What Happens If A Star Explodes Near Earth?

Exploding stars are one of the hazards life faces in the universe, but a nearby explosion could cause anything from a total calamity to a beautiful view, depending on the scale you’re using to define “near”.  Before we get to that, let’s talk about what is meant by “explode”.

Do stars actually explode?

If you want to stretch the definition, the Sun explodes frequently, particularly at the moment when it is at solar maximum. Solar flares and coronal mass ejections are types of explosions, after all. Most of the time all these do is interfere with radio transmissions a little, push a few satellites out of orbit, and give fortunate people a display of pretty lights in the sky. We know explosions like this can be dangerous to technologically advanced civilizations if they haven’t prepared. If you want to know more about the potential consequences of a direct hit from a really large coronal mass ejection, we’ve covered that before, but for most people asking the question, that isn’t what they mean.

Some stars can explode in much more dramatic ways than the Sun ever will. The Sun has at least eight times too little mass to become a core-collapse supernova. Other sorts of explosion, including novae and Type Ia supernovae require two stars in very close orbits around each other, so that is not a fate the Solar System needs to fear either. Kilonovae require both; two very massive stars that are close enough that, after undergoing independent supernova explosions, they eventually collide.

If a star as close to us as the Sun were to undergo any of these life on Earth would wither under the intense radiation, and in some cases, the whole planet would be ejected from the Solar System. The benefit of orbiting a medium-mass star with no stellar companion is there is not the slightest threat of this happening to us. 

On the other hand, even the next closest star, Proxima Centauri, undergoes explosions vastly larger than the Sun experiences and we never noticed until recent advances in telescopes. 

What If A Nearby Star Goes Supernova?

When a star more than eight times the mass of the Sun reaches the end of its life it becomes a supernova. As noted, this has dire effects on anything in its orbit. On the other hand, as Douglas Adams reminded us, space is really, really big, and distance lends safety.

On a cosmic scale, the Large Magellanic Cloud is very nearby, less than a hundred thousandth of the distance to the furthest galaxy we have seen. Yet when a supernova exploded there in 1987, it was only just visible to the naked eye, and physicists were surprised we could detect products such as neutrinos at all.

Even within our galaxy, several supernovae have been seen in recorded history, with none of them doing us any more harm than a crisis of faith among some observers. The most famous of these was in 1054, leaving a legacy of the Crab Nebula. A light in the sky brighter than Venus may have slightly altered the balance between nocturnal predators and prey, but there were certainly no lasting consequences. 

How Close Is Too Close?

Where things get interesting is in the in-between zone: the area far enough from Earth to contain stars that could plausibly become supernovae, but close enough that we might need to worry.

Deep ocean sediments reveal spikes in radioactive elements, particularly iron-60 approximately 3 million and 8 million years ago. Although this is still contested, many scientists attribute these to surges in radiation from supernovas.

Arguments remain about how much damage such events did. Although some attempts have been made to link the likely explosions to changes in climate and some species’ disappearance, the connection is murky. Certainly, we know there was no mass extinction like the end of the dinosaur era at the time. 

We’d expect a repeat to have modestly negative effects – increased radiation might induce more cancers, for example – but the consequences would be much smaller than those of humanity’s own actions.

We don’t know how far away these prehistoric supernovae were, assuming they even happened, but distances like 100-600 light years have been proposed. 

For things to get really serious, therefore, a supernova would have to be even closer.

The Danger Zone

It is thought a supernova within 30 light-years of Earth would present serious risks. If we’re close enough, the heat and light released could significantly raise the planet’s temperature, like suddenly having a second Sun. Meanwhile, higher energy radiation could have very negative effects on the upper atmosphere. Both would fade after a month or so, but by then it could be too late for a great deal of life on Earth. For thousands of years thereafter, very fast protons and electrons would pepper the Earth, destroying the ozone layer.

The effects of a supernova follow the inverse square law, so one 100 light-years away would have a tenth the impact of one at 20 light-years. By 200 light-years the effect would be one-fortieth of one at 30 light-years, which should probably be small enough that we’d be pretty safe. Yet in a galaxy 100,000 light-years wide, 200 light-years is really quite close.

Betelgeuse is the closest star considered a short-term supernova candidate, although probably not for a million years. Measuring its distance has presented some unusual challenges, but estimates range from 530 to 900 light years, putting it well outside the range we need to worry about.

There are several other stars with the mass to become supernovae that are closer than Betelgeuse, including Spica and Alpha Cruxis. However, they’re all millions of years from that point, and about 300 light-years away anyway.

The closest supernova candidate isn’t a giant star at all. IK Pegasi is a binary system that has at least some of the characteristics required to create a Type 1a supernova. However, systems like this evolve much more slowly than those that become core-collapse supernovae, and the danger is very distant indeed.

Are There Special Cases?

Although generally speaking, stars exploding more than 30-50 light-years away don’t pose much threat, there are some possible exceptions.

Some supernovae release much more of their energy as X-rays than others. Being within 100-160 light-years of one of the high X-ray supernovae would devastate the ozone layer even more effectively than chlorofluorocarbons. Without an ozone layer, Earth’s surface would be exposed to lethal quantities of ultraviolet light from the Sun, causing a mass extinction. 

Hypernovae and kilonovae are dramatically more powerful than ordinary supernovae, although they are also much rarer. Consequently, the danger zone for these would be considerably larger. Some astronomers think the giant star Eta Carinae is a hypernova candidate, possibly even quite soon. After all, in the 19th century, it put out as much energy as a typical supernova over a period of years instead of months and survived. However, Eta Carinae is 7,500 light-years away. If a hypernova were to occur at that distance, it would probably look like a supernova hundreds of light-years away, certainly a comfortable distance from our perspective.

One additional concern is that some star explosions are thought to release a lot of their energy in jets a few degrees wide creating gamma ray bursts, This means that if Earth happened to lie within the cone created by these jets, we would experience a much more powerful burst than our distance would lead us to expect. In such an unfortunate case, being within a few hundred light-years, or even several thousand could lead to a surge of gamma ray radiation that once again would destroy the ozone layer.

The flip side of this is that for anyone outside those cones, the radiation would be even less than that expected if the burst was spherical. It’s quite possible we could be 20 or 30 light-years from such an explosion and not be greatly affected if we were outside the cone.

We’re still learning about events like these – the first kilonova was only seen in 2017, so there could be other classes of events, or special types of the ones we know about, that pose a risk at greater distances. However, it seems likely that anything currently unknown would be very rare indeed.

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