Death from the Skies! (13 page)

Supernovae are a bit brighter than the dentist’s X-ray machine, though. However, the X-rays from an exploding star can only hurt you if they can
reach
you. As it happens, we have a built-in shield.

You’re sitting in it.

The Earth’s atmosphere is very good at absorbing these types of light. Many astronomical sources emit X-rays, but astronomers didn’t even know about them until the 1960s because of the Earth’s atmospheric absorption. X-rays are blocked while still high in the atmosphere, so they never reach the ground, and even mountaintop telescopes can’t detect them. It wasn’t until the advent of the Space Age that it was found that stars, galaxies, and other objects emit X-rays.

So we here on Earth are pretty safe from exposure. X-rays, even from a nearby supernova, are absorbed by our atmosphere, posing little threat. But what about any humans
above
the atmosphere? Astronauts orbiting the Earth in the International Space Station are in fact at risk. Given typical X-ray emission from a supernova explosion, the astronauts will receive a lethal dose if the star is closer than about 3,000 light-years or so. That’s quite a long way! There are
many
stars capable of exploding within that distance of us. Astronauts are clearly our most serious casualties from the prompt (that is, immediate) radiation from a supernova.

Gamma rays, which are higher-energy than X-rays, have pretty much the same story. They are absorbed by our atmosphere, and pose little threat to human tissue for landlubbers. However, they actually make things worse for our spacebound crew. The absorption of the gamma ray by a piece of metal—say, the hull of a space station—can lead to the metal emitting many X-rays in response; it’s like electromagnetic shrapnel. A solar flare (as discussed in chapter 2) can generate enough gamma rays to do serious harm, and a supernova within a few thousand light-years can still generate enough gamma rays to equal or surpass the amount created in a big solar flare. Direct exposure to these gamma rays can be lethal. The “secondary radiation” from metal absorption can also be very high, lethal in its own right to unprotected astronauts.

Don’t forget that our satellites are also sensitive to this event (see chapter 2). Not only that, but the flash of gamma rays and X-rays from a nearby supernova would ionize the upper atmosphere, creating a cascade of subatomic particles. This would create a strong pulse of magnetic energy that can damage our power grid in the same way a solar coronal mass ejection can (see chapter 2 for details on this kind of event). Communications, television, global positioning, high-flying aircraft, and even the supply of electricity by power lines could be severely damaged by this pulse of supernova radiation.

Again, there are several stars ready to pop within that distance. The odds of any one blowing in the near future are incredibly low, but we are now a spacefaring race, and highly dependent on our orbiting infrastructure. The good news is that if governments take the threat from solar outbursts seriously and fortify our infrastructure against that, we’ll be safe from supernovae as well.

At least, safe from
that
particular threat. We’re not done touring the arsenal quite yet.

Before you start to breathe too easily, sitting under this ocean of air, you should realize we’re forgetting something. It’s true that we ground-based humans are safe from direct exposure to high-energy radiation because the atmosphere absorbs this radiation. But then it’s fair to ask,
how does this affect the atmosphere itself?

This is potentially the greatest threat a supernova poses.

Our atmosphere is a many-layered thing. We sit at the bottom, where there’s plenty of oxygen mixed in with nitrogen, as well as traces of other gases like carbon dioxide and argon. But up higher, things are different.

As covered in chapter 2, between heights of about 10 to 30 miles above the Earth’s surface sits a layer of ozone, which absorbs dangerous ultraviolet (UV) radiation from the Sun. Unimpeded, this UV light would reach the ground and do all sorts of damage, including causing sunburn and skin cancer in humans. Moreover, many protozoa and bacteria, the basis of the food chain on the planet, are very sensitive to UV.

Obviously, the ozone layer is critically important to life on Earth, and as far as a supernova is concerned, it has a big fat bull’s-eye painted on it.

When the X-rays and gamma rays from a supernova hit the Earth’s atmosphere, they can destroy ozone molecules, leading to the cascading series of events described at the beginning of this chapter. The critical factor, as it has been all along, is
distance.
How close can a supernova be before it damages the ozone layer enough to affect life on the surface?

This is an important issue, and many scientists have taken it very seriously indeed. Some have set up computer simulations to see how much damage a nearby supernova can inflict on our atmosphere. They used a mathematical model of the atmosphere, which includes such effects as the height of the supernova over the horizon, the time of year, the distance, and so on.

Different models yield different answers, but the result seems to be good for us: a supernova would have to be at most 100 light-years away before there would be enough damage to the ozone layer to kill off the base of the food chain. Some models indicate it would have to be even closer, perhaps 25 light-years.

There are no massive stars ready to explode that are that close, so we once again appear to be safe . . . or do we?

I have some more bad news: massive stars are not the only kind that can explode. Low-mass stars like the Sun lack the mass to create the conditions needed for a core collapse, but it turns out core collapse is not the only way to blow up a star.

In a massive star, helium piles up in the core and eventually will fuse into carbon and oxygen. But in a low-mass star, that doesn’t happen: there just isn’t enough pressure from the weight of the overlying layers in the star to get the helium nuclei to fuse. Instead, helium just accumulates in the very center of the star, forming a dense ball. This helium sphere is degenerate; degeneracy is that weird quantum mechanical state discussed earlier that occurs when too many particles—in this case, electrons—of one type are squeezed together very tightly. As more helium piles on, the degeneracy increases, and the temperature soars (though in this case still not enough to actually fuse the helium into carbon and oxygen).

As we also saw earlier, the low-mass star expands and cools, becoming a red giant. If it’s massive enough it might yet fuse helium into carbon, with carbon eventually building up and the cycle repeating. If the star doesn’t have the mass to fuse carbon, the fusion process ends there.

But the red-giant star’s life is not quite over just yet. While all this is going on deep in the core, at its surface the situation is different. The star’s vastly increased size means that gravity at the surface is much lower; the gas there is not held on as tightly as it was before. Remember too that the star’s brightness has increased greatly. Any gas particle at the surface is bombarded with light coming up from below. The gas absorbs this light, which gives it a kick upward. That kick can easily overcome the weakened gravity, giving the gas enough momentum to break free of the surface and be launched out into space.

A dense stream of material is emitted from the star. Astronomers call this a
stellar wind,
like a solar wind on steroids. Red-giant winds can be very dense, blowing off thousands of times as much gas as the star did before its core became degenerate; the stream can be so thick that the star’s outer layers can be entirely blown off in just a few thousand years. In just a short time compared to the star’s life span, it loses as much as half its mass.

When this happens, the degenerate core is eventually exposed to space, and is called a
white dwarf.
Although it can contain the mass of an entire star, it is so dense that it’s no bigger than the Earth. The surface gravity is unimaginable, hundreds of thousands of times stronger than the Earth’s. A cubic inch of white-dwarf material would have a mass of several
tons,
like compressing dozens of cars into the size of a sugarcube. It’s also very hot, glowing at a temperature of over 100,000 degrees Fahrenheit.

After the outer layers are shed in the stellar wind, this ball of ultra-compressed superhot material is now sitting in the center of an expanding cloud of gas. The white dwarf is so hot that it emits a flood of ultraviolet light that energizes the gas in the expanding wind, setting it aglow. Seen from Earth, these gas clouds look like pale, ghostly disks, glowing a characteristic green color due to oxygen in the gas. Astronomers named them
planetary nebulae
because of their resemblance to distant planets seen through the eyepiece, but that’s a misnomer: they are the dying gasps of medium-mass stars, and someday the Sun will go through this stage as well (making life here very uncomfortable, so you just know there’s a whole chapter later on devoted to this).

From there on out, though, the star’s life is rather boring. Eventually the gas expands away, dissipating entirely and mixing with the lonely cold gas that exists between stars. Over billions of years the white dwarf cools, dims, and simply fades away.

But for some white dwarfs, the story does not end there.

Something like half of all the stars in the sky are a part of binary or multiple-star systems: stars that orbit each other because of their mutual gravity. Imagine now such a binary star, with two stars in mutual orbit. Both have roughly the mass of the Sun. One ages somewhat faster than the other; perhaps it is slightly more massive than its companion, and so fusion progresses a bit more quickly. It becomes a red giant, blows off its outer layers, and becomes a dense helium white dwarf.

Eventually, the other star begins to go through the same process. But when it expands into a red giant, its partner star is already a white dwarf, with its commensurate strong gravity. If the dwarf is close enough to this new red giant, its intense gravitational pull can essentially draw material off the other star, literally feeding off it. This gas, which is almost entirely hydrogen, then falls on the surface of the white dwarf and accumulates like snow on the ground.

Things get dicey from there. The gravity of the white dwarf is incredibly strong, squeezing the mass accumulating on its surface immensely. If the mass is raining down too quickly, it piles up on one spot, and the pressure builds there beyond the breaking point. The hydrogen in the pile fuses in a flash, detonating like a thermonuclear bomb—except one with 100,000 times the energy output of the entire Sun.

There is an immense flash, and the accumulated matter blows off the surface of the star despite the intense gravity. Like belching after eating too much food too quickly, this takes the pressure off the white dwarf, and after things settle down, the matter begins to accumulate again, resetting the cycle.

The energy released is gigantic on a human scale, but still much smaller than a supernova, and this event is called simply a
nova.
The white dwarf is largely unaffected by the blast—the amount of matter blown off in the event is only a few hundred times the mass of the Earth,
²⁶
which is far, far less than the mass of the star—and therefore the cycle can repeat as long as the red giant feeds the white dwarf.

 

However, if the red-giant matter stream is on the slower side, things are very different. The gas won’t pile up as quickly and explode in one spot. Instead, it will get spread out over the entire surface of the white dwarf, forming a shell of inert hydrogen all around it. But this time there is no pressure release, no ability to burp. Since the matter is spread out, the pressure is lower than in the earlier case, and the material continues to build up, getting thicker and thicker everywhere on the white dwarf’s surface. Eventually, however, when enough matter piles up, it will still reach that fusion flashpoint.

In this case, it’s not just the hydrogen in one small spot that fuses in a thermonuclear flash; it’s
all the matter over the entire surface of the star.
The explosive energy released is much, much larger, and eats its way
down
into the white dwarf as well as up into space. The energy release is so titanic that it can disrupt the structure of the star itself, causing a catastrophe on an epic scale. The entire star explodes like one enormous thermonuclear bomb the size of Planet Earth. It is literally a disaster: the star goes supernova.

By a cosmic coincidence, the total energy released in this event (called a Type I supernova) is very similar to the energy emitted by a massive star going supernova (called a Type II), even though the two physical processes are completely different. In fact they look so similar that it took astronomers quite some time to figure out that the two events were actually entirely separate. But both release huge amounts of energy, and both are very dangerous if they happen too close to us.