Astronomers have observed what could be the ‘perfect explosion’ caused by the merger of two very dense neutron stars.
Researchers on Wednesday described for the first time the contours of the perfect spherical explosion, called a kilonova.
It occurs when neutron stars merge into a rapidly expanding fireball of luminous matter shortly before the combined entity collapses to form a black hole.
The two neutron stars, with a combined mass of about 2.7 times that of our sun, had orbited each other for billions of years before colliding at high speeds and exploding.
This unfolded in a galaxy called NGC 4993, about 140-150 million light-years away from Earth in the direction of the constellation Hydra.
The existence of kilonova explosions was proposed in 1974 and confirmed in 2013, but what they looked like was unknown until it was detected in 2017 and intensively studied.
“It’s a perfect explosion in many ways. It is beautiful, both aesthetically, in the simplicity of the shape, and in its physical significance,” said astrophysicist Albert Schneppen of the Cosmic Dawn Center in Copenhagen, lead author of the study published in the journal Nature.
“Aesthetically, the colors emitted by the kilonova literally look like helium – except, of course, that it is a few hundred million times larger in surface area. Of course, this globular explosion contains the extraordinary physics at the heart of this merger,” added Sneppen.
The researchers expected that the explosion would perhaps look like a flattened disk – a colossal bright cosmic pancake, possibly with a jet of material coming out of it.
“To be honest, we’re really going back to the drawing board with this one,” said Cosmic Dawn Center astrophysicist and study co-author Darach Watson.
“Given the extreme nature of physical conditions—much more extreme than a nuclear explosion, for example, with densities greater than an atomic nucleus, temperatures of billions of degrees, and magnetic fields strong enough to distort the shapes of atoms—there may well be fundamental physics here we don’t understand yet,” Watson added.
The kilonova was studied using the European Southern Observatory’s Very Large Telescope based in Chile.
The two neutron stars began life as massive regular stars in a two-star system called a binary. Each exploded and collapsed after running out of fuel, leaving behind a small and dense core about 20 km in diameter, but more massive than the sun.
Gradually, they approached each other, orbiting at a rapid clip. Each stretched and pulled apart in the final seconds before merging due to the strength of the other’s gravitational field.
Their inner parts collided at about 25% the speed of light, creating the strongest magnetic fields in the universe. The explosion released the luminosity of about a billion suns for a few days.
The two briefly formed a single massive neutron star that then collapsed to form a black hole, an even denser object with gravity so fierce that even light cannot escape.
The outer parts of neutron stars, meanwhile, stretched into long streamers, with some material being thrown into space.
During the process, the densities and temperatures were so intense that heavy elements such as gold, platinum, arsenic, uranium and iodine were forged.
Scientists are trying to find the reason behind the “fundamentally surprising” spherical shape of the explosion, calling it “a fascinating challenge for any theoretical and numerical simulations”.
MORE: Incredible moment two neutron stars collide captured on world’s first video
MORE: NASA finds ‘clearest evidence’ of ancient lake on Mars, raising hopes for alien life