The Washington Post

‘Kilonova’ is first one seen in detail by astronomer­s


Two neutron stars moving at 100 million meters per second rammed into one another in space. In what experts called a “cosmic car crash,” the stars merged and collapsed to form a black hole — while throwing out fragments that produced a perfectly spherical fireball of blue and red.

The collision — known as a “kilonova” — occurred in a galaxy about 140 million light-years away.

The ball of optical ultraviole­t and infrared light has since been intensely observed by astrophysi­cists at the University of Copenhagen.

A new study from the group of experts published last month 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 find the host galaxy where the stars collided, and observe the optical, ultraviole­t 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.

 ?? ALBERT SNEPPEN/REUTERS ?? An illustrati­on of a cosmic collision of two neutron stars — known as a “kilonova” — that occurred in a galaxy about 140 million light-years away.
ALBERT SNEPPEN/REUTERS An illustrati­on of a cosmic collision of two neutron stars — known as a “kilonova” — that occurred in a galaxy about 140 million light-years away.

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