What If History’s Brightest Supernova Exploded In Earth’s Backyard?

A thought experiment

The Crab Nebula (NASA)

From the vantage point of almost four billion light years away, the brightest known supernova looks a lot like they all do. Last year, the automated telescopes in Ohio State’s ASASSN network saw the peach-fuzz blob of a distant galaxy. By June 2015, that galaxy was gone, all of its stars outshone by a piercing blue point.

Now observations of the aftermath published this week in the journal Science show that the new explosion, ASASSN-15lh, is a few hundred times brighter than your garden-variety supernova, and two or three times brighter than the previous record holder. It’s in a class by itself.

“If you walked outside and saw a person who was six feet tall, and then someone who was six thousand feet tall, you would notice,” says team member Todd Thompson of Ohio State University. “You begin to question whether this is even a person.”

Since June, the supernova hasn’t once wavered below the wattage of every star in our Milky Way galaxy combined. In an average week, the glowing, cooling shrapnel releases more energy than our Sun has emitted ever. If it weren’t so far away, we’d have front row seats to appreciate the spectacle of this superlative natural firework.

So let’s pull it closer.

Astronomers talk in terms of parsecs, with one parsec equal to three and a quarter light years—a little less than the distance between the Sun and Alpha Centauri, the nearest star.  Within a thousand parsecs of us, the supernova would be a point brighter than the full moon, though it would cast an eerie glow much bluer than moonlight.

Exchange it with the star Procyon, the brightest star in the constellation Canis Minor—still farther away than stars like Alpha Centuari and Sirius—and it would outshine the daytime Sun. Let’s settle on a distance in between, at the edge of survivability: 12 parsecs. Make it an even Kessel run. Arcturus, the reddish, fourth-brightest star in the night sky, is at about this distance.

First, a few caveats: Regular supernovae are rare, with a massive star popping its guts out somewhere in the vast Milky Way every century or so. Super-luminous supernovae are rarer, and this thing, whatever it is, is presumably rarer still.

But rare doesn’t mean impossible. In 1996, grad student Brian Fields and his adviser listed out the radioactive elements blasted into space by a supernova that you might be able hunt down on Earth. A supernova close enough to leave a trace, they reasoned, might also have been close enough to pose a serious threat to life.

“If we were really lucky we could connect it to a mass extinction,” says Fields, now of the University of Illinois. “That’s sort of the Holy Grail, or the unholy grail, of the field.”

Happily for life on Earth, the fossil record didn’t offer any evidence of a recent, supernova-fueled mass extinction. But in 1998, German deep-sea researchers emailed Fields after finding a layer of “live,” still-decaying iron isotope at the bottom of the ocean. After decades of follow-up work, Fields now thinks that the iron came from an explosion within a few dozen parsecs in the last two or three million years.

Now consider ASASSN-15lh at the too-close-for comfort distance of the star Arcturus. A few hours before we notice anything amiss, underground detectors on Earth would start picking up neutrinos: tiny, slippery particles that carry away the lion’s share of the energy trapped between a dying star’s hardened core and its atmosphere. Light struggles to escape, but neutrinos slide right out, traveling just below the speed of light.

Photons would soon follow. The supernova would hang as a blinding point in our sky, like a smaller, but much more dangerous Sun. Dangerous because in addition to the visible light, the exploded star would pour  X-rays, gamma rays, and hard ultraviolet radiation into Earth’s atmosphere, obliterating its ozone layer.

Every ecosystem fueled by sunlight—which is all of them, except for deep-sea vents—would feel the pain for decades, as the ozone layer heals. “If you’re a small beastie, you can’t just put on suntan lotion,” Fields says. It would likely trigger a mass extinction. From hard radiation alone, researchers estimate the kill zone of an ordinary supernova to extend out to around 10 parsecs.

Surprisingly, it isn’t clear whether ASASSN-15lh’s extraordinary luminosity would sterilize a wider radius of space than your average supernova. But there are clues. For one, it’s very blue, suggesting it cranks out a lot of UV rays. Using only ASASSN-15lh’s heightened brightness, Thompson estimates it could be deadly even up to three hundred parsecs away.

Alternatively, its insane brightness might actually make it less dangerous. Perhaps it glows so much in visible wavelengths because its expanding cloud of debris is catching harmful X-rays and ultraviolet photons, and transforming them into showier but less-harmful visible light. “It may be so luminous because it has shielded us from the harder radiation,” says Adrian Melott, a physicist at the University of Kansas who has argued that near-Earth supernovae may even have played a role in one of Earth’s “Big Five” mass extinctions.

Melott also said a supernova could trigger sleep disruption throughout the animal kingdom. Its overwhelming blue light would blare from the night sky, like a giant insomnia-inducing laptop screen.

Even if humans managed to survive such an event, we’d be watching its aftermath for generations. Hundreds or thousands of years after the light from the supernova had faded, something like the Crab Nebula would loom over more and more of the sky.

But we probably wouldn’t survive. When the blast front finally hit, the Sun would protect us as best it can. Our Sun emits its own wind, too, which acts as a protective bubble between the solar system and interstellar space by deflecting charged particles. But in the face of the supernova, this protective bubble around the Sun would shrink, exposing outer planets to a bath of cosmic rays. Even if the Earth stays on the inside of the bubble, un-magnetized dust grains slip through, depositing iron like the samples Fields found.

And then for tens of thousands of years, we sit in a sea of hot gas while trapped cosmic rays loop and zigzag along magnetic field lines, steadily barraging the ozone layer, until at long last the cloud of shocked gas wisps away into nothingness and our biosphere is left to pick up the pieces.

It’s all very bleak, but the caveats still apply: ASASSN-15lh is far, far away and supernovae are exceedingly rare.  “We know where the nearest massive stars are,” Fields says. “So they’ll be fun to watch, good fireworks, but nothing to be scared of.”

But for any inhabited planets or intelligent aliens inside ASASSN-15lh’s blast radius, it would have been far worse than a fireworks show. “They had a very bad day,” says Fields.

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