Death from the Skies! (17 page)

If you wanted to launch a rocket to the Moon from this newly shrunken globe, you’d have to give it a lot more power to overcome the Earth’s gravity. If you shrank the Earth more, the rocket would need even more power, and so on. Eventually, the Earth would shrink so much that its gravity would be literally impossible to overcome.

You might think you just need to add more thrust to the rocket, but when matter gets this dense, Einstein has something to say about the situation. He postulated that gravity is really just a manifestation of bent space. What you feel as a force downward, toward the center of the Earth, is actually a bending of space, like the way the surface of a mattress would bend if you plunked a bowling ball down on it. Roll a marble across the bed, and the path of the marble bends, just the same way an asteroid’s path bends because of gravity when it passes near the Earth.

This is more than just a model, more than mere speculation. Its consequences are very real: if too much matter is packed into too small a volume, the bending of space can become so severe that it literally becomes an infinitely deep pit. You can fall in, but you can never climb back out.

An object like this is like a hole in space. Nothing can escape it, not even light. Since it cannot emit light, this hole would be black. What would
you
name such a thing?

And so it goes in the core of the exploding star. If the core is too massive to form a stable neutron star, it collapses.
All
the way down. It shrinks to a mathematical point, space gets bent to the breaking point, and a black hole is born.

The gravity of the hole is intense. Any matter close by will be drawn inexorably into it. But there’s a hitch. Stars spin, and so do their cores. As the core collapses on its way to forming a black hole, that rotation increases, the same way an ice skater can increase her spin by drawing her arms in. Once the black hole is created, it will be spinning very rapidly, and any matter falling in will
also
revolve around it, like water going down a drain. The closer to the black hole it gets, the faster that matter will swirl around it.

So matter falling into a black hole doesn’t just fall straight in—plonk!—and disappear; it
spirals
in. The matter just outside the black hole begins to pile up, and it forms a flattened disk called an
accretion disk
(accretion is the process of accumulating matter). This will happen for any star that is spinning before it collapses, but models have shown that GRB progenitors may be spinning even faster than normal. These rapid rotators form an accretion disk much more readily than a slowly rotating star. And once the disk forms, the ferocious gravity of the black hole will get the inner part of the disk moving very close to the speed of light, and even matter farther out from The Edge will still be moving incredibly rapidly.

When a black hole forms, spin and gravity are not the only things to get amplified. Stars also have magnetic fields, like giant bar magnets (see chapter 2). Just as gravity increases as the star shrinks, so does the magnetic field. A typical star may have a magnetic field not much stronger than the Earth’s: just enough to make a needle in a compass move. But if you take a star a few million miles across and squeeze it into a ball just a few miles across, the magnetism increases hugely as well, getting billions and even
trillions
of times stronger.

Any matter trying to fall into the black hole is therefore under the influence of a witches’ brew of forces. Gravity tries to draw it in, but its angular momentum counteracts that, forming the disk. The magnetic fields also get twisted up like a tornado as the matter spins around the disk. And on top of it all, there’s just plain old heat, created, oddly, by something familiar amid all these exotic forces: friction. As the matter in the disk swirls madly around under the force of the black hole’s gravity, the particles in it slam together at incredibly high speeds, which generates immense quantities of friction. This heats the disk to millions of degrees.

The sheer heat tends to drive particles away from the black hole. If a particle tries to move outward in the plane of the disk, it slams into other particles and cannot escape. But if it goes
up,
out, it’s free to travel—there’s less material in that direction. Moreover, the monstrously amplified magnetic fields can also accelerate the particles up and out. The heat and magnetism combine to focus a pair of tight beams, like two ultra-mega-superflashlights glued together at their bases. These twin beams shoot out from directly above and below the black hole, firing outward, away from the black hole in directions perpendicular to the disk.

What happens next is a vision of hell so apocalyptic that it’s difficult to exaggerate. Moments after the black hole is created and the disk forms around it, all that energy—a billion
billion
times the Sun’s output—is focused into twin beams of unmitigated fury. So much energy is packed so tightly into the beams that they blast outward in opposite directions, eating their way through the star at the speed of light. Within seconds, the beams have chewed their way out to the surface and are free. Any matter in their way is torn apart, heated to billions of degrees, rendered into its constituent subatomic particles, and accelerated to within a hairbreadth of the speed of light. Ironically, by the time they punch their way through the star, perhaps only a few hundred Earth-masses of matter are in the beam, which is huge on a human scale, but tiny on a cosmic one. But that also is a key to their power: since the total amount of matter in the beams is relatively small, it can be accelerated to incredible speeds.

Clouds of gas still surround the doomed star, echoes of past eruptions before the final explosion. The beams of energy and matter slam into this material, creating vast shock waves, sonic booms in the material, but on a mind-numbing scale.

There are also shock waves generated inside the jet itself as parts of it move faster than others. When these collide, the awesome energy of the jet churns up the matter inside them, creating unimaginable turbulence, which in turn adds greatly to the energy emitted. The ensuing conflagration emits gamma rays, huge amounts of them, as the magnetic fields and raw energy of the beams bombard the matter.

 

A gamma-ray burst is born.

The beams continue on. Behind them, the rest of the star finishes its collapse, forming what would otherwise be a normal supernova. Before the discovery of GRBs, a supernova was considered the most violent, the most energetic single event in the Universe. But a decent GRB can dwarf the energy of even a supernova. Because of this, astronomers coined a new word to describe the event:
hypernova.

Once the beams pass through the gas, they continue on, leaving behind superheated matter that begins to cool. As it does, it emits light for some time after the beams have moved on. This is the source of the afterglow sought so dearly by scientists on Earth. The matter can get extremely bright—one GRB in 2008 was nearly 8 billion light-years away, but was visible to the naked eye! But the afterglows fade rapidly, dropping in brightness by factors of thousands in just a few minutes. That’s why the optical afterglow was initially so difficult to detect. Even the titanic energy of a GRB is mitigated by raw distance.

But we now know that GRBs are created in a hypernova, when a massive star explodes . . . and we see massive stars in our own galaxy. Sure, all the GRBs we have ever seen have been at terribly remote distances, billions of light-years removed from Earth.

But what happens if one goes off that’s
not
far away? What if a nearby star becomes a GRB?

An object that finds itself in the path of the beams of a nearby GRB will have bad things happen to it.

Very bad things.

But before I scare the pants off you, remember that if you are far enough away they are no danger at all. The only reason we can see GRBs at all is
because
we are in the path of the beams: since all the light of the GRB is focused into those beams, if they miss us we don’t see anything. So if they are far enough away you just see a faint blip, and it’s gone. But if you’re too close . . .

The effects from a GRB are very similar to those of a supernova, which isn’t surprising. They are related phenomena, with GRBs being produced in supernovae, and they both emit huge amounts of energy in the form of gamma rays, X-rays, and optical light.

Where they differ is how well they sow their destruction over different distances. With a supernova, which emits radiation and matter in all directions, the effects die down rapidly with distance. As we saw in chapter 3, they appear to be mostly harmless from a distance of more than 25 to 50 light-years or so.

But GRBs are beamed. Their luminosity does not decrease as rapidly with distance, and this makes them dangerous from farther away.
Much
farther away.

Every GRB is different, making prognostication difficult. But enough have been observed to do a little averaging and get the effects from a typical GRB, whatever “typical” means when you’re dealing with Armageddon focused into a death ray.

Let’s set the scene.

Why fool around? Let’s say a GRB went off
really
close: 100 light-years away. Even from that very short distance, the beam of a GRB would be huge, 50 trillion miles across. This means that the whole Earth, the whole
solar system,
would be engulfed in the beam’s maw like a sand flea in a tsunami.

GRBs, mercifully, are relatively short-lived, so the beam would impact us for anywhere from less than a second to a few minutes. The average burst lasts for about ten seconds.

This is short compared with the rotation of the Earth, so only one hemisphere would get slammed by the beam. The other hemisphere would be relatively unaffected . . . for a while, at least. The effects would be worst for locations directly under the GRB (where the burst would appear to be straight up, at the sky’s zenith), and minimized where the burst was on the horizon. Still, as we’ll see, no place on Earth would be entirely safe.

The raw energy that would be dumped onto the Earth is staggering, well beyond the sweatiest of cold war nightmares: it would be like blowing up
a one-megaton nuclear bomb over every square mile of the planet facing the GRB.
It’s (probably) not enough to boil the oceans or strip away the Earth’s atmosphere, but the devastation would be beyond comprehension.

Mind you, this is all from an object that is
600 trillion miles away.

Anyone looking up at the sky at the moment of the burst might be blinded, although it would probably take several seconds to reach peak optical brightness, enough time to flinch and look away. Not that that would help much.

Those caught outside at that moment would be in a lot of trouble. If the heat didn’t roast them—and it would—the huge influx of ultraviolet radiation would instantly give them a lethal sunburn. The ozone layer would be destroyed literally in a flash, allowing all the UV from both the GRB and the Sun down to the Earth’s surface unimpeded. This would sterilize the surface of the Earth and even the oceans down to a depth of several yards.

And that’s just from the UV and the heat. It seems cruel to even
mention
the far, far worse effects of gamma rays and X-rays.

Instead, let’s take a step back. GRBs are incredibly rare phenomena. Although they probably happen several times a day somewhere in the Universe, it’s a really big Universe. The odds of one happening 100 light-years away are currently zero. Zip. Nada. There are no stars close to us that are
anywhere near
the capability of becoming a burst. The nearest supernova candidate is farther away than that, and GRBs are far rarer than supernovae.

Feel better? Good. So let’s try to be more realistic. What
is
the nearest GRB candidate?

In the southern sky is a star that looks unremarkable to the naked eye. Called Eta Carinae,
³³
or just Eta for short, it’s a faint star in a crowded field of brighter ones. However, its faintness belies its fury. It’s actually about 7,500 light-years away, and is in fact the most distant star that can be seen with the unaided eye.

The star itself
³⁴
is a monster: it may have 100 or more times the mass of the Sun, and it emits 5 million times the energy of the Sun—in one second, it gives off as much light as the Sun does in
two months.
Eta suffers periodic spasms, blowing off huge amounts of matter. In 1843, it underwent such a violent episode that it became the second brightest star in the sky, even at its vast distance. It expelled huge quantities of matter, more than 10 times the mass of the Sun, at over a million miles per hour. Today, we see the aftereffects of that explosion as two huge lobes of expanding matter, each looking like the blast from a cosmic cannon. The energy of the event was almost as powerful as a supernova itself.