Cosmic Cataclysms: The Threat of Supernovae and Neutron Star Collisions to Earth

The universe is a vast expanse of celestial bodies, each following their own cosmic journey. Stars, like our sun, are remarkably constant, varying in brightness by only 0.1% over years and decades. This stability is due to the fusion of hydrogen into helium, a process that powers stars and will keep our sun shining steadily for about 5 billion more years. However, when stars exhaust their nuclear fuel, their deaths can lead to spectacular pyrotechnics.

The Death of a Massive Star

Stars more than eight times more massive than the sun die violently in an explosion called a supernova. These cataclysmic events happen across the Milky Way only a few times a century, and are usually remote enough that they go unnoticed on Earth. For a dying star to have any effect on life on our planet, it would have to go supernova within 100 light years from Earth.

When a star massive enough to die in a supernova does so, it briefly rivals the brightness of billions of stars. The dying star emits high energy radiation as gamma rays and releases a torrent of high-energy particles in the form of cosmic rays: subatomic particles moving at close to the speed of light.

Radiation Damage

If a star goes supernova close enough to Earth, the gamma-ray radiation could damage some of the planetary protection that allows life to thrive on Earth. Astronomers have found evidence of a supernova 300 light years away that exploded 2.5 million years ago. Radiation from gamma rays eroded the ozone layer, which protects life on Earth from the sun’s harmful radiation. This event would have cooled the climate, leading to the extinction of some ancient species.

When Neutron Stars Collide

Supernovae aren’t the only events that emit gamma rays. Neutron star collisions cause high-energy phenomena ranging from gamma rays to gravitational waves. Left behind after a supernova explosion, neutron stars are city-size balls of matter with the density of an atomic nucleus, so 300 trillion times denser than the sun. These collisions created many of the gold and precious metals on Earth.

A neutron star collision generates an intense burst of gamma rays. These gamma rays are concentrated into a narrow jet of radiation that packs a big punch. If the Earth were in the line of fire of a gamma-ray burst within 10,000 light years, or 10% of the diameter of the galaxy, the burst would severely damage the ozone layer. It would also damage the DNA inside organisms’ cells, at a level that would kill many simple life forms like bacteria.

The most extreme astrophysical events have a long reach. Astronomers were reminded of this in October 2022, when a pulse of radiation swept through the solar system and overloaded all of the gamma-ray telescopes in space. It was the brightest gamma-ray burst to occur since human civilization began. The radiation caused a sudden disturbance to the Earth’s ionosphere, even though the source was an explosion nearly 2 billion light years away. Life on Earth was unaffected, but the fact that it altered the ionosphere is sobering—a similar burst in the Milky Way would be a million times brighter.

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