So while I’m running with the theme of supernovae, here’s an interesting thought… What would happen if a supernova happened near us. Not too near, obviously. As delightful as writing about the planet being vapourised may be, the probabilities of a supernova occuring that close are mercifully small. In fact, the probability of a supernova occuring within 50 light years varies (according to who you ask) between 1 every 100 million years, to once every 10 billion years. Not really that frequent, then.
So it might be a surprise to learn that there have been a handful of supernovae near Earth, albeit all in prehistoric times. There’s evidence to suggest that a nearby supernova may have caused the Pliocene extinction 2 million years ago. Likewise, one is believed to have occurred 13 million years ago, and a near-Earth supernova has even been suggested as the cause for the Permian extinction 250 million years ago. How do we know? Well, we don’t, to be honest — especially as supernova remnants only last for a million years or so, which is extremely short lived as astrophysical phenomena go. The isolated neutron stars they leave behind are so faint that only 8 have ever been found. We do, however, have some interesting geological evidence. Geological evidence shows such things as enrichment of Iron-60 (regular iron is lighter, at 56) in oceanic crust, trapped extraterrestrial noble gasses in fullerene cages, shocked quartz crystals, star dust and others. During the permian extinction in particular, things such as increased volcanism, anoxic oceans and a global increase in the carbon-13/carbon-12 ratio may possibly be linked to said supernova.
In fact, supernovae might have been a common sight in the skies during the Pliocene era 2-5 million years ago. A huge molecular cloud known as Scorpius-Centaurus drifted as close as 130 light years from us. Sco-Cen, as it’s abbreviated was then, and still is now, a site of massive star formation. At the time, a huge OB association (a cluster of heavyweight O-type and B-type stars) was in the final stages of it’s life, and they began bursting like cosmic popcorn. Some of humankind’s earliest ancestors might have looked up at the sky, wondering what these strange new lights were. As it happens, the end of the Pliocene saw a huge die-off of UV-sensitive marine creatures, with no obvious cause. The die-off seemingly coincides with iron-60 enrichment in the sediment.
So where does all of this leave us? Well, the estimated minimum safe distance from a supernova is around 160-200 light years. Just for the sake of argument, let’s suppose a type II happened around 100 light years away, well within the danger zone. A type II, incidentally, is the less dangerous of the known varieties, but even so, being that close it could have catastrophic effects. What would happen? What might we see?
Well, the first thing to happen would be a huge burst of neutrinos released as the star’s core collapsed. These would be released at the speed of light, and would show up in neutrino observatories worldwide. Most would pass straight through the Earth, without us ever realising. The danger would be that with the energies contained in these neutrinos, any that scattered off organic tissue could easily damage DNA and cause cellular mutation.
A couple of hours later, the time taken for the shockwave to propagate from stellar core to surface, an immensely bright light would flare into existence in the sky. Accounts by ancient chinese and arabic astronomers of SN 1006 describe it as appearing with the brightness of “half a moon” and being bright enough to cast shadows and read manuscripts by. A supernova as close as 100 light years would be a spectacular sight, probably bright enough to make looking at it uncomfortable. But together with the spectacular beauty, would come a dark twist. A supernova explosion gives off broad spectrum electromagnetic radiation. Visible light would be accompanied by it’s troublesome siblings. While the atmosphere would protect us from the brunt of it, the whole planet would be bathed in gamma and x-ray radiation. This would alter the chemistry of our atmosphere, forming nitric oxide and destroying a significant amount of the ozone layer. The Earth would take over a hundred years to rebuild it’s ozone, allowing harmful UV from the sun to get through in the meantime. The burst of infrared from the supernova might be enough to cause atmospheric warming, causing our atmosphere to expand and possibly increase in pressure slightly. Finally, the radio waves would cause havoc with communications and potentially damage any unshielded magnetic data storage. Any satellites in orbit around Earth would be instantly fried, and any astronauts in space could receive a fatal dose of radiation. The electromagnetic pulse could even affect the Earth’s magnetic field, temporarily disrupting it, and causing increased aurorae in the North and South.
The supernova itself, at a mere 100 light years, could remain visible during the day for weeks. Possibly months, and even after it started to fade, it would remain visible in the night sky for years. Witnesses of SN 1054 (the crab nebula, 6500 light years away) reported that it remained visible in daylight for 23 days and was visible in the night sky for almost 2 years.
But that wouldn’t be the end. Not if that star had exploded close enough.
Accelerated to relativistic speeds of 10% the speed of light, other supernova debris would follow. Nearly a millenium later, the blast wave would finally strike the solar system. We’d get some protection from the Oort cloud and the Sun’s heliosphere, but some would still get through. The first thing we’d see would be a shower of cosmic rays. These would fragment atoms and molecules in the atmosphere into radicals and new elements. Fantastic aurorae may be seen across the globe, as the cosmic rays puncture the atmosphere. Once again, these would cause nitric oxides to form and cause ozone depletion. This time though, the cosmic rays would cause clouds to form, increasing global cloud cover significantly. The planet’s albedo would increase, reflecting more and more sunlight into space, causing a period of global cooling and perhaps even an ice age. This ice age would be a “cosmic ray winter” lasting thousands of years.
Finally with the wave of particles, with enough energy, the solar system would be peppered with energetic atoms and grains of star dust. Debris from the supernova itself, and interstellar dust swept along with it. This would appear to us like a huge global meteor shower. The atmosphere would again protect us from the damaging effects. All we’d have left to do would be to sit back and watch in wonder.
Of course, the probability of a supernova happening that close to us is… well… astronomical. In fact, at a safe distance of 500-1000 light years, I’d actually love to see one. Who knows… If I stay alive for another 75 years or so, there’s a fair chance I might!