A long time ago in a galaxy far away—NGC 4993, to be exact—two neutron stars collided and created a spectacular light show.
After billions of years spent slowly circling each other, in their last moments the two degenerate stars spiraled around each other thousands of times before finally smashing together at a significant fraction of light-speed, likely creating a black hole. The merger was so violent it shook the universe, emitting some 200 million suns' worth of energy as perturbations in the fabric of spacetime called gravitational waves. Those waves propagated out from the merger like ripples on a pond, eventually washing over Earth—and into our planet's premiere gravitational-wave detectors, the U.S.-built LIGO and European-built Virgo observatories.
Yet gravitational waves were not the merger's only products. The event also emitted electromagnetic radiation—that is, light—marking the first time astronomers have managed to capture both gravitational waves and light from a single source. The first light from the merger was a brief, brilliant burst of gamma rays, a probable birth cry of the black hole picked up by NASA's Fermi Gamma-Ray Space Telescope.
Hours later astronomers using ground-based telescopes detected more light from the merger—a so-called "kilonova"—produced as debris from the merger expanded and cooled. For weeks much of the world's astronomical community watched the kilonova as it slowly faded from view.
According to a 2016 study, supernovae occurring as close as 50 light-years from Earth could pose an imminent danger to Earth's biosphere—humans included. The event would likely shower us in so much high-energy cosmic radiation that it could spark a planetary mass extinction. Researchers have tentatively linked past instances of spiking extinction rates and plummeting biodiversity to postulated astrophysical events, and in at least one case have even found definitive evidence for a nearby supernova as the culprit. Twenty million years ago, a star 325 light-years from Earth exploded, showering the planet in radioactive iron particles that eventually settled in deep-sea sediments on the ocean floor.That event, researchers speculate, may have triggered ice ages and altered the course of evolution and human history.
The exact details of past (and future) astrophysical cataclysms' impact on Earth's biosphere depend not only on their distance, but also their orientation. A supernova, for instance, can sometimes expel its energy in all directions—meaning it is not always a very targeted phenomenon. Merging black holes are expected to emit scarcely any radiation at all, making them surprisingly benign for any nearby biosphere. A kilonova, however, has different physics at play. Neutron stars are a few dozen kilometers in radius rather than a few million like a typical stars. When these dense objects merge, they tend to produce jets that blast out gamma rays from their poles.
"[W]hat it looks like to us, and the effect it has on us, would depend a lot on whether or not one of the jets was pointed directly at us," Frank says. Based on its distance and orientation to Earth, a kilonova's jets would walk the fine line between a spectacular light show and a catastrophic stripping away of the planet's upper atmosphere. If a jet is pointed directly at us, drastic changes could be in store. And we probably wouldn't see them coming. A kilonova begins with a burst of gamma rays—incredibly energetic photons that, by definition, move at light-speed, the fastest anything can travel through the universe. Because nothing else can move faster, those photons would strike first, and without warning.
Don't let all this keep you up at night. Kilonovae are relatively rare cosmic phenomena, estimated to occur just once every 10,000 years in a galaxy like the Milky Way. That's because neutron stars, which are produced by supernovae, hardly ever form as pairs. Usually, a neutron star will receive a hefty "kick" from its formative supernova; sometimes these kicks are strong enough to eject a neutron star entirely from its galaxy to hurtle at high speeds indefinitely through the cosmos. "When neutron stars are born, they're often high-velocity. For them to survive in a binary is nontrivial," Fruchter says. And the chances of two finding each other and merging after forming independently are, for lack of a better term, astronomically low.
For the unabridged report click here: : A Nearby Neutron Star Collision Could Cause Calamity on Earth
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