A short history of nearly everything - Bill Bryson [18]
In 1987 Saul Perlmutter at the Lawrence Berkeley lab in California, needing more Ia supernovae than visual sightings were providing, set out to find a more systematic method of searching for them. Perlmutter devised a nifty system using sophisticated computers and charge-coupled devices—in essence, really good digital cameras. It automated supernova hunting. Telescopes could now take thousands of pictures and let a computer detect the telltale bright spots that marked a supernova explosion. In five years, with the new technique, Perlmutter and his colleagues at Berkeley found forty-two supernovae. Now even amateurs are finding supernovae with charge-coupled devices. “With CCDs you can aim a telescope at the sky and go watch television,” Evans said with a touch of dismay. “It took all the romance out of it.”
I asked him if he was tempted to adopt the new technology. “Oh, no,” he said, “I enjoy my way too much. Besides”—he gave a nod at the photo of his latest supernova and smiled—“I can still beat them sometimes.”
The question that naturally occurs is “What would it be like if a star exploded nearby?” Our nearest stellar neighbor, as we have seen, is Alpha Centauri, 4.3 light-years away. I had imagined that if there were an explosion there we would have 4.3 years to watch the light of this magnificent event spreading across the sky, as if tipped from a giant can. What would it be like if we had four years and four months to watch an inescapable doom advancing toward us, knowing that when it finally arrived it would blow the skin right off our bones? Would people still go to work? Would farmers plant crops? Would anyone deliver them to the stores?
Weeks later, back in the town in New Hampshire where I live, I put these questions to John Thorstensen, an astronomer at Dartmouth College. “Oh no,” he said, laughing. “The news of such an event travels out at the speed of light, but so does the destructiveness, so you'd learn about it and die from it in the same instant. But don't worry because it's not going to happen.”
For the blast of a supernova explosion to kill you, he explained, you would have to be “ridiculously close”—probably within ten light-years or so. “The danger would be various types of radiation—cosmic rays and so on.” These would produce fabulous auroras, shimmering curtains of spooky light that would fill the whole sky. This would not be a good thing. Anything potent enough to put on such a show could well blow away the magnetosphere, the magnetic zone high above the Earth that normally protects us from ultraviolet rays and other cosmic assaults. Without the magnetosphere anyone unfortunate enough to step into sunlight would pretty quickly take on the appearance of, let us say, an overcooked pizza.
The reason we can be reasonably confident that such an event won't happen in our corner of the galaxy, Thorstensen said, is that it takes a particular kind of star to make a supernova in the first place. A candidate star must be ten to twenty times as massive as our own Sun and “we don't have anything of the requisite size that's that close. The universe is a mercifully big place.” The nearest likely candidate he added, is Betelgeuse, whose various sputterings have for years suggested that something interestingly unstable is going on there. But Betelgeuse is fifty thousand light-years away.
Only half a dozen times in recorded history have supernovae been close enough to be visible to the naked eye. One was a blast in 1054 that created the Crab Nebula. Another, in 1604, made a star bright enough to be seen during the day for over three weeks. The most recent was in 1987, when a supernova flared in a zone of the cosmos known as the Large Magellanic Cloud, but that was only barely visible and only in the southern hemisphere—and it was a comfortably safe 169,000 light-years away.
Supernovae are significant to us in one other decidedly central way. Without them we wouldn't be here. You will recall the cosmological conundrum with which we ended the first chapter