170817 that scientists are still reviewing it and will be looking into it for a while. This led astrophysicist Albert Snappen of the Niels Bohr Institute and his colleagues to the project to determine the shape of the kilonova.

This is because the geometry of the explosion is determined by the properties of the extremely dense matter that makes up neutron stars, and can help scientists better understand the energy of the explosion and other properties of coalescence. They thought they knew roughly what they were going to find and their job would be to place more elaborate restrictions on known features. The spherical explosion they actually detected shows that we don’t understand neutron star mergers.

“You have two supercompact stars orbiting each other 100 times per second before they collapse. Our intuition and all previous models suggest that the explosion cloud created by the collision should have a flat and highly asymmetrical shape,” says Snappen.

“The most likely way to make the explosion spherical is to release a large amount of energy from the center of the explosion and flatten an otherwise asymmetrical shape. So the spherical shape tells us there was probably a lot of unexpectedly high energy at the center of the collision.”

There is a possible explanation for this. Neutron stars are stars that can transform into stars of a certain mass after they have consumed all the fusion fuel in their cores. When the star reaches this point, it ejects its outer material and the core collapses into a super-dense object. Smaller stars become white dwarfs with a mass of about 1.4 times the mass of the Sun. Mid-range stars turn into neutron stars with a mass of about 2.4 times that of the Sun. And larger stars turn into black holes.

When two neutron stars collide, the combined mass causes the newly formed object to collapse further gravitationally and turn into a black hole. But in the short time before that happens, the object could become a hypermassive neutron star with an extremely strong magnetic field. A recent analysis shows that this happened with the GW170817. For just a second, the object was a hypermassive neutron star. This could explain the global kilonova, the researchers say.

“Perhaps a kind of ‘magnetic bomb’ is created when the energy of the massive magnetic field of a hypermassive neutron star is released as the star transforms into a black hole,” Watson explains.

“The release of magnetic energy can cause the matter in the explosion to disperse more globally. In this case, the birth of a black hole can be very energetic.”

But some questions remain unanswered, especially about how heavy elements are hammered in a kiloforge. We know this happens; After the explosion, scientists clearly detected strontium in the emission of kilotons of matter.

In their kilonova analysis, Snappen’s team found an almost spherically symmetrical distribution of one of the lightest heavy elements, strontium. But models suggest that heavier elements, such as gold and uranium, must form separately from lighter ones in the kilonova. According to the researchers, this indicates that neutrinos are at work.

“An alternative idea is that during the milliseconds a hypermassive neutron star lives, it emits very powerful radiation, possibly containing an enormous amount of neutrinos,” Snappen says.

“Neutrinos can cause neutrons to turn into protons and electrons, thus forming lighter elements overall. This idea also has flaws, but we think neutrinos play a much more important role than we thought.”

It is possible to have more than one mechanism in the game. We hope to be able to detect more neutron star collisions in motion in the future.