The debris from a supernova spreads out too. If you are on a planet near the explosion, more matter will slam into you than if you’re farther away. In this case, the amount of impacting material will drop with the square of your distance: if you double your distance, you’ll get one-quarter as much material hitting you. But how far away is far away enough?
Just to assume a worst-case scenario, let’s take an improbably close distance of 10 light-years for the supernova. That means it would be about 60 trillion miles from the Earth.24 Let’s further assume the total ejected mass is 20 times the mass of the Sun, about typical for your run-of-the-mill supernova. In this case, the amount of matter that would hit the Earth is about 40 million tons.
Yikes! Duck!
But just how much is that really?
That sounds like a lot of material, but it really isn’t; a small hill about 1,200 feet tall would have about that much mass. If that hit all at once it would be bad—chapter 1 made that very clear—but remember, this would be spread out over the surface area of the entire Earth. In fact, it’s far less than an ounce per square foot over the whole Earth: once spread out, it’s more like a single raindrop falling in your backyard.
And we know it wouldn’t be an extinction-level event, since we’ve survived asteroid impacts of this size and larger before. We might notice a slight diminution of sunlight, but no real long-term effects.
We have a more realistic situation, with the explosion of the star in 1054 that formed the Crab Nebula. At 6,500 light-years away, how much debris will impact the Earth? It turns out to be about 100 tons.25 And again, while 100 tons sounds like a lot, the Earth gets hit by 20 to 40 tons of meteoric material a day. Debris from the Crab is just a bump on top of our normal daily influx. But you needn’t worry anyway: at typical ejection speeds of one-twentieth to one-tenth the speed of light, it will take 100,000 years for that material to hit us—and the event was only 1,000 years ago. Not only that, but the material will certainly never reach us anyway: gas and dust between the stars will slow down and stop the Crab ejecta before it ever gets close.
Optical light
Another obvious feature of supernovae is that they’re bright. The Crab was about as bright as the planet Venus, even from 6,500 light-years away. How close would a supernova have to be for the light to be too bright?
We have to think for a moment about what “too bright” means. Some animals, for example, time their cycles to the Moon. Breeding, feeding, hunting, and so on are timed or at least aided by lunar light. Having a supernova as bright as the Moon hanging in the sky day and night could in theory affect some species.
For a supernova to get that bright, it would have to be at a distance of about 500 light-years. There are in fact one or two stars that close that could explode, notably again the blue giant Spica in Virgo. If it blew up, it would be easily visible in broad daylight, and at night would rival the Moon in the sky, bright enough to read by and to cast sharp shadows! But this extra light would be more of an annoyance than anything else. Bright as it is, the supernova would still just be a point of light in the sky, difficult to look at directly without making your eyes water. However, there wouldn’t be any actual physiological damage to your eyes. You’d just learn to avoid looking at it (or wear sunglasses at night).
There would be no added heat from this new source of light; the supernova would still be too far away to actually warm us up. Think of it this way: the Moon doesn’t heat the Earth noticeably, so a supernova as bright as the Moon wouldn’t either.
One possible problem would be the disruption of some animal cycles, but the effects of this are hard to determine. They might very well be minimal, since even the fury of a supernova dies down with time. Within a few months the explosion will have faded to more tolerable levels. Animal cycles timed with the Moon may be disturbed, but likely would recover.
It’s worth noting that the closer a supernova is, the brighter it is. To get as bright as the Sun, it would have to be much closer: about a light-year. Not only are there no stars that close capable of exploding, there are no stars that close to us at all (except, of course, the Sun itself).
Neutrinos
What about all those neutrinos, created when electrons in the core of the star merged with protons to form neutrons? The total energy emitted is huge. Are we in danger from that?
The answer is a little bit difficult to ascertain, actually. Physically, the direct absorption of the energy of a neutrino by a human cell is not terribly worrisome. Neutrinos are incredibly slippery; in fact, just while you are reading this sentence several trillion neutrinos have passed right through your body, and odds are very high that not a single one was absorbed by you. A supernova would have to be impossibly close—as close as the Sun is to the Earth—to be able to directly kill a human being through neutrino absorption.
But before you sigh in relief, there’s more to consider. Neutrinos can bounce off the nuclei in atoms, and deposit their energy that way, rather like hitting a bell with a hammer. It turns out that this method of depositing energy is more efficient—that is, more likely to have an effect. If a neutrino did this, a cell nucleus (specifically the DNA there) could be damaged, potentially leading to the development of cancer.
Once again, the exact danger from this is difficult to calculate, but mathematical simulations have shown that a supernova would have to be improbably close to do any damage in this manner. The effects are minimal for a supernova farther away than about 30 light-years, and again it’s worth noting that there are no potential supernovae this close to Earth. Your DNA is safe.
Direct exposure to gamma rays and X-rays
Things get stickier when we consider other forms of light. You’re almost certainly familiar with X-rays from visits to the dentist’s office, or if you’ve ever broken a bone. Medically, X-rays are wonderful because they can penetrate the soft tissue of our skin and muscles; as far as an X-ray photon is concerned, those cells are transparent. But bones are denser, and more likely to absorb the X-ray. If you put film underneath an arm, X-rays will pass right through soft tissue and expose the film, while bones absorb the X-rays, leaving only a shadow on the film.
However, soft tissue does absorb some X-rays, and that’s part of the danger. If a cell absorbs the high-energy X-ray, it’s like shooting a bullet into an egg. The energy released as the tissue absorbs the energy can destroy the cell. Low-energy X-rays can also damage DNA, potentially causing a cell to become cancerous. While this sounds alarming, it should be noted that a typical medical X-ray procedure is quite safe—Space Shuttle astronauts, for example, who stay in space for two weeks receive a dose of radiation from the Sun equivalent to about fifty medical X-rays with no ill effects. Digital technology has made it possible to lower the dose even more, since digital detectors are far more sensitive to X-rays than film.
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 distanc
e 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.
Gamma and X-rays, redux
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?
SIRIUS DANGER?
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.
Death From the Skies!: These Are the Ways the World Will End... Page 9