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Astronomers have long theorized about the existence of a cosmic explosion called a kilonova, and in 2017, they finally witnessed one. Located in a galaxy known as NGC 4993, about 140-150 million light-years away from Earth, the kilonova was the result of two neutron stars colliding with each other at high speeds. The event was so intense that it released the luminosity of about a billion suns for a few days.
Researchers have now published a detailed analysis of the perfectly spherical explosion in the journal Nature. They describe the kilonova as “a perfect explosion in several ways,” both aesthetically and in terms of its physical significance. The colors emitted by the kilonova are said to resemble a sun, but on a much larger scale.
The kilonova explosion occurred when two neutron stars, with a combined mass about 2.7 times that of our sun, merged into a rapidly expanding fireball of luminous matter. This happened shortly before the combined entity collapsed to form a black hole, which is an even denser object with gravity so strong that not even light can escape.
The kilonova explosion was studied using the European Southern Observatory’s Chile-based Very Large Telescope. Researchers had expected the explosion to look like a flattened disk, possibly with a jet of material streaming out of it. However, they were surprised to find that the explosion was perfectly spherical.
Astrophysicist Albert Sneppen of the Cosmic Dawn Center in Copenhagen, lead author of the research, stated that the kilonova is “beautiful, both aesthetically, in the simplicity of the shape, and in its physical significance.” The explosion contains “extraordinary physics” at the heart of the neutron star merger, said Sneppen.
The extreme physical conditions of the neutron star merger created the most intense magnetic fields in the universe. The densities and temperatures were so intense that heavy elements were forged during the process, including gold, platinum, arsenic, uranium, and iodine.
The two neutron stars began their lives as massive normal stars in a two-star system called a binary. Each star exploded and collapsed after running out of fuel, leaving behind a small and dense core about 20 km in diameter but packing more mass than the sun.
As the two neutron stars drew nearer to each other, they orbited at a speedy clip. Each star was stretched out and pulled apart in the final seconds before the merger because of the power of the other’s gravitational field.
The kilonova explosion has left scientists with a new challenge to understand the fundamental physics behind its spherical shape. With extreme physical conditions, far 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 physics that we don’t understand yet.