Supernovae are some of the most spectacular events in the universe, capable of releasing in one cosmic flash the energy produced by the Sun in its lifetime. So, if you care about your wellbeing, you shouldn’t be near one.
Unfortunately, there’s no precise cut-off point where astronomers are certain the danger zone ends. Currently, they estimate that if a star goes supernova 40 to 50 light-years away, we should be safe. However, there are many uncertainties. As is often the case with astrophysical sources, things are not as simple as we would like them to be.
“We think that cosmic rays might play a bigger role than photons over a certain distance,” Professor Adrian Melott told IFLScience. He and his team estimated the 50 light-year safety limit in a study in 2017.
With cosmic rays coming into play, magnetic fields need to be taken into account. The Earth's magnetic field plays a role, as do the ones related to the Solar System and the local area of the Milky Way. In addition, both the direction of the cosmic ray stream and what’s going on its path affect the amount of cosmic rays that could get here.
Let's say they did get here: While the light from the supernova would pass instantaneously, cosmic rays – a mixture of extremely fast protons and electrons – would stream from the source for 10,000 years. Once they reach our planet, they would ionize the ozone layer, and without the ozone layer, more UV light would reach the ground.
This might not sound like a global catastrophe with boiling oceans and melting mountains, but 10,000 years of higher-than-average cosmic radiation would be enough to send many species to an early grave. This happened in the past and it might happen once again. But we don't have to worry about it just yet because there are no dangerous stars nearby about to go supernova.
Stars go supernova when their core suddenly cannot hold back the weight of the star itself. Some go boom because they are too massive and they've consumed so much of their fuel that there’s no energy to keep them up anymore – these are the core collapse ones. Others instead are formed by a white dwarf stealing material from a companion, until they collapse on themselves. These are known as Type Ia.
Picking the 50 light-year limit as our boundary, there are about 2,000 stars in this region of the Milky Way. Most of these are extremely dim stars. Only 133 are bright enough to be visible, and none of them appear to be getting ready to explode.
The closest “candidate” for an explosive demise is IK Pegasi 150 light-years away. This is a Type Ia supernova, but it will take so long for the star to actually explode that it will have gone even further away from Earth.
For core-collapse candidates, we need to look at supergiants, which are the vast majority of the core-collapse supernovae. Still don’t expect them to explode in a matter of hours. They can take up to 30 million years before they explode. Antares and Betelgeuse are believed to be the closest supergiants to Earth, but again we are safe from them as both are over 600 light-years away.
Our current safety shouldn’t make us underestimate just how violent and powerful these events are, though. For example, if we make the amount of light we get from an average supernova to be no more than the light we get on Earth from the Sun, then this exploding star has to be 3,600 light-years away. Obviously, a supernova releases a lot of energy very quickly, so at that distance, we might see a bright blip before it gets dimmer and dimmer.
But what if a supernova were to explode at the same distance as the nearest star to the Sun? Well poor us. With such a catastrophe only 4 light-years away, every square meter of Earth would receive about 4.66 billion watts of power. Back-of-the-envelope calculations (the full blast delivered in just 1 second and the Earth made of a single uniform component) suggest an increase in temperature of thousands of degrees.
A more sophisticated analysis would need to take into account the fact that our planet is made of different materials, the absorption of the atmosphere, and the amount of time for the energy to be delivered, among more complex astronomical considerations. However, we know full well that even an increase of a few degrees is catastrophic. We wouldn’t want one side of the planet to be suddenly hundreds of degrees hotter.
But if roasted by a supernova is how Earth will go, the danger is truly in the distant future. No exploding stars will kill us in our lifetime, and we hope that this is at least a little bit comforting.