It seems to be a relatively rare phenomenon that two neutron stars in close orbit collide. In the entire Milky Way Galaxy, scientists believe there are only ten pairs of neutron stars destined to collide, out of a hundred billion stars. In addition, there have only been a handful of kilonova (here for more info) explosions in the aftermath of such a collision, and none in our galaxy. What would happen if there were a kilonova in the Milky Way? What would it mean for life on Earth?
According to a team of researchers led by Haille Perkins, a physicist at the University of Illinois at Urbana-Champaign, there would be little to be happy about if the explosion were to occur too close to the Earth. Sometimes space, seemingly a calm and serene sea of stars and galaxies, can be downright devious and deadly!
Neutron stars: colliding would produce deadly cosmic rays
The results of the study, uploaded to the arXiv preprint server, suggest that the greatest danger from cosmic rays comes if we are not in the path of the kilonova gamma-ray burst, up to a distance of 35 light-years. If we were in the path, we would have a lethal proximity of 300 light-years, but we should be in exactly the right place.
This will help us assess the threat to our own existence, but it will also allow scientists to determine the likelihood of life on alien worlds surviving in the vicinity of these disruptive events, which emit powerful, atmosphere-destroying radiation.
Binary star collisions
Binary neutron star collisions - based on the small number we have observed - have several components. There is usually a short-lived gamma-ray burst (supernovae of collapsing single stars are longer) that erupts a pair of narrow jets on either side of the colliding stars, and gamma-ray cocoons around each jet that form as the jet tries to pierce the material released during the kilonova.
Powerful X-rays that leave no escape
As Science Alert explains: "When the jets hit the surrounding interstellar medium, they produce powerful X-rays; this is known as the residual X-ray glow. And as this evolves over years or centuries, a bubble of cosmic rays expands out into space from the center of the collision. Perkins and his team looked at how these events might affect a planet, based on what we know from the first neutron star collision ever detected, known as GW170817.
They found that any living thing within the narrow radius of the jet, up to 91 parsecs - 297 light-years - away, would likely be incinerated by the powerful gamma radiation. Outside this narrow range, however, the situation is somewhat safer. One would have to be at a distance of about 13 light-years to be hit by gamma radiation from the cocoon structures.
The Consequences for the Earth
Both of these threats would only last for a short period of time; they would destroy the ozone in the Earth's stratosphere, and it would take about four years to rebuild it. X-rays, the team found, are much more deadly because the residual glow lasts much longer than gamma rays. However, it would have to be relatively close, within about 16 light-years.
Biggest threat
The researchers found that it is the long-term cosmic ray bubble that poses the greatest threat. Accelerated by the rest of the kilonova, these would destroy the ozone layer, leaving the Earth vulnerable and permeable to radiation, possibly for several thousand years. This would cause a devastating mass extinction. In order for this to happen, we would have to be within about 35 light years of the source.
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