When two neutron stars collide, very bad things happen

There has been a growing focus on celestial explosions called kilonova events. These eruptions happen when two dense stars, known as neutron stars, crash into each other and release bursts of matter, light, and subatomic particles.

Some scientists believe these events might pose serious risks to all life anywhere in the Universe if they ever occur close to inhabited planets, Earth included.

Astronomers have long studied other cosmic hazards like supernova and active galactic cores, but kilonova incidents are less common and less understood.

This perspective is shared by Dr. Haille M. L. Perkins from the University of Illinois, who cautions that any source emitting powerful ionizing radiation could threaten Earth-like biospheres under the right conditions.

Understanding neutron stars – the basics

Neutron stars are the dense, burnt-out cores of massive stars that exploded in supernovae. When a star much bigger than our Sun reaches the end of its life, it collapses under its own gravity.

The outer layers blast off, and the core compresses so tightly that protons and electrons smoosh together to form neutrons.

What’s left behind is a city-sized object with more mass than the Sun – so dense that a single teaspoon of it would weigh about a billion tons.

These stars may be small, but they pack a punch. Some spin hundreds of times per second, shooting out beams of radiation like cosmic lighthouses – we call those pulsars.

Their magnetic fields are ridiculously strong, and in some extreme cases, they become magnetars, which are basically the most intense magnets in the universe.

Even though they’re incredibly far away, neutron stars give scientists clues about the fundamental laws of physics. They’re like nature’s ultimate stress test for matter.

Neutron stars and kilonovas

A kilonova is linked to the smashing together of two neutron star remains. These stars are made of matter packed so tightly that a handful of it outweighs a skyscraper.

When they merge, they emit streams of gamma-ray and X-ray radiation that might strip away protective atmospheric layers on a planet.

Ionizing light is measured by how easily it can disrupt molecules. This kind of radiation is concerning because it can damage the ozone.

Once the ozone thins, harmful solar ultraviolet can reach the surface and damage living organisms.

Kilonova energy strips planet protection

Experts discuss the potential harm from these events by focusing on total energy delivered over a few years. The energy is tallied in kilojoules per square meter.

When it surpasses a certain level, the atmosphere starts losing its ability to block solar ultraviolet rays. This threshold can be reached if a nearby kilonova releases high-energy photons toward a planet.

“Ionizing radiation from these sources can be dangerous for life on Earth-like planets when located too close,” wrote the authors.

Multiple bands of the electromagnetic spectrum can hit an orbiting world, but the biggest threat is often linked to intense flashes of gamma or X-ray energy.

Neutron star beams can still harm

Short bursts of gamma rays, sometimes called short gamma-ray bursts, often come from these collisions. Many earlier studies focused on planets directly aligned with the beam from such a burst.

Researchers now address the less obvious danger lurking at wider viewing angles. Even if the planet is not in the direct path of the beam, scattered emissions might still be strong enough to matter.

Observations of one event called GW170817 showed that off-axis viewers still detect measurable radiation.

The brightness depends on how the explosion interacts with surrounding gas. Lower-density regions will produce weaker afterglows than denser zones.

This means damage estimates can vary a lot from one collision to another.

Why cosmic rays matter

Months or years after the collision, faster jets stop dominating, but a slower blast wave can produce cosmic rays.

These energetic particles can penetrate deeply into atmospheres, form reactive molecules, and further harm ozone layers. They can also create particles called muons that reach the ground.

Some scientists suggest cosmic-ray exposure may cause widespread harm to cells, prompting genetic mutations.

Calculations show that if a kilonova is close enough, this late-stage wave might outdo the earlier gamma-ray flash in terms of long-term biological risk.

Rarity vs. impact

The distance at which these effects become lethal is small on galactic scales. Because kilonova events occur very infrequently, few worlds are ever inside the zone of concern.

The odds of one striking near Earth seem slim. Scientists often compare their kill distance to that of a supernova, but supernova eruptions happen far more often.

While the energies involved could be comparable, the scarcity of kilonova collisions means they pose less threat on average.

Researchers believe the chance of a planet like ours being hit by damaging radiation from one is significantly lower than the chance of being exposed to cosmic rays from a supernova.

Possible dangers to technology

A short gamma-ray surge could trigger an atmospheric ionization cascade. This might set off strong electrical currents that overload power grids.

Auroras bright enough to see in daytime skies might appear, and satellites could experience major outages. Astronauts beyond Earth’s shielding would be especially vulnerable to incoming high-energy particles.

On the ground, any meltdown of electronics might be overshadowed by the biological hazards. Elevated radiation at the surface can threaten livestock, crops, and water sources if the influx of ionizing particles persists.

Future neutron star data may reveal more

Only a single event of this type, GW170817, has been observed in enough detail to model possible harm. More detections will add clarity.

Ongoing advances in space-based and ground-based observatories will help astronomers gather more data on these rare outbursts.

Scientists involved in modeling kilonova activity hope that future studies will settle lingering questions about how fast each type of emission fades.

Refined calculations will clarify if cosmic rays pose the strongest hit or if X-rays and gamma rays dominate the danger.

The study is published in The Astrophysical Journal.

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