Kilonova, the clash between two stars is a show: «It’s the perfect explosion»- Corriere.it

One in 10 billion. the estimate that scientists make of the percentage of stars that may end their lives in thekilonova explosionwhich happens when two neutron stars collide giving rise to a colossal explosion. For the first time, astronomers were able to accurately describe the contours of the explosion, which produced a rapidly expanding luminous and fiery sphere defined as perfect, as if it had been drawn with a compass, at least in its first moments.

The collision

The collision occurred between two neutron stars with a total mass equal to 2.7 times that of the Sun in the galaxy NGC 4993, which is located in the direction of the constellation Hydra at distance of about 150 million light years from us. The existence of kilonovas was theoretically proposed in 1974 but only in 2013 was it confirmed and the first was seen on August 17, 2017 with the Very Large Telescope of the European Southern Observatory in Chile, also detected by the gravitational waves produced and by X-rays It took almost six years to study what had happened and now the analyses, by an international group led by Albert Sneppen of the University of Copenhagen, have been published in Natures
and confirm the highly spherical nature in the first moments of the explosion.

The sphericity of the explosion

At their birth, the two space objects were two normal stars of a binary system which, once their fuel was exhausted, exploded creating two supercompact neutron stars with a diameter of no more than 20 kilometers. The two neutron stars continued to orbit a common center of mass for billions of years before colliding at high speed (75,000 kilometers per second, a quarter of the speed of light) releasing enormous amounts of energy and creating the most intense magnetic fields in the Universe. For the first time, the perfect roundness of the explosion was studied, while previous studies hypothesized a discoidal shape associated with a linear outflow for the material expelled at the moment of impact.

The heaviest elements are born

The kilonova explosion has such a power, combined with very high temperatures and pressures and intense magnetic fields, from give birth to heavier metallic elements, such as gold, platinum, silver and uranium, by nuclear fusion, which are expelled and shot into space. In a few days, the explosion reaches a luminosity comparable to that of a billion suns. The two stars then became a single neutron star, then collapsed into a black hole. What happened was extraordinary, Sneppen said, because to make an explosion spherical, a huge amount of energy would have to be released from the center, smoothing out regions that would otherwise be asymmetrical. Also because, adds co-author Darach Watson, the conditions of the explosion are such that physical conditions that we do not yet understand could develop.