Will we all die if Betelgeuse goes supernova?

The red supergiant star Betelgeuse, the ‘shoulder’ of Orion, has been dimming of late. As we saw last week, that’s possibly a sign it might be about to go supernova. Odds are on that won’t happen. But what if it does? One of the actual ways an apocalypse could descend upon Earth is when a nearby supernova explodes. These detonations are among the largest known and deliver massive amounts of energy to their near vicinity. By ‘near’, of course, I’m referring to astronomical distances.

That energy takes the form of neutrinos (which pass through just about everything without affecting it) and electromagnetic radiation – everything from radio waves through heat, light, ultraviolet, on up to x-rays and gamma rays. The more dangerous are the shorter wavelengths, because they contain much more energy.

If Earth was struck by such a blast, our magnetic field and the atmosphere would protect the surface from quite a bit of the damage. But not all of it; and there’s a chance that the ozone layer would take a real blasting; calculations indicate a Type II supernova some 26 light years away would soak Earth with enough energetic radiation to reduce the ozone layer by half.  Along the way, chemical reactions triggered in the atmosphere could create a planet-wide smog of nitrous oxide (yup, laughing gas), reducing the surface to darkness for a short time. It’ll dissipate, but the ozone layer will take longer to recover. All that is bad news for plants, particularly.

But that isn’t the only problem. Every element heavier than iron comes from one place, and one only: supernova detonations. The energies involved in a supernova are so colossal that all the heavier elements are formed by them through fusion. And all of them are violently radioactive at first.  They are blown out by the supernova detonation into a nebula that expands at a high speed – radically below light speed, but still high by interstellar standards – in directions driven by the way the explosion developed. Not all are symmetrical: the Crab Nebula, which formed from a supernova detonation in 1054, gets its name for good reason.

The Crab Nebula; a 6-light year wide nebula formed by a supernova burst in 1054 AD, which was observed on Earth. Credit: NASA/HST.

Those radioactive elements will shower whatever they hit as they travel into the vast yonder. Luckily the radioactivity dissipates fairly quickly, except for the elements that are, themselves, naturally radioactive. However, it doesn’t dissipate quickly enough to prevent everything near the supernova being dusted with them.

It takes time for such particles to move interstellar distances. But arrive they will, long after the original explosion, depending on how distant the supernova is and how fast the particles are travelling. If Betelgeuse exploded, for instance, it’s been calculated the dust would reach us around 20,000 years later.

Earth has been ‘dusted’ before. In fact, slightly radioactive iron has been found on the sea bed, indicating that the planet was showered with supernova detritus at some time in the past. There is speculation that some of the ‘great deaths’ known in our geological history were caused, in part, by nearby supernova detonation.

In this calculation, the good news is that radiation operates by the inverse-square law: double the distance and you reduce the intensity to a quarter. So it doesn’t take much distance – astronomically speaking – for the ‘lethal zone’ to fade. And the dust cloud, similarly, dissipates with distance as it spreads out, and the particles themselves become less hazardous as time reduces the levels of radioactivity.

As an aside, here’s Betelgeuse in the same position as the Sun in our solar system. I made this with my trusty Universe Sandbox software. Jupiter isn’t far from the surface in this simulation – estimates of radius actually vary. Of more import are the orbital paths; were Betelgeuse to suddenly turn up where the Sun is, the planets wouldn’t have the velocity to orbit it, and would all plunge into the star.

The real question is what classes as ‘nearby’.  Astronomers have a very different vision of ‘near’ than everyday people on Earth, but when it comes to supernovae the calculation is variable anyway, because it all depends on the type of supernova as much as distance. The figures I’ve seen, though, suggest that a Type II supernova within 30 light years would probably cook us. Beyond that, the consensus seems to be that we’d be OK if it was 50 light years away (remembering that inverse-square tail-off). However, some astronomers suggest 100 light years is a better safety margin. (It’s 5.879 <exp> 14 miles, since you ask…)

The good news is that there are about 800 known stars within 100 light years of Earth, and not one of them is able to blow as a Type II supernova.

The better news, as far as Betelgeuse is concerned, is that at minimum it’s 450 light years away, and at maximum maybe 800. The usual consensus is around 650.  The huge uncertainty is due to the difficulty measuring its parallax – which I’ll explain in another post. However, any of those distances mean that when the star blows, we’ll get a light show and lots of science data. The star itself will be bright enough to see in daylight, and maybe hurt your eyes – magnitude will likely be -12.4, and it’ll be a tiny dot. For comparison the full moon at perigee has a magnitude of -12.9, spread out across the 0.52 degrees average width, seen from Earth.

But there’ll be no danger to Earth when Betelgeuse blows. Luckily for us. If, of course, it blows any time soon. Odds are on it won’t – the current unusual dimming might be a precursor to a blast. Or not. Its estimated remaining life is over 100,000 years, so if it blew tomorrow, we’d be lucky. I should add that, technically, if we DO see it blow up tomorrow it means it actually blew around 1370 and the light has only just reached us.

Personally I think the bigger danger facing humanity in 2020 is humanity. But maybe that’s just me.

Copyright © Matthew Wright 2020

11 thoughts on “Will we all die if Betelgeuse goes supernova?

    1. Yeah, no danger from Betelgeuse if it blows. Signs are it won’t – this current dimming seems to be just an extreme example of its usual fluctuations. The star to watch for, in point of fact, is Eta Carinae A, which is about 10-11 times further away but also liable to make a bigger bang. It already half blew up – the blast was observed here in the mid-nineteenth century – and now it looks like a small nebula. Again, no real danger here but it’ll yield a lot of science and likely a spectacular light show when it goes. If it goes any time soon…

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  1. I started reading with ‘fear and trepidation’! Bushfires, Corona virus and now Betelgeuse? Can’t tell you how relieved I am that we’re safe. Then again, with human beings, safety is a relative thing.

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    1. I think it would be a very ‘human’ thing if Betelgeuse exploded in the next months, in the sense that it would be received as a harbinger of something given what’s happening in the world just now. What worries me about the coronavirus strain is less the reality of it than the rumour; world economic systems are built on froth and imagination and it doesn’t take much to destabilise them (much the same cognitive principle as a supernova, utterly disconnected from the solar system, being received as a harbinger of human fortune).l

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      1. You’re so right. I was watching something on the ABC news channel last night, and one of the talking heads was saying that if the disruption to manufacturing [in China] continued at its present pace for a few more weeks, the economic effects would be ‘terrible’. I can’t imagine that the whole world has all its eggs in China’s basket, but Australia certainly does. Expect to see the hype turn to a local panic soon. 😦


        1. Not to panic or anything… but the economic impact’s going to be worse than just the fact that China is entwined in everybody’s supply chains and trade links – I have a blog post coming up on it in a couple of weeks. What worries me is the effect on the abstract markets (the ‘financial markets’). My background is that before I was full-time writing, I worked in communications for NZ’s central bank, which gave me an in-your-face view of all this plus close exposure to economics. I wrote a historical-economic academic paper for their peer-reviewed economic journal comparing the GFC of 2008 with the great depression in NZ: https://www.rbnz.govt.nz/research-and-publications/reserve-bank-bulletin/2009/rbb2009-72-03-02 (I ended up editing that journal, before I left).

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          1. Wow…I had no idea you were involved in so many vital areas. Yes, if the markets get spooked…:/
            If by great depression you mean the one in 1930’s, I most sincerely hope we don’t repeat /that/ awful period.

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            1. As Xena once said, ‘I have many skills’. My rather mercurial life has led me into some interesting places. I never stopped learning. And yeah, that problem of the Great Depression of the 1930s is still with us. It’s a human thing, and for consistent reasons. Sigh…

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