Knock 'em dead: how does one extinguish life on Earth? Let me count the ways

Natural History, May, 2005 by Neil deGrasse Tyson

You want more? Astrophysicists have imagined a nearly endless spectrum of awesome catastrophes. Right now, for instance, the Milky Way Galaxy and the Andromeda Galaxy, a near twin of ours 2.4 million light-years up the road, are Falling toward each other. In about 7 billion years they may collide, causing the cosmic equivalent of a train wreck. Gas clouds would slam into one another; stars would be cast hither and yon. If another star swung close enough to confound our gravitational allegiance to the Sun, our planet could get flung out of the solar system, leaving us homeless in the dark.

That would be bad.

Two billion years before that hap pens, however, the Sun itself will swell up and die of natural causes, engulfing the inner planets--including Earth--and vaporizing all their material contents.

That would be worse.

And if an interloping black hole comes too close to us, it will dine on the entire planet, first crumbling the solid Earth into a rubble pile by virtue of its unstoppable tidal forces. The remains would then be extruded though the fabric of space-time, descending as a long string of atoms through the black hole's event horizon, down to its singularity.

But Earth's geologic record never mentions any early close encounters with a black hole--no crumbling, no eating. And given the number of neighborhood black holes and their rate of formation, I'd say we have more pressing issues of survival before us.

How about getting flied by waves of high-energy electromagnetic radiation and particles, spewed across space by an exploding star?

Most stars die a peaceful death, gently shedding their outer gases into interstellar space. But one in a thousand--the star whose mass is greater than about seven or eight times that of the Sun--dies in a violent, dazzling explosion called a supernova. If we found ourselves within thirty light-years of one of those, a lethal dose of cosmic rays--high-energy particles that shoot across space at almost the speed of light--would come our way.

The first casualties would be ozone molecules. Stratospheric ozone ([O.sub.3]) normally absorbs damaging ultraviolet radiation from the Sun. In so doing, the radiation breaks the ozone molecule apart into oxygen (O) and molecular oxygen ([O.sub.2]). The newly freed oxygen atoms can then join forces with other oxygen molecules, yielding ozone once again. On a normal day, solar ultraviolet rays destroy Earth's ozone at the same rate as the ozone gets replenished. But if there were an overwhelming high-energy assault on our stratosphere, the ozone would be destroyed too fast--leaving us in desperate need of sunblock.

Once the first wave of cosmic rays took out our defensive ozone, the Sun's ultraviolet would sail clear down to Earth's surface, splitting oxygen and nitrogen molecules as it went. For the birds, mammals, and other residents of Earth's surface and airspace, that would be bad news indeed. Free oxygen atoms and free nitrogen atoms would readily combine. One product would be nitrogen dioxide, a component of smog--which would darken the atmosphere and cause the temperature to plummet. A new ice age might dawn even as the ultraviolet rays slowly sterilized Earth's surface.