Death from the Skies! (10 page)

However, we have survived many small oscillations. The Little Ice Age came and went, with people taking it in stride—they really did skate on the Thames. Huge flares have wreaked havoc on our power grids, and with a little care, foresight, and a pile of money, we can avoid total disaster.

As for the big swings . . . well, we’ll see. They may not happen for centuries or even millennia, and by then we may be able to take action. But the time to start thinking about it all is right now; and we are. Smart people are working on these very topics, and while it may take time to figure out all the angles, and there may be lots of arguments along the way, I think in the end we’ll figure a lot of this stuff out.

In the meantime, I’ll still enjoy the occasional sunny afternoon . . . but I’ll also be mindful that over my shoulder, just an astronomical stone’s throw away, is a vast and mighty star. And it has a temper.

Gamma rays from the supernova destroy vast amounts of ozone, which is quickly reduced to half its normal amount. When the Sun rises in the morning, its ultraviolet light will stream all the way through the atmosphere nearly unabated. Severe sunburn will be the least of the problems faced on Earth as the UV radiation kills off the ocean’s phytoplankton, which make up the base of the food chain. Animals that feed off phytoplankton find their food source dwindling and eventually disappearing in mere days, and animals that feed off
those
animals face the same dire problem a few days later. This die-off marches up the food chain, and it won’t stop until it reaches the top: us.

If you go outside on a dark, clear night, far from city lights, you can see thousands of stars sprinkled across the sky. They may seem unchanging, fixed—some people even refer to the night sky as the “starry vault,” implying a strength and permanence. Sure, the stars rise and set, but that is a reflection of the Earth’s motion, not theirs. They twinkle too, but again the fault lies in ourselves and not the stars—they flicker because the ocean of air above our heads blurs their light.

Even if you go out night after night, week after week, you may not see any changes in the stars. A sharp-eyed observer may note that some stars subtly and periodically change their brightness; these so-called
variable stars
wax and wane over days and weeks. But the stars themselves neither appear nor disappear, and do very much seem as permanent as the night sky itself.

But the Universe is deceiving. Things do change, and sometimes that change can be dramatic. On July 4, 1054, a new star appeared in the sky in the constellation of Taurus. Chinese astronomers recorded this “guest star,” noting that it appeared to be even brighter than the planet Venus, which is third only to the Sun and the Moon in our sky. There are records scattered throughout the world of the appearance of this new object, though they are spotty and not without some controversy, but there is no doubt about the reality of the event.

Today, a thousand years later, if you use a pair of binoculars to scan the sky in the constellation of Taurus between the horns of the bull, you might note a faint fuzzy blob that is clearly not a star. A small telescope can back up this observation. A big telescope—especially one equipped with a camera capable of taking long time exposures—reveals this object to be a gaseous and filamentary cloud. It
looks
like the aftermath of an explosion. In fact, images taken many years apart reveal the gas cloud (called a
nebula
by astronomers, from the Latin word for “cloud”) to be expanding; filaments and knots in the cloud have obviously moved outward in the intervening time. Backtracking the expansion shows that all the gas originally came from one point in the sky, the position of which is marked by a star very near the center of the cloud, indicative of a single explosive event. By measuring the
speed
of the expansion, the
date
of this event can be estimated. Remarkably, that date is the mid-eleventh century, suspiciously close to when the Chinese guest star was observed. Today, no astronomer on the planet doubts that the two events are the very same thing.

 

What the Chinese saw was one of the largest and scariest events in astronomy: a
supernova.
It might not have seemed all that scary at the time—after all, it was just a light in the sky! But upon closer examination the magnitude and scale of the event are revealed.

The gas cloud—called the Crab Nebula because of its current dubious resemblance in a small telescope to a crustacean—is the expanding remnant of this stellar explosion. In the subsequent millennium since its creation the cloud has grown to trillions of miles in diameter. The gas is still ferociously hot, heated to thousands of degrees by the shock waves generated as it expands supersonically and rams into the cooler gas surrounding it. Energy also continues to be poured into the cloud by the emanations from the central star, the cinder left over from the explosion.

The distance to the Crab is an estimated 6,500 light-years, or about 40
quadrillion
(40,000,000,000,000,000) miles, and yet even at such a distance, and after ten centuries, it is one of the brightest nebulae in the sky. At the time, the supernova event itself was bright enough to be seen in full daylight, indicating that in just a few weeks the explosion released awesome amounts of energy into space—as much as the Sun will put out
over its entire lifetime of 12 billion years.
In fact, a typical supernova can easily outshine the combined light from all the hundreds of billions of stars in an entire galaxy, and persist that way for weeks.

Our eyes can only see visible light, a very narrow slice of the energy band of light called the electromagnetic spectrum, which includes radio waves, infrared, ultraviolet, X-rays, and super-high-energy gamma rays. If you had X-ray eyes, the Crab would be one of the brightest objects in the sky. The same is true of radio waves, and if you could see in gamma rays, the Crab would be the single brightest persistent object in the sky.

I’ll gently remind you that the Crab is
400 million times farther away than the Sun.

Clearly, supernovae are awesome events capable of wreaking destruction on a vast scale. At its remote distance, the explosion that generated the Crab Nebula was little more than a pretty light in the sky, but not all supernovae are so far away. In fact, the Earth has had close shaves with exploding stars in the past, and there will certainly be more in the future.

But how close is
too
close? To understand just what a supernova can do to its environment and just how these events can be a danger to us on Earth, we’ll have to understand what would make an otherwise stable star explode.

 

While ancient astronomers were baffled by the stars in the night sky—were they holes in the vault of the sky, letting the radiance of the Sun through?—we have a pretty good understanding of them now.

Stars are not just points of light—each is a sun in its own right, most smaller but some fantastically larger and more luminous than our Sun. What a revelation that must have been, the first time someone realized that stars are suns, but terribly far away!

As astronomers studied stars, slowly and inevitably they learned more about them. Some stars are red, and some are blue (you can see this with your own eyes by examining a handful of the brightest ones), which indicates that they have different temperatures: red stars are cooler, blue stars hotter. Many stars are not individuals, but instead are pairs of stars orbiting each other in what are called
binary systems,
only masquerading as single stars because of their remote distance. Using laws of physics established by the astronomer and mathematician Johannes Kepler in the seventeenth century, astronomers could determine the masses of the stars in binaries, opening the door to an understanding of the physical processes inside them.