What happens when two very dense stellar remnants collide? A perfect explosion. Astronomers have now observed one of these perfect explosions that was colossal and the blast was utterly spherical in nature, which happened as two neutron stars merged together.

The two neutron stars collapsed to form a black hole as astronomers looked on. The explosion is called a kilonova, which is one of the most powerful explosions to occur in the universe. It happens when two very dense objects, neutron stars, come together for a merger.

The kilonova was studied using the European Southern Observatory’s Chile-based Very Large Telescope.

Astronomers have since then marveled at the rapidly expanding fireball of luminous matter and the details have defied their expectations. The collision was billions of years in the making as the two neutron stars, with a combined mass about 2.7 times that of our sun, had been orbiting each other for eons.

The two neutron stars began their lives as massive normal 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 12 miles (20 km) in diameter but packing more mass than the sun. Each was stretched out and pulled apart in the final seconds before the merger because of the power of the other’s gravitational field.

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 not even light can escape.

“It is a perfect explosion in several ways. It is beautiful, both aesthetically, in the simplicity of the shape, and in its physical significance,” said astrophysicist Albert Sneppen of the Cosmic Dawn Center in Copenhagen, lead author of the research published in the journal Nature.

The event occurred in a galaxy called NGC 4993, about 140-150 million light years away from Earth in the direction of the constellation Hydra. A light year is the distance light travels in a year, nearly 9.5 trillion kilometers. The researchers had expected the explosion to perhaps look like a flattened disk – a colossal luminous cosmic pancake, possibly with a jet of material streaming out of it.

The existence of kilonova explosions was proposed in 1974 and confirmed in 2013, but what they looked like was unknown until this one was detected in 2017 and studied intensively. “Aesthetically, the colors the kilonova emits quite literally look like a sun – except, of course, being a few hundred million times larger in surface area. Physically, this spherical explosion contains the extraordinary physics at the heart of this merger,” Sneppen added.

The researchers offered some hypotheses to explain the spherical shape of the explosion, including energy released from the short-lived single neutron star’s enormous magnetic field or the role of enigmatic particles called neutrinos.


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