We've got drought in California, an economic crisis in Europe, random mayhem all over, and you're fretting about supernovas? Well, somebody has to. Luckily, one of our top individuals has already done the spadework. Let me introduce astronomer Martin Beech, author of a 2011 paper, "The Past, Present, and Future Supernova Threat to Earth's Biosphere." Short version: You can renew your magazine subscriptions for another year.
To set the table for the long version, let's talk about existential threats. These can be categorized as follows:
First order. We may think of these as hundred-year perils, indicating the timeframe within which the cloacal discharge has a decent chance of encountering the rotary distributor. Examples abound: nuclear war, global warming, resource exhaustion. If the most alarming predictions have yet to become reality, that's partly because some of the biggest brains in our species have devoted a good chunk of the available clock cycles to heading them off.
Second order. These are the thousand- to hundred-thousand-year threats. The classic example, brought to mind by my recent column on the subject, is the next ice age, once thought to loom but now indefinitely postponed. Modern humans—people who, with a little cleaning up, you could invite over for an evening of spare ribs and pinochle—have endured at least one such trial already. For now, global warming has pushed the return of the glaciers to the back of the freezer, as it were. But there's much we don't understand. Who the hell knows?
Third order. These are perils that may come to pass in the millions or tens of millions of years. Here we're talking about giant asteroid strikes, great extinctions (I realize one can cause the other), and other cataclysms that permanently affect the planet and everything living on it. So far as we know, no such event has occurred within human experience. That said, there's good evidence these things have happened, and they might again.
Fourth order. These are disasters that may strike within the hundreds of millions to billions of years—"very low probability, high-consequence problem[s]," as Professor Beech puts it. This is the territory he's staked out. In the densely argued paper referred to above, he makes the following observations:
A typical supernova must occur within 10 parsecs (about 33 light-years, or 192 trillion miles) of Earth to have any noticeable effect on terrestrial life. The closest star that's on track for supernovadom, IK Pegasi (a binary pair, actually), is 46 parsecs away; Betelgeuse, considered the star likeliest to go supernova next, is 197 parsecs distant. Arcturus, since you asked, is 11 parsecs away. Seeming implication: don't worry about it.
Not so fast. No supernova candidates are within striking distance of us now. However, as the stars wheel through the cosmos over astronomical time, some may come within range.
Another cosmic peril is gamma-ray bursts (GRBs), focused beams of high-energy radiation thought to be thrown off by, among other things, massive supernovas. GRBs have a lethal range of 1,000 parsecs. The GRBs we detect (they're quite common) come from galaxies much further away and so are harmless. However, if: 1. a massive star within 1,000 parsecs of us were to go nova, and 2. said nova produced a GRB, and 3. said GRB were pointed in our direction, Earth would be smoked.
What are the chances? Professor Beech calculates that, "over the remaining life of the biosphere"—that is, the roughly 2 billion years he thinks we've got left before our dying sun snuffs out earthly life—we may get belted by one GRB and 20 supernovas, or roughly one every 100 million years. Indeed, Earth may have already gotten smacked a few times.
But—a very big but, if I may speak frankly—we don't know this for a fact. Although scientists have conjectured that this or that turning point in Earth's history stemmed from the planetary surface having been crisped by a close-up supernova, evidence confirming this is lacking. (A few supernovas have been spotted in historical times, but all were at harmless distances.) In other words, the risk posed by supernovas and GRBs is entirely theoretical.
Granted, theorizing is what scientists do. Professor Beech concedes the threat is "not one of immediate concern," immediate being defined as within the next several million years. However, in his view that's no reason to put the matter out of our minds. He think it's worth looking near other pre-nova or just-past-nova stars for signs of large-scale astroengineering projects constructed by advanced civilizations attempting to shield themselves from incineration.
We'll let the professor brood about that. As for you, Trevor: If you want to have the occasional palpitation about giant asteroids, go for it. But cross supernovas off your list.
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