What would happen if a supernova detonated close to Earth?

How devastating would a supernova explosion be if it were too close to Earth, and could our planet’s magnetic field stifle the blast?

By Lewis Dartnell

Published: Thursday, 16 March 2023 at 12:00 am

Supernovae are one of the Universe’s most cataclysmic events. When massive stars run out of nuclear fuel in their core, they can no longer hold themselves up against the immense crush of their own gravity and so suddenly collapse.

This creates enormous shockwaves through the star and the outer layers explode violently outwards.

The expanding shell of material sweeps through interstellar space and can trigger the formation of new stars.

But some of these particles end up with a huge amount of energy and are shot across the galaxy as cosmic rays.

""
Supernova remnant LMC N63A. Credit Enhanced Image by Judy Schmidt (CC BY-NC-SA) based on images provided courtesy of NASA/CXC/SAO & NASA/STScI.)

Earth is being constantly bombarded by a background flux of such galactic cosmic rays and they are one of the major hazards to astronauts venturing beyond the shielding effects of the planet’s atmosphere and magnetic field.

But if a supernova were to detonate particularly close to Earth, the point-blank blast of cosmic radiation could have serious consequences on a global scale.

Computer simulations have shown that the pulse of energetic particles would drive a lot of chemistry in the atmosphere, creating compounds like nitrogen dioxide that cause acid rain and chemically attack the ozone layer (and so lead to greater levels of ultraviolet radiation from the Sun reaching Earth’s surface).

Some of the energetic particles created during these interactions – such as muons – also make it to the surface and so can pose a direct radiation hazard to life.

Could our magnetic field save us?

""
Could Earth’s magnetic field save us from the devastating effects of a supernova blast? Credit: Elen11 / iStock / Getty Images Plus

But there’s been an important detail missing from previous studies on the terrestrial effects of cosmic rays from nearby supernovae.

Due to the fact that they are charged, cosmic ray particles are deflected by magnetic fields; an effect that is more pronounced for the lower-energy radiation particles.

This means that as the pulse of particles blasting out from a supernova travels to Earth, the lower-energy particles are scattered more by the magnetic fields threading through interstellar space and are effectively suppressed.

Brian Thomas and Alexander Yelland, at Washburn University in Kansas and MIT respectively, ran computer simulations that include this filtering effect.

They found, as they had expected, that it reduces the impact of the cosmic ray surge on Earth.

""
The dynamo effect of the Earth’s spinning molten core produces our planet’s magnetic field, which prevents the solar wind from stripping away our atmosphere. Credit: Naeblys / Getty ImagesCredit: Naeblys / Getty Images

The filtering effect is much more significant for more distant supernovae.

At, say, 320 lightyears from Earth, a supernova probably wouldn’t have any noticeable terrestrial effects.

But for a supernova around 160 lightyears away, the enhanced stream of cosmic rays hitting Earth is still 10 times higher than background levels and lasts for centuries.

For an especially close supernova, at 65 lightyears, Thomas and Yelland calculate that the flood of cosmic rays would surge to around 200 times higher than normal.

This would have dire consequences for the planet, with at least 30% of the ozone layer destroyed around the planet, and as much as 87% in polar regions.