Death from the Skies! (12 page)

The events in the core reverberate throughout the star. The core was supporting the outer layers of the star, and when the core collapses, for them it’s a real-life Wile E. Coyote moment: just as when the cartoon character suddenly realizes he is no longer over solid ground and starts to fall, the gas from the star’s outer layers suddenly finds itself hovering over a vacuum and comes crashing down. The incredible gravity of the core accelerates the gas hugely, and it slams into the compressed core at a significant fraction of the speed of light.

This creates a
huge
rebound effect that reverses the direction of the inbound gas and starts to blow it back out. This rebound, as vast as it is, is amazingly not enough on its own to blow up the star; the explosion would stall, and the outer layers would begin to fall once again onto the core. But the star has one more surprise up its sleeve.

Even after the initial collapse, the core is still loaded with electrons. The tremendous heat and pressure from the collapse applies a huge force on these electrons, squeezing them together into the protons in the core. When this happens, the electrons plus protons create more neutrons. But they also create ghostly subatomic particles called
neutrinos,
and these are what spell disaster for the star.
²³

Neutrinos are extremely tenuous particles, able to penetrate huge amounts of material without getting absorbed; to them even the densest material is nearly transparent. They blast out of the core, carrying away vast amounts of energy from the collapse. The energy they carry out is nothing short of staggering: it can equal the Sun’s
entire lifetime
output of energy
! In fact, the solid majority of the energy released in a supernova event is in the form of neutrinos; the visible light we see, blinding though it is, only adds up to a paltry 1 percent of the released energy.

The core generates neutrinos in unbelievably prodigious quantities: some 10
⁵⁸
(that’s a 1 followed by 58 zeros, folks) of the particles scream out of the core over the course of about ten seconds. This is just around the same time that the outer layers of the star fall onto the core and begin their failed rebound. Just as the gigantic bounce fails, and all that material is about to fall back on the core, all those countless neutrinos slam into the gas.

Even though neutrinos tend to pass right through normal matter, the stellar gas is incredibly dense. Plus, there are just simply so many neutrinos that some fraction of them get absorbed no matter what—it’s like driving through a swarm of bugs in your car; no matter how much they avoid you, you’re still going to get some goo on your windshield.

Only a tiny fraction, maybe 1 percent, of the neutrinos get absorbed by the gas, but it’s still an epic event: the total energy dumped into the gas is
huge.

This,
this
is what destroys the star.

It’s like setting off a bomb in a fireworks factory. The energy of a hundred billion
billion
Suns rips into the star’s outer layers, reversing their course, literally exploding them outward. Octillions of tons of doomed star tear outward at speeds of many thousands of miles per second. The event is so titanic that even the tiny fraction of it that is converted into light can be seen clear across the visible Universe.

And that’s just visible light. Other forms of light—X-rays, gamma rays, and ultraviolet light—also pour out of the newly formed
supernova.
As the shock wave of the explosion tears through the outer layers of the star, pressures and temperatures get so high that nuclear fusion can be triggered. In fact, elements heavier than iron can finally be created in this way, since the conditions in the blast wave are, incredibly, actually
more
violent than in the core of a star. Radioactive versions of elements like cobalt, aluminum, and titanium are created in the expanding debris, and they emit gamma rays when they decay. The gas, already hellishly hot, absorbs this energy and becomes even hotter, heated to millions of degrees. It glows in X-rays and ultraviolet light. Also, these explosions are rarely perfectly smooth. Some materials will be accelerated faster than others, and the inevitable collisions between them generate even more tremendous shock waves, similar to sonic booms inside the expanding material. This can also generate X-rays.

All in all, a supernova is a seething cauldron of power, chaos, and violence. It is one of the most terrifying events in the visible Universe.

Needless to say, anything close to the exploding star is facing upwind in a flaming hurricane. Any planet orbiting the nascent supernova is a goner: having your primary star explode in a billion-degree conflagration can end in only one way, and it’s not pretty. The planets will be torched, sterilized, and any air or water is stripped away by the sheer energy of the explosion.

The sudden decrease in mass of the star weakens its gravity severely, thus ejecting any planets from the system. It’s possible that there are thousands or even millions of scorched rogue planets wandering the Milky Way, their birth stars long since dead. Space is so vast, however, that we may never find such planets even if the galaxy is loaded with them.

Clearly, supernovae are
dangerous.
Your best bet is to stay as far away from them as possible. But
how
far away? If a star in our galaxy explodes, how close is
too
close?

In the appendix is a table that lists all the known stars within 1,000 light-years that have the potential to go supernova. The closest, Spica, a blue giant in Virgo, is about 260 light-years away, and most of the others are considerably farther off. While we can’t give the specific date any one of these stars will explode, it is a dead cold fact that they all
will
blow up, and some in the next few thousand years.

How much should we worry about this?

It depends on what it is we should be worried about, actually. At first glance, you might think that just the sheer enormity of the event is all you need to consider. An entire star just exploded! But in fact there are many weapons in a supernova’s arsenal. Some are not cause for concern. But others . . .

If you’re standing near an explosion, the most obvious danger is from debris. That’s bad enough if you’re near, say, a grenade, but a supernova takes this quite a bit further: the launch of a few octillion tons of gas into space at a significant fraction of the speed of light sounds more than a little dangerous. And it is! But only if you’re relatively close by. A planet circling the doomed star is itself doomed, of course, but what if you’re watching from the cheap seats, around another star?

To simplify the situation somewhat, let’s imagine that all that matter is ejected from the supernova in one instant. We’d see a thin shell of gas expanding outward, its diameter increasing with time. Almost all the mass of the original star is in that shell (the outer layers that explode outward may outweigh the core by several times). As it expands, the area of the shell increases, and so the amount of mass in a given area decreases—it’s very much like light emitted from a lightbulb; the farther you are from it, the more the light gets spread out and the dimmer it appears.

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.

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).

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.

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.