Good morning, Dr. Zoomie! So…I read something online where an astronomer said that this big star, Betelgeuse (don’t say it three times!) is about ready to blow up and that it’s pretty close to us (Is Betelgeuse About to Become a Supernova? (msn.com)). What’s it going to do to us here on Earth? Should I quit my job and start working on my bucket list?
Well…for starters you need to realize that when an astronomer says “soon” they mean “sometime in the next million years” and when they say “close” they mean “even traveling at the speed of light it’ll take a few lifetimes to reach.” So, by astronomical standards, Betelgeuse is just down the street and it’s going to blow up in the next couple of days. Don’t hold your breath…and definitely don’t quit your job. Working on your bucket list – that one’s your decision. But this is the easy stuff – let’s talk about what happens to “nearby” planets when a star explodes.
I won’t go into the nitty-gritty details of how stars age and why they explode – for this, let it suffice to say that an exploding star (a supernova) outshines an entire galaxy for a few weeks and they pour a ton of energy and radiation into space. Anything nearby (in this case, “nearby” means anything withing about 10 light-years is going to have a rough time.
The reason for this is that a supernova is a huge event and it’s going to spread some damage. If a hand grenade goes off, everyone within about 10 meters is likely to be killed and injuries will occur even further away. In the case of a supernova, the “zone of death” is measured in tens of light-years and the radiation can be damaging (i.e. some deaths and a lot of injuries) to at least three times as far away – to about 100 or so light years under some conditions. But further than that…even something as massively energetic as a supernova isn’t going to have much of an effect. Or, rather, it might produce a deadly dose of radiation to anything that’s in space (think of bacteria hitching a ride inside a meteor or comet traveling between stars), but once the radiation has filtered through an atmosphere the dose is so low that it’s not going to kill most organisms.
When a star explodes it gives off a lot of energy in a variety of forms. There’s the kinetic energy of the bits of the star that are blasted into space, the light it emits (again, this single stellar explosion will outshine an entire galaxy for a few weeks), there’s the energy of the neutrinos it emits, ultraviolet light, and then there’s the radioactive material the supernova forms. Let’s take these one at a time.
Kinetic energy: A supernova explosion blasts much of the material that made up the star into space at high velocities. Just as the kinetic energy in a quickly thrown baseball can sting your hand, the kinetic energy of a star’s outer layers blasted off into space can also damage small objects (like planets) in their path. Luckily, the density of this material falls off quickly with distance so the impact is not very substantial even a few light years away – and certainly isn’t an issue with a star that’s several hundred light years distant. So the kinetic energy from a supernova isn’t going to pose a threat.
Neutrinos: When a star explodes it emits a LOT of tiny particles called neutrinos – when I say “a lot” I mean it’s a number that even impresses astronomers (a billion, trillion, trillion, trillion, trillion of them). Unlike other forms of radiation, neutrinos fly unimpeded from the star’s core because they barely interact with matter and they pour into space. But the same property that lets them escape the star also keeps them from being a threat – neutrinos can pass through the entire earth without slowing down and our bodies certainly won’t affect them (nor they, us) in the slightest. Lethal Neutrinos (xkcd.com)
Direct ionizing radiation (UV, x-ray, gamma): Among other things, when a star blows up it gives off a lot of UV, x-ray, and gamma radiation…that’s the bad news. The good news is that much of this is absorbed by the clouds of gas and dust the star started giving off millions of years before it died as well as by the layers of gas blasted into space by the explosion. The bad news is that enough can escape to still cause problems for a planet that’s close enough. In particular, the massive UV blast can destroy a planet’s ozone layer and can cause photochemical reactions in the atmosphere, causing the planet to become blanketed in a brown smog that will lower global temperatures for years or decades. And some unusually “x-ray luminous” supernovae can do the same (https://arxiv.org/abs/2210.11622).
Delayed gamma radiation: What’s impressive is that supernovae continue producing radioactivity for a few years after they explode, although not in a way you might expect. Specifically, one type of supernovae produces radioactive nickel-56 when they explode and the nickel decays to form radioactive cobalt-56; each of these emits gamma radiation when they decay. That’s sort of cool, but here’s what’s impressive – the amount of these radioactive materials produced weighs as much as our Sun. That’s a lot of radioactivity and the total energy of the gamma rays produced is about 1% of the energy released by the explosion itself and it comes out over the course of just a handful of years. Luckily for life on nearby planets, over 90% of this gamma radiation is absorbed by the supernova debris, but even the 1% that escapes can be dangerous to a distance of a few lightyears.
And still more radioactivity! A supernova produces much more than Ni-56 and Co-56 (both of which will decay away in a relatively short period of time). It also produces radioactive iron (Fe-60), manganese (Mn-53), and plutonium (Pu-244), each of which will survive long enough to retain their radioactivity for the millions of years it takes to reach nearby planets. But these, too, are too sparse to cause problems when they arrive. In fact, scientists have found traces of each of these on Earth, presumably from nearby supernova…at least one of which seems to have been close enough (about 100 light-years away) that it might be linked with the extinction of some of North America’s megafauna about 2-3 million years ago (https://www.universetoday.com/140856/a-supernova-2-6-million-years-ago-could-have-wiped-out-the-oceans-large-animals/).
In addition to the Pliocene extinction, astrophysicist Adrian Melott has found evidence suggesting that a supernova might also have been involved in an even earlier mass extinction – the Ordovician extinction about 443 million years ago. So there’s no doubt that exploding stars can have a significantly bad impact on life on our planet (although only twice that we know of in at least 500 million years, so it’s not an everyday occurrence). But the question here is what Betelgeuse could do to us – and the answer is “None of the above” because it’s between 500-600 lightyears away. By the time the radiation and debris reaches us it’s far too diffuse to cause any harm.
So the bottom line…supernovae can – and have – had traumatic effects on life on Earth in the past. But when Betelgeuse explodes, whether it’s next year or long after we’re all gone, its only impact will be giving us a blazingly bright star, bright enough to see even during the day, for a couple of years.