What Supernova Distance Would Trigger Mass Extinction? (script) (Patreon)

Around two and a half million years ago, a human-ish creature looked up at the sky confused at a constellation familiar to her, but unfamiliar to us. She was confused because its brightest star was getting brighter, and quickly. Over around a day, it blazed to become brighter than the full moon, easily visible during daytime. For weeks this young australopithecus and her people-your ancestors-wondered at this new celestial body. And then it faded and vanished entirely, leaving a void in the ancient constellation. Since then, the stars have shifted into patterns that would be as unrecognisable to your great-times-a-hundred-thousand grandmother as hers would be to you. But maybe, relatively soon, astronomically speaking, we’ll share a celestial experience—the wonder and the terror of seeing a familiar star go supernova.

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Yad al-jawzā - the hand of the giant in Arabic - now bastardised by medieval monks and the Tim Burton film to Betelgeuse. It’s the bright red star on the shoulder of Orion. It’s a red supergiant—a star 11 times the mass of our Sun and nearing the end of its life. The thing about supergiants is that we can’t know how far along the death sequence they are. Betelgeuse could have a 100,000 years, or its gamma ray-bright death flash may already be travelling the short 640 light years between us.

Probably more like the former, but recently Betelgeuse has been teasing us. In 2019 it dimmed dramatically for several months—which seemed foreboding, but we now understand was due to it coughing up obscuring material from its own atmosphere. It seemed to recover, but over the past several weeks the star has brightened, and is now half again brighter than normal. Sadly, at least for astronomers, this is probably just a .. fit as Betelgeuse settles back into its regular 400-day cycle. Our best estimates give the star between 10-100,000 years, and besides, a Betelgeuse supernova wouldn’t be close enough to damage the earth. But these events remind us that our night sky and local stellar neighbourhood is NOT a static thing.

And we know a very visible supernova must have occurred around 2.7 million years ago—we know it because we find a layer of unstable isotopes like iron 60—beneath the ocean floor and in the Antarctic snow and even on the Moon. This must have been produced in a recent stellar catastrophe. We also see that the Sun inhabits a bubble of relatively low density material that was carved out by supernova explosions over the past 20 million years or so. Supernovae have lit up our sky recently on astronomical and geological timescales. So how long before we get another? Which star will it be? And should we wish for it?

Let’s start by talking a little about the different types of supernova, because some are much much more dangerous than others—as in they kill ya from farther away.

The deaths of massive stars result in type II supernovae—and I’ll come back to type Is. We also call these core collapse supernovae, because they happen when the star’s core … wait for it … collapses. For a star like the Sun, when it’s done fusing hydrogen into helium in its core, it’ll puff out into a red giant, then go through an extra fit of helium burning, then shed its outer layers non-explosively leaving behind a burned-out ball of hot carbon and oxygen - a white dwarf.

But stars around 8 times the mass of the sun have enough gravitational crush to fuse heavier elements—all the way up to iron. At that point, fusion ends and layered core of heavy elements collapses under gravity, stopping only when the innermost ball of iron transforms into a neutron star. This releases an explosion of neutrinos, that are so numerous and energetic that they blast the surrounding layers back out. That’s the supernova. Many neutrons are also blasted out, slamming into the infalling nuclei to produce more heavy elements—including the iron-60 that led us to our Pleistoscene supernova.

The vast majority of the energy of a supernova is carried away in neutrinos, but by the time these reach us they’re diffuse, and just do what neutrinos do—pass through everything. If a supernova went off in our vicinity, our vast neutrino detectors would go crazy for a bit. Then the light would arrive. Energetic light—gamma rays, X-rays, as well as plenty of visible light. This is when the star increases in brightness 10 to 100,000 times. Our atmosphere prevents the most dangerous photons from reaching the ground, with most of them intercepted by our ozone layer. But if the supernova is close enough, a lot of that ozone can be destroyed, and can take a long time to recover. In that recovery period, a lot more ultraviolet radiation from our own Sun will reach the ground, damaging surface life.

Scientists estimate that the destruction of 30-50% of the ozone layer would result in a mass-extinction event. But in order to do that a supernova would have to be within 15-20 light years.

Lagging behind the wave of neutrinos and light is the actual shock-front of the supernova, which takes 100s to 1000s of years to reach us depending on distance. This is potentially much more dangerous. That shockfront is a wave of particles, kinetic energy, and magnetic fields that leads the expanding supernova remnant. The magnetic fields act as a giant particle accelerator, accelerating particles to near light speed. These become cosmic rays, and may ultimately contain up to 10% of the energy of the supernova. Cosmic rays have an easier time penetrating the atmosphere than do gamma rays, and so a nearby supernova can bombard the planet with a huge increase in this dangerous radiation for thousands of years. And the supernova doesn't have to be so close to us for its cosmic rays to be deadly—within 30 to 50 light years to trigger a mass extinction, depending on the local interstellar medium and the supernova type. This is the real kill-zone for a supernova.

But they can still be dangerous from further out. For example, the pleistoscene supernova is thought to have been 50 to 100 light years away—just outside the kill zone. But there’s evidence that many animals living in shallow layers of the oceans went extinct at around that time, while deeper-dwelling animals were spared. This has been attributed to a massive increase in the muon flux from the supernova. Unlike most cosmic rays, this heavy cousin of the electron can penetrate some distance into water, causing damage to surface layers before being blocked. Cosmic rays can also alter atmospheric chemistry, including destroying ozone. This 2.7 million years ago corresponds to the beginning of the Pleistoscene epoch—a period of cooler climate compared to the previous Plioscene epoch. We also call it the Ice Age with its periodic glaciations, which may or may not have ended yet. It’s highly plausible that the subtler effects of that supernova kicked off the Pleistoscene.

So supernovae can influence the Earth even if we’re outside the kill zone. But there are also ways to extend the kill zone of an exploding star. In a study that came out in April this year, a team of astronomers investigated a new source of supernova unpleasantness. As I mentioned, the quick blast of gamma rays doesn’t last long enough to do substantial damage at any distance likely for a future supernova. But in cases where the progenitor star was embedded in a relatively thick region of gas, that gas can continue to radiate X-rays for hundreds to thousands of years after the initial explosion. Such a supernova could bake our ozone layer to extinction-level depletion from much further distances than a regular supernova - up to 150 Light years.

The other way to extend the kill-range is with a hypernova, in which the gamma rays are beamed in a particular direction. If that direction is us we see a gamma ray burst. We can see these from the other side of the universe, but if one occurs within several thousand light years it can significantly deplete the ozone layer. These are much rarer than regular supernovae, but there’s evidence that we do get smacked every billion years or so.

We’ve covered gamma ray bursts in another episode, so I won’t go into more detail now. Today we’re figuring out the prospects of getting hit by a regular supernova. Which we can now do, because we know the size of the kill-zone. Within 30 or so light years for death by cosmic rays, or 150 light years in special cases for a long X-ray bake.

Betelgeuse is 640 light years away, so no problem there. There are currently just a handful of stars closer to us than Betelgeuse that will one day go supernova. The nearest is Spica at 220 LY. Spica is a blue giant and is just big enough to eventually go supernova. But it’s still burning hydrogen in its core, so is nowhere near death. It’s also too far to cause problems when that inevitably happens.

Before you relax completely, there is an impending supernova that’s closer than these. Just within the maximum range of dangerous at 150 light years we have IK Pegasi. This isn’t a massive star that’s about to die. It’s a hydrogen-burning star a bit more massive and hotter than our Sun in a binary orbit with a white dwarf. Remember white dwarfs? Remnants of stars not massive enough to go supernova. When the living star in the IK Pegasi system approaches the end of its life it’ll start to puff up. Its outer layers will start to stream onto white dwarf, which itself will grow in mass.

When it reaches a critical mass—the Chandresekhar limit at 1.4 times the mass of the Sun, a runaway fusion reaction will rip through the white dwarf interior and it’ll explode as a type 1a supernova. If this were to happen at IK Pegasi’s current distance, we’d get a celestial lightshow comparable to the Pleistoscene supernova, and presumably the whales would have a bad time. Fortunately all of this is a couple of billion years away, and by then IK Pegasi will have drifted to a much safer distance of a few hundred light years.

We seem to be safe for now, but if all stars are moving relative to each other, then eventually a dying massive star or a feeding white dwarf will find its way into the neighbourhood of the Sun. So how long before this happens?

There are different ways to estimate this. One is by seeing how often this has happened in the past. There are historical, geological, and astronomical records of past supernovae in and around the Milky Way. Historically, there was the one we saw in the large Magellanic cloud in 1987, and the one observed by Johanne Kepler in 1604, and several in ancient records, like the one recorded by Chinese and Arab astronomers in 1054 AD, which we now see as the Crab nebula. Geologically there’s the Pleistocene supernova, and also similar evidence for another supernova at around the 7 million year mark. Astronomically we see the local bubble, indicating multiple supernovae in the past 20 million years or so, and in that bubble there’s even a couple of supernova remnants and neutron stars.

We can also look at the frequency of supernovae in other galaxies, which we observe relatively frequently because we’re actively watching 1000s of  galaxies for these things. And the last way to make this estimate is just by counting the massive stars in our galaxy and using our understanding of stellar evolution to project a supernova rate.

All this put together, we get that a supernova of one type or another goes off on average every 50 years inside the Milky Way. Most of these are many thousands of light years away.

Supernovae occur in the 15-20 light years needed to directly roast our ozone layer on the order of a billion years or so—a similar timescale to being hit by gamma ray burst. These may have never happened, but it’s also possible that one or the other or both could have been responsible for some of the extreme mass extinctions in Earth’s long history.

Within the 30-50 light years of the cosmic-ray-death-zone, we expect that to happen on the timescale of a hundred million years or so, with a similar scale for the rarer but longer-range X-ray-bright supernovae. These must have happened before, and will happen again on a similar timescale. We don’t know when, but not tomorrow. We’re currently in a region of the Milky Way disk with a low star formation rate. But when we enter one of the spiral arms that rate goes up, and so does the danger.

The next supernova that you’ll be able to see with your own naked eye could well happen in your lifetime, but it’ll be very distant—likely just a bright star where there previously wasn’t one. The next truly spectacular supernova will probably be Betelgeuse. It’s not entirely impossible, but we’re unlikely to catch that one ourselves, despite its recent rumblings. It’s more likely that this spectacle will be enjoyed by a distant future generation, who may be different from us as we are from the australopithecus proto-astronomer with whom we started this story. For better or worse, we live in a temporarily quiet period, supernova-wise, but we see the evidence and importance and frequency of these stellar cataclysms everywhere, from the irradiated ocean floor to the supernova-blasted bubbles of our local interstellar space time.