What Are Sun Spots?

"Hey, younger brother, do you know what sunspots are?"

"Like, spots on the Sun?"
"Yeah"
"Not really"
"Tell me everything you know about them."
"They're, um, little dark spots that appear on the Sun. I don't know if it's part of the surface of the star or if it's our perspective from Earth looking at the star and there's stuff in the way. It could be stuff just floating nearby."

This post is to explain to my brother, a substitute teacher, what sunspots are.

Little Dark Spots

Sunspots are, in fact, on the surface of the Sun and other stars. Stuff (planets, meteors, dust, etc) floating in front of the Sun is a really good guess, though, and certainly happens a lot. The way we know it's not stuff between us and the Sun is that we see the spots rotate around the Sun. The material at the equator of the Sun makes one rotation every 25 Earth days, so if a sunspot is at the equator, we see it move across the surface in 12-ish days, then it moves behind the Sun for another 12-ish days, and reappears. Stuff between us and the Sun wouldn't have that rotational pattern, so we know the spots are, in fact, on the Sun.

Differential Rotation

You'll notice above that I specified the rotational period at the equator of the Sun. At the poles, the material actually takes over 34 days to make one rotation. This isn't normal for planets - it takes one Earth day to make one rotation whether you're in Brazil or Greenland because the Earth's surface is a solid mass. On the surface of the Sun, though, the material (plasma) can glide and slide and isn't stuck together like a solid, so the increased rotational speed at the equator lets the material there spin faster around than the material towards the poles.

Normally I would just bring this up as a fun fact and wouldn't try to explain it in so much detail, but in fact this phenomenon (called differential rotation) is where sunspots come from! But there's one more piece to the puzzle that makes almost all astronomers moan...

Magnetism

The Earth has a magnetic field thanks to the rotation of material, specifically iron, in its core. For similar yet different reasons, the Sun also has a magnetic field. Its magnetic field travels from pole to pole, and also around the equator with its rotation. This might not be too much of a problem, except that the material on the surface has an electric charge (it's made of ions). The electric charge interacts with the magnetic field (via the electromagnetic force, so you know they're linked), so as the material rotates, it can start to drag the magnetic field with it.

The magnetic field interaction and differential rotation all come together to make some crazy magnetic field lines that look like this:

Differential rotation of the Sun drags the magnetic field to create sunspots.

Notice in the final image that there are little bumps. This is where the magnetic field actually leaves the surface of the Sun, loops around, and enters into the Sun again! The exit and entrance points of these magnetic lines is where we see sunspots!

Why are sunspots dark?

Let's first establish that the surface of the Sun is bright because of it's temperature. The short answer to why sunspots are dark is because these areas are cooler than the surrounding solar surface. Granted, it's still extremely hot, but compared to the rest it appears dim; this is similar to how a projector screen is white, but when the projector shows a white page with black text, the text only appears black because the surrounding is illuminated, even though the "black" is actually the white screen.

But why is it cooler? The magnetic field exiting and entering the surface drag ionized material up and out of the surface, the same way that same ionized material drags the magnetic field. When this loop of magnetic field and ionized material are seen from the side, we call it a prominense, and it looks like this:

When viewed face on, it's the classic sunspots we're familiar with.

Some other fun facts about sunspots: They begin to appear close to the equator, and then more and more gradually occur further away from the equator in the shape of an arrow pointing in the direction of the magnetic field's rotation. After about 11 years of this activity, the Sun's magnetic field flips from all of the stress of being wound up. This snaps the magnetic field lines back into place, and the sunspots suddenly disappear. But the differential rotation is still going on, so the magnetic field is wound up again, creating more sunspots around the equator and then outward, for another 11 years until another flip happens. 

I specifically mention that the sunspots form an arrow pointing in the direction of the magnetic field's rotation, but not necessarily the rotation of the Sun. That's because after a flip, the magnetic field lines are wound opposite the rotation from the previous 11-year cycle since North and South swap. So really, this is a 22-year cycle, but you can see the height of solar activity and sunspots towards the end of the 11-year cycle.

One last thing I liked that my brother said is that sunspots are on stars. And this is true! Other stars experience differential rotation that drags at the magnetic field, and will also have a sunspot cycle! Hopefully most of this didn't go over my brothers head, or yours. See you in the next post! 🌠