What would happen if the Sun went supernova?

If the thought of our Sun going supernova ever occurred to me, I always assumed that Earth, from crust to the last inhabitant, would be instantly terminated. We’d see a gigantic fireball on the sky, and before we could say “supernova!”, we’d turn into hot vapour.

Not long ago I read a short story that made me question this image. I won’t tell you which story, as it’d be a major spoiler (if you don’t mind spoilers – you can check at the end)*. In this story, the protagonists have ample time to panic, seek solace in drink and even buy crackers and foie gras while waiting for the flaming shock wave.

While I liked the story a lot, I found this picture unlikely: after all, the Sun is only 8 light-minutes away, surely we’d be dead long before we could get the change from the barman? As so often with simple questions, it turned out to be – not so simple. So let’s see it closer: what would happen if the Sun went supernova?

What is a supernova?

We all know what a supernova is: a star collapsing in on itself and causing a helluva big explosion. It takes a specific kind of binary star or a very massive star. The heavier the star, the faster it’s blazing through its lifecycle, burning a lot of candles at both ends. It’s fusing heavier and heavier elements together to produce energy to resist the gravitational pull to shrink. But it cannot continue to the more exotic edges of the periodic table: when it reaches iron, the fusion reaction takes more energy than it produces. From this moment, there’s nothing holding back the mass of the star from collapsing inward.

How long does all this take? It’s surprisingly fast, even on human scale. The last phase of dying, fusing heavier and heavier elements together, lasts a few million years. If we can observe the spectrum of the star, it’s very noticeable, as the fusion reaction is using a sequence of different elements. Carbon-fusing could take 600 years, but neon is done in six months, oxygen-fusing in 6 days, and the final silicon-fusing phase in a single day. Once the silicon-fusion phase is over, the star’s core collapses in less than a quarter of a second. It’s a violently, unimaginably fast process, where the collapsing outer layers are moving at almost ¼ of the speed of light!

The resulting shockwaves reach the star’s surface in a few hours. The star goes on brightening for a few months, though as I said, it depends on variables – the longest brightening stage we’ve observed so far took 3 years. After peak brightness, it’s around a few years for it to fade away. In the meantime, matter from the explosion continues to expand rapidly, mixing with the solar system’s gas and matter. This remnant of the supernova will go on glowing through X-ray wavelengths for thousands of years.

What if the Sun went nova?

Supernova explosions are among the most massive energy-producing reactions of the universe. All this energy goes into a mix of photons (light and heat), the kinetic energy of expanding stellar material, and neutrinos. Any of these three would potentially terminate all life of Earth – but which one would get to us first? And would there be any way to survive?

1. Boiled alive?

The most obvious way to die from a next-door supernova explosion is to be vaporized by heat. The massive wall of heat hitting the daytime side of Earth would blow the atmosphere and oceans off the planet, and would scorch the surface to ashes. Correction – not even ashes would remain, and some of the surface itself would likely be destroyed. Depending on variables, the supernova may not incinerate the whole Earth, but the planet’s temperature could temporarily rise to well above the Sun’s current temperature.

The heat surge would reach us 8 minutes after the explosion, and would be so prompt that we’d hardly notice anything. We wouldn’t even hear the explosion, as sound travels much slower than light, and in any case it propagates too badly in space. We’d be disintegrating before getting frightened at all. The night side of Earth would last a few minutes longer, but unlikely as long as in the referred piece of fiction – so have the crackers and pate ready in your cupboard.

Could anyone survive?

There’s a lot of uncertainty around the estimates, and views range from a) the inner planets, including Earth and Mars, would be totally incinerated to b) Earth, perhaps reduced in mass, would continue to exist, but nothing and nobody on the planet’s surface would survive.

Could you build some kind of underground, heat-shielded bunker? Perhaps – but how you would survive in longer term on a planet devoid of atmosphere, water, ozone shield, vegetation etc. is a tough one. Better spend that effort on building spaceships. We’d need a lot more calculations to determine how far we’d need to go to escape – to Mars? Jupiter’s moons? Pluto? Icy moons, planetoids and dwarf planets are all options for our survival – if only we can get the modelling right and build enough space vessels.

2. Blown to pieces?

Following on the heels of the heat tide, the stellar material would also reach us – trillions and trillions of tons of debris, plus the kinetic shockwave from the explosion. On a normal day, this would sound alarming, but most views concur that we’d be long dead for other reasons before we’d need to worry about the blast. So there aren’t many calculations available for this kind of impact. 

Could anyone survive?

If you survived the heat surge, it’s probable that you would also survive the blast.

3. Ripped by neutrinos?

Many sources agree that we don’t need to worry about scenarios 1) and 2), because neutrinos would finish us off first. During a supernova explosion, neutrino emissions increase by a factor of never mind how much (ten quadrillions, if that means anything to you), and they also become a lot more energetic than normal. As neutrinos move near the speed of light, and they can penetrate and travel through anything, they would reach us first. It doesn’t matter which side of the Earth you are, your body would boil inside out within seconds.

Sources vary when it comes to the safe distance from the supernova: some believe the explosion would deliver a lethal dose to humans up to a distance of 16 astronomical units, about the distance of Uranus from the Sun. Some are more pessimistic, taking the assumption that the lethal radius of a supernova is on the order of 3 parsecs, about 10 light years.

To be sure, there are other views (in minority), who point out that neutrinos are so averse to interact with matter that we need to build extremely costly underground water tanks just to catch a few – so how would these ghost particles kill you, even if they are super high in numbers and energized?

These opinions sustain that in order to receive a harmful dose of neutrinos, you would have to stand right inside the Sun during the midst of a supernova explosion.

Could anyone survive?

Given the range of views above, it’s hard to say with certainty. If the pessimists are right, neutrinos would certainly kill any living creature on Earth: there is no way to build a bunker to stop particles that can easily travel through a kilometer-wide lead wall. We’d need to escape to Uranus, Pluto or beyond for safety.

4. Knocked off orbit?

An interesting variation of doom scenarios is the possibility of Earth being kicked out of its orbit due to the exploding solar shell’s momentum. Or, with similar effect, the Sun losing so much mass that it would not keep Earth on orbit any more. In either case, whatever remained of Earth would wander off into space, and become a rogue planet. While this scenario is less likely than the planet being completely annihilated, astronomers have evidence of a good number of rogue planets around in interstellar space – so it can happen (though not all of them would originate from supernova explosions, obviously).

Could anyone survive?

Supposing that they survived the heat impact/neutrino radiation in their underground bunker, people would face the problem of lack of water/atmosphere/vegetation after Earth as scorched by the massive heat. In addition, survivors would face a permanent night. On the bright side, Earth’s core would stay hot for a very long time (millions, possibly billions of years) due to the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from planet formation. If they tapped into this source, remaining humans could construct some kind of habitat and carry on living indefinitely. Going into the realm of highly imaginary, they could even try to invent some kind of propulsion to navigate Earth towards another star to orbit.

Can the Sun go supernova?

All this being said, the probability of the Sun going supernova is extremely low. A star would need to have at least 8-10 times the Sun’s mass to trigger this kind of cataclysm. Our Sun is expected to go on the way of the red giants: in about 5 billion years, it is expected to swell into a mammoth red star and swallow all the inner planets, including Earth, during its expansion.

For the Sun to become a supernova, our current knowledge of stellar astronomy would need to be fundamentally flawed. Or there should be some dramatic intervention by an unknown factor, such as black monoliths multiplying on the Sun’s surface until it reaches the required mass,** or the Sun colliding with another star. Of these three options, only the probability of stellar collision was calculated: 1 in 10²⁸ years. As this is several magnitudes more than the possible date for the Big Rip, it’s firmly in the “nothing to worry about category.”

Still, we’ve seen that supernovae can pose a threat even at interstellar distances. Is Earth at any risk from explosions by other stars? [coming up soon]

*The short story is Larry Niven’s “Inconstant Moon”

**Black monoliths on Jupiter – reference to Arthur C. Clarke’s “2010: Odyssey Two”