Death from the Skies! (6 page)

Of course astronomers—and I count myself guilty here as well—do love to use diminutive adjectives when describing low-mass stars:
dinky, tiny, feeble.
But that’s hardly fair, either: even the smallest star is far, far larger than Jupiter, and Jupiter is pretty big; three hundred Earths would fit inside it, so even a small star is a huge object.

And yet the Sun is larger in size than the majority of stars in the galaxy: their median diameter is about a tenth that of the Sun. So even on a cosmic scale the Sun is
big.

On a
human
scale, as you can imagine, it’s a scary, scary place.

The Sun is about 93 million miles away. If you could build a highway and drive there, it would take over 170 years. Even an airplane would take two decades to fly to the Sun if it could.

And yet . . . imagine it’s summer and you’re standing outside. You turn your face up to the Sun. Feel the warmth? Sure! The Sun is so bright you can’t even look at it. And if you stand there for more than a few minutes you risk damaging your skin.

The Sun’s fearsome power is generated deep in its core, where a controlled nuclear reaction is taking place: the Sun is continuously fusing nuclei of hydrogen together to create helium nuclei. Every time this reaction occurs a little bit of energy is given off, and in the Sun’s core the reaction happens a
lot:
every second of every day, the Sun converts
700 million tons
of hydrogen into 695 million tons of helium.

The missing 5 million tons get converted into energy, via Einstein’s famous equation
E = mc
²
, which shows that mass and energy can be converted back and forth into one another, and that a tiny bit of matter produces a whopping amount of energy. Five million tons is a huge amount of matter, the equivalent weight of seven fully loaded oil supertankers . . . and the Sun chews through that much hydrogen
every second.
⁶

The energy created every second in the core of the Sun—equal to the energy it emits from its surface—is the equivalent to the detonation of
100 billion one-megaton nuclear bombs.
This is 200
million
times the total explosive yield of every nuclear weapon ever detonated on, below, and above the surface of the Earth . . . and the Sun does this every second of every day, and will continue to do so for billions of years yet to come.

Some people like to say the Sun is essentially a giant nuclear bomb, but that’s misleading: a bomb explodes.
⁷
But the Sun doesn’t explode, because it has a lot of mass. This means it has a lot of gravity, which balances the energy it generates. The heat produced makes the Sun want to expand (like a hot-air balloon expands), but the Sun’s own gravity holds it together. It’s a balancing act; in fact, a good definition of a star is a ball of gas with nuclear fusion in its center held together by its own gravity.

But just because the entire Sun doesn’t explode like a bomb doesn’t mean that explosions don’t happen. In fact, the Sun is capable of epic eruptions; but they’re not nuclear in nature, they’re magnetic.

When I was a kid (and sure, I’ll admit it: even today), I was fascinated by magnets. I had a few different kinds, and I would play with them constantly. I read a lot about magnetism, and in one of my books it said magnetism could be destroyed by heat. I (carefully!) held a bar magnet in a candle flame for a few minutes, and sure enough, after that it wouldn’t attract nails or needles anymore.

I was also something of an astronomy geek even then, and I had a book that talked about the magnetic field of the Sun. I remember being confused by this: how could the Sun have a magnetic field if it was so hot?

What I didn’t understand is that there is more than one way to create a magnetic field. Simply put, a magnetic field can be generated by moving electrical charges. When you turn on a light, for example, electrons (subatomic particles with a negative charge) flow through a wire from the wall to the light. This motion produces a local (temporary) magnetic field around the wire. When you turn off the light, though, the flow of electrons stops, and the magnetic field collapses.
⁸

This has a very interesting—and useful—effect. If an electrically conductive object like a wire moves through a magnetic field, an electric current will flow along the wire. This current, in turn, generates its own magnetic field. If the current moves in just the right way, its magnetic field will reinforce the magnetic field already there and you get a self-sustaining system.

However, this only works if there is an outside source of energy to make things move. For example, you could use a crank to make a coil of copper wires rotate inside a magnetic field (generated by a permanent magnet). Your arm supplies the outside energy. Or, if you’re smart, and you want to make a lot of electricity, you stick this getup near a source of flowing water—say, inside a dam—and make giant turbines composed of copper that spin as water flows past them . . . which is precisely how hydroelectric power plants work. A system that converts mechanical energy to electromagnetism in this way is called a
dynamo.

The Sun is just such a dynamo. Its interior is hot: so hot, in fact, that electrons are stripped off their atoms, allowing them to flow more or less freely. An atom that is missing one or more electrons is said to be
ionized.
As these electrons move in the ionized gas, they generate magnetic fields.

If the Sun were just sitting there in space, a nonmoving and non-rotating ball of hot gas, the electrons inside would move around higgledy-piggledy, and all those individual magnetic fields generated would be oriented in random directions and cancel each other out. But the motions of the electrons in the Sun are far from random. For one thing, the Sun spins on its axis once a month, and that can create streams of gas in its interior. This preferred direction of motion for the electrons means that their individual magnetic fields can build on one another like creeks all flowing into a river, creating a larger magnetic field.

If it were just that simple, scientists would understand everything about how the Sun works. But in reality the Sun is incredibly complicated, with a vast system of moving gas inside it. The heat from the core makes gas above it rise,
⁹
generating towering conveyor belts of gas over 100,000 miles high, moving up and down inside the Sun. Other rivers of gas move around it like the jet stream does on Earth, and yet another set of streams flows north and south as well. When taken all together, the Sun more closely resembles a ball of writhing worms than a simple sphere of gas. It’s like a street map of Tokyo, but in three dimensions and changing with time as well. Because of this, the magnetic field of the Sun is a nightmare as well, making it ferociously difficult to understand. On the positive side, though, it also keeps a lot of solar physicists off the streets.

All of this together is what creates the Sun’s dynamo. The charged particles inside the Sun are moving in currents. These currents move inside a magnetic field, so the currents themselves generate a magnetic field, and the whole thing is self-reinforcing. The crank, in this case, is the Sun itself, with its own rotation providing the mechanical input energy needed to generate the dynamo. The Sun is huge and massive, so there is a vast amount of rotational energy to tap into. The solar magnetic field is created at the cost of the Sun’s spin, but it will take billions of years for the energy loss to result in a noticeably slower spin.

The Sun’s magnetic field is complicated and interesting, and by interesting, of course, I mean dangerous.

Or had you forgotten the title of this book?

Earlier, I mentioned that a star can be defined as an object with fusion in its center, whose tendency to expand due to energy production is balanced by its gravity.

Stars are a study in balance in this way. If gravity were weaker, they’d expand or explode. If their energy generation were a little weaker, they’d shrink or collapse (more about both of these in later chapters). Their rotation, chemical composition, gravity, heat, pressure, and yes, magnetic field all combine in exquisite balance to produce a stable star.

But sometimes things get out of whack.

When a simple magnetic field is illustrated, you usually see a set of lines emerging from the poles of the magnet, connecting one pole to the other. The field lines of a bar magnet, for example, look something like a doughnut. These magnetic field lines are useful to visualize the strength of a magnet: where the lines are bunched up together (like near the poles of a bar magnet), the magnetic field is stronger; where they are spaced out the field is weaker. If you place a small bar magnet inside the magnetic field of a larger magnet, the smaller one will align itself along the larger’s field lines. That’s why a compass points north; the needle is a magnet, and it aligns itself along the Earth’s magnetic field lines.

Things get complicated if the magnet is not a simple shape. If you bend a bar magnet, the field lines will bend as well. If you take a dozen magnets, a hundred, and throw them together, the field lines can get very distorted, because each bit of the magnetic field is attached to the object generating it. Mess with one and you affect the other.

The magnetic field of the Sun is generated by moving currents of gas—currents that get twisted, distorted, and bent around just like rivers on the Earth. These field lines may be generated beneath the surface of the Sun, but they don’t stay down there; they pierce
through
the surface, looping upward and back down into the interior in an incredibly complex, interwoven, and interconnected way. These magnetic field lines can really get their knickers in a twist, becoming entwined and entangled. When this happens, there are profound changes on the surface of the Sun.

For one thing, since the field lines and the gas are coupled, when the lines get tangled and compressed, the gas has a harder time moving around as well. It’s like a giant net is thrown over the gas, preventing it from moving freely. Hotter gas welling up from below can’t reach the surface, and regions where the lines are particularly dense begin to cool off. Since the brightness of the Sun is due to its temperature, a cooler region becomes dimmer, forming a dark area on the Sun called a
sunspot.
Because sunspots are inherently magnetic phenomena (they are really a cross section of the magnetic field lines where they intersect the surface of the Sun), they always come in pairs with reversed magnetic polarity: one is like a magnet’s north pole, and the other is the south pole.

Sunspots can be small, barely visible to telescopes on Earth, and they can be huge, dwarfing the Earth itself, with some so large that they can be seen by the naked eye when the Sun is on the horizon.
¹⁰

In fact, it was the observation of sunspots that first keyed astronomers into the Sun’s magnetic field. Heinrich Schwabe was a solar observer in the early nineteenth century who counted the number of sunspots every day for decades. He discovered that the number of spots waxes and wanes with a period of about eleven years from peak to peak—we now call this the
sunspot cycle.
At the time of the maximum, there can be well over a hundred sunspots on the Sun, but at the minimum that number drops to essentially zero.

Schwabe decided to publish his results in 1859, and it was quickly determined that the times of peak sunspot number also corresponded to the times of peak magnetic activity on the Earth, indicating a connection between sunspots and magnetism. In 1908, the astronomer George Ellery Hale discovered that the magnetic fields in sunspots can be thousands of times stronger than the Earth’s, indicating the presence of intense energies being stored there.

 

Which brings us back to balance. As the magnetic field lines tangle up, there is a balance struck between the pressure built up by the magnetic energy stored in them and the tension that exists in the lines. Imagine the magnetic field lines are like steel coil springs, all tangled together and interconnected. The springs are compressed and want to expand, but the tension of the intertwined mess keeps them from springing back. Now keep compressing them and adding more springs, again and again. The energy stored up would get pretty impressive.

What happens if you take a bolt cutter and snip one of the springs?

Right. Better stand back.

The same thing happens in a sunspot—in fact, much of the physics is pretty similar to a convoluted mess of coiled springs, with the analogous tension and pressure. As the field lines get more entangled, and more are added, the pressure builds up. Sometimes the pressure is relieved early in the process, and not much happens. But other times it builds, and builds . . .