What happens when two neutron stars collide? A ‘perfect’ explosion.
A new study from the group of experts published Wednesday in the journal Nature provides details from an in-depth study of the 2017 celestial event, the first kilonova ever observed by astronomers using ripples in space-time called gravitational waves. Experts across continents also used traditional telescopes to watch the signals reach Earth, compiling the first detailed data on a kilonova. In an article from October 2017, The Washington Post described the detection as “an astronomical marvel.”
It was a “perfect” explosion, Albert Sneppen, the lead author of the study, said in an email, because of the “simplicity of the shape and in its physical significance.”
“I was quite surprised by how simple the story hiding behind the curtain of complexity in the data,” Sneppen continued. “You have this immensely complex physics, unimaginable dense stars and the birth of a black hole — and then it all reduces to this beautiful sphere.”
The neutron stars that crashed into each other are “dense and compact,” Sneppen said. They only measured around 20 km in diameter — about 12 miles — but they are “heavier than the sun,” he said. “A teaspoon of neutron star matter weighs more than Mount Everest.”
Sneppen said the sphere after the collision started out “from a size much smaller than the Earth, but [expanded] at a fraction of the speed of light.” It grew to a size “hundreds of million times larger in surface area than the sun itself.”
It took the team of experts years to understand the data produced by the 2017 kilonova, which Sneppen said changed in color, beginning with “very blue tones” but transitioning into "progressively redder colors as the days passed.” It was weeks before the kilonova faded from view, Sneppen said.
Large international search teams and collaborations allowed astronomers to locate the host galaxy where the stars collided, and observe the optical, ultra-violet and infrared light from the kilonova, Sneppen said.
“We looked at a range of colors for this analysis, from the ultraviolet, over the visible colors your eye can see (e.g. blue, green, yellow, red), to the infrared colors,” Sneppen said. While the first measurement of a kilonova was recorded in 2013, experts were unable to glean such detail until advances in gravitational wave astronomy in 2015 enabled researchers to “detail and in systematic fashion find these rare explosions in the cosmic haystack,” Sneppen said.
Sneppen said that if a kilonova were to occur in the Milky Way — less than 30,000 light- years away — it would be the brightest star in the night sky, making it discoverable to the human eye.
“But given the next ones will probably be in galaxies hundreds of millions of light-years away — you need large telescopes and advanced equipment,” he said.
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