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.
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.
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