Do you recognize the name Galileo Galilei?
Galileo was the classical astronomer who made the drawing above. I have little idea what his writing actually says—it’s in Latin—but it’s clear enough what this early diagram is all about.
It’s a drawing of his observations of the sun.
And it’s proof, discovered way back in Galileo’s time but not accepted until much later, that the sun actually rotates.
How do we know that?
You’re probably wondering what those sort of splotchy markings are, in that drawing. They’re sunspots, and you can see them on the sun most days of the year.
Here’s an image of real sunspots, taken through a telescope several years ago.
In case you’re thinking of going outside and trying to look at some sunspots for yourself, let me give you a brief safety advisory…Galileo paid a price for his solar observations. Protective solar equipment hadn’t been invented yet, and he eventually went blind.
So…what are sunspots, anyway?
Well, in a previous post, I explained what they are and how they form. They’re basically slightly cooler regions of the solar surface, where tangled ropes of the sun’s magnetic field block the flow of heat from below.
Don’t let that whole “slightly cooler” spiel fool you, though. If you took the sun away and kept the sunspot, it would look brighter than the full moon.
Galileo was the first to recognize something interesting about sunspots, though. They actually move across the solar surface.
This series of images clearly shows the progression of one sunspot across the solar surface over the course of a week. See how this monstrous sunspot formation starts out on the left “limb” (edge) of the sun and travels all the way across to the right?
Now there’s one essential question we have to ask…what’s moving, the sunspot or the sun?
The simpler answer is the sun. It would make sense and fit the general equation of the rest of the universe—there isn’t a single celestial body we know about that doesn’t spin on its axis, even galaxies.
Galileo was the first astronomer to suggest this, and he got a lot of hate from the Church for it. At the time, the sun was believed to be perfect, fixed and motionless, and any suggestion that it could move was in defiance of the Church.
But, in the end, it doesn’t matter what the Church thinks (or, honestly, what anyone thinks). Science relies on evidence, not belief, and the sun is indeed rotating.
Sunspots are handy things. They’re like little landmarks on the surface of the sun. They migrate toward the equator a little bit over time, but they’ve helped us learn a lot about the sun…including its magnetic field.
There’s one thing all cosmic magnetic fields seem to have in common—the dynamo effect.
Wait…what the heck does that mean?
It’s a common effect, and it’s not limited to outer space. Rotate any conductor of electricity and stir it by convection, and you get a dynamo effect, which produces a magnetic field. It happens in the Earth, and it happens in the sun, too.
So what’s the difference between the Earth’s magnetic field and that of the sun, you ask?
Well…two things. First, our magnetic field is produced deep in our core. The sun produces its magnetic field just below the convective zone, near the visible “surface”—which, surprise, is where you see sunspots.
The other difference is vitally important. The sun is not a solid object.
In fact, it doesn’t have an ounce of a “solid” state of matter in its body. Or a fraction of an ounce, for that matter. The sun is entirely plasma, a fourth state of matter that’s even more energized and gaseous than, well, a gas.
Which is why sunspots don’t just appear to migrate around as the sun rotates. They do something weird…
They don’t all move at the same speed.
Literally. Sunspots near the poles travel a good deal slower than sunspots closer to the equator. Why? Because the gases near the sun’s poles rotate slower than the gases closer to the equator.
As you might guess from the diagram, this is called differential rotation. And, like the dynamo effect, it’s not that uncommon. Earth has it, too. So does Jupiter. So does Saturn. So does any other body large enough to produce a magnetic field.
What do I mean by “large enough”? Um…let’s cover that later.
So, what happens when the sun’s surface literally rotates at different speeds?
The magnetic field is going to get tangled up.
Every 22 years, the magnetic field starts out straight—and looks a lot like what you would expect from a bar magnet, or even from a magnetic field as large as Earth’s. But then, as time passes and the surface rotates, the magnetic field gets dragged along with it.
This is because the gas at the sun’s surface is so hot, it’s ionized. That’s what you call it when electrons and protons—the parts of an atom—are literally stripped apart from one another, and electrons are free to move.
This makes ionized gas a very good conductor of electricity…and the magnetic field will always stick around with the best conductor.
It means that regardless of how the surface gases of the sun stir and twist around, the magnetic field is along for the ride.
As a consequence, the magnetic field gets really tangled up over time. And I mean…really tangled up.
Tangled enough that loops of the magnetic field are bound to burst out from the solar surface…and take all that electricity-conducting gas with it.
Now…does anything look familiar about this twisted loop of magnetic field lines?
How about now?
Sunspots usually occur in groups or pairs. And just like you would expect with a bar magnet, one of them in the pair is going to be magnetic north, while the other sunspot is magnetic south.
And here’s another fun fact—at any one time, any pair of sunspots in the sun’s northern hemisphere will have the same polarity. But in the southern hemisphere, that polarity will be reversed.
It gets better, though. After years and years, the magnetic field becomes so tangled up that it can’t sustain this neat polarity organization system. Magnetic loops right next to one another end up pointed in different directions.
And after about 11 years of this, the magnetic field tries to reorganize itself. And in its quest to settle into a simpler pattern, it ends up completely untangling itself.
That’s right—11 years into the magnetic cycle, the magnetic field completely unwinds, returning itself to the original factory setting.
Well…maybe not quite like that.
Seriously, though. The untangling process is quicker than you’d think—you’d almost think the sun literally hit its own personal “factory reset” button.
However…all its problems aren’t solved. That factory reset went a little awry, and it ended up with a reversed magnetic field.
Yeah. Everything that was once north…is now south.
Not to worry, though! 11 years of field tangling later, it’ll hit that factory reset button again, and restore its polarity to the original setting. Everything will be as it was.
Anyway, that’s enough on our sun’s magnetic cycle for now. Next week I’ll talk about sunspots and magnetic cycles on other stars…and after that, I’ll conclude our talk on the sun with tales of its own personal weather systems.