This diagram is a tiny bit misleading.
Here, it looks like the chromosphere is the visible surface of the sun, with the photosphere just below. Really, we never see the chromosphere. If you ever look through a solar telescope at the sun, the photosphere is the surface that you see.
The sun is structured a lot like the Earth, just in that it has a core, a dense region between the core and the surface, a “surface” layer, and a few atmospheric layers. The chromosphere is part of that solar atmosphere. And you never see it.
A great time to get a good look at the chromosphere is during a total solar eclipse.
See that pink stuff right around the edges? That’s the chromosphere.
Now…wait a second here. What exactly are we even looking at? I mean, I’ve seen the sun before, every day basically, and that doesn’t look like the sun. It’s all…dark and silhouette-y.
It’s an eclipse of the sun.
What you’re looking at is the silhouette of the moon creeping right between the Earth and the sun as it orbits the Earth. This doesn’t always happen perfectly—most of the time the moon passes too high or too low, or appears too small to cover the sun’s disk.
But every once in a while, the moon covers the sun perfectly, and the bright photosphere is hidden from sight. With its light blocked, we can see the chromosphere peek out from behind the moon.
Here’s the question, though. If we can only see the chromosphere during a total solar eclipse, how do we study it?
We may not be able to see it with our eyes, but we can study the spectrum it produces.
Okay, at this point, I seriously recommend checking out my post on spectra, if you haven’t already. I’ll do my best to give you the short version here, but spectra are so important that I’ll be referencing them in a lot of posts from here on out.
Basically, you’re looking at a strip of color that includes every color in the rainbow…and more. The rainbow is just the visible bit of a vast electromagnetic spectrum, which, yes, recurs in my posts almost constantly. You’ll want to familiarize yourself with that, too.
The electromagnetic spectrum is a spectrum of all radiation, including visible light.
Everything we know about the universe, we know from analyzing the radiation from this spectrum that reaches Earth. We use visible light to see what an object looks like, but we can take advantage of other facets of this spectrum to learn even more about it.
That’s because everything we can see in the universe is made up of matter, which is made of atoms—the smallest building blocks of the universe. And each different atom produces a specific set of lines on a spectrum…like this.
There are absorption spectra and emission spectra, as you can see here. An absorption spectrum is produced by most stars. It means that the star is emitting all electromagnetic wavelengths, but certain atoms in the star are blocking certain wavelengths.
As you might imagine, that produces dark lines where you would otherwise see color on these spectra.
Sometimes, though, we see emission spectra. The same atom might produce both an absorption spectrum or an emission spectrum, depending on whether it’s absorbing or emitting light.
In the case of the sun’s chromosphere….
The gases there aren’t dense enough to emit the entire spectrum of light. The atoms in those gases emit light at a few choice wavelengths, producing an emission spectrum—a dark line with bright bands where light is being emitted.
And at this point, I’ll redirect your attention to our graph of the chromosphere’s emissions…
Notice that this emission spectrum is mostly dark save for bright lines of color here and there.
I know that there’s enough bright lines to make it look a bit like an absorption spectrum, but take a close look at the bright red line on the left—if this were an absorption spectrum, it would be a black line on a rainbow background.
Another way the untrained eye can easily tell that this is an emission spectrum is by looking at the graph above.
This graph is the way astronomers actually analyze data from a spectrum. The spectrum itself doesn’t tell you much, and it can’t show anything beyond the visible spectrum of light.
Translate that data to a graph, though, and you can see tiny little imperfections in the curve that aren’t easily noticeable otherwise. Notice that the spikes in the graph correspond to the bands of color on the spectrum.
This graph tells us a number of things about the chromosphere—and the first thing, we know just because this is an emission spectrum.
There’s a set of laws that govern spectra just like physics governs the universe. And one of them is that emission spectra are produced by exited, low-density gases.
What do I mean by that? This gas is hot, hot enough to generate its own light. But it can’t be very dense, because it’s not producing the entire electromagnetic spectrum—it’s only producing sharp spikes of radiation at choice wavelengths.
Now take another look at that graph…but this time, we’re going to focus just on the curve part, not the spectrum.
Look closely at those spikes in radiation. See how they’re all labeled with weird numbers and letters?
If you’re familiar with the periodic table, you’ll recognize those letters as the symbols for certain elements. We’re talking about atoms, those little building blocks of the universe that the chromosphere—and everything else in the universe—is made out of.
We already know that the chromosphere must be made up of low-density gas that’s emitting light at choice wavelengths. But which gases?
Because astronomers understand spectra, they know exactly which gases. This graph tells us which ones they are. But this graph is showing us something weird.
I see a bit of hydrogen and helium on this graph, which makes sense. That’s what stars are made of for the most part. But what’s up with those other elements? There’s some barium, sodium, magnesium…even iron?
Um…that’s starting to look more like the Earth’s surface than the sun.
You might think this is a case of the astronomers being wrong—this is the kind of data astronomers in the 1920s had, and it took one astronomer’s understanding of how a star’s temperature affects its atoms to prove the data was misleading.
This time, though…no. The graph we’re looking at has already been corrected for those issues. We’re really seeing iron in the atmosphere of the sun.
All this emission spectrum stuff is pretty cool…but we’re forgetting another way we can study the chromosphere.
Remember where the chromosphere is?
It’s right above the photosphere, the visible surface of the sun. And light from the photosphere has to pass through the chromosphere first to reach us.
The chromosphere is mostly transparent to that light. But what if certain specific atoms in the chromosphere blocked the photosphere’s light at certain wavelengths?
That’s exactly what happens—and we get an absorption spectrum. But that’s not all.
What if we could image the sun at only the wavelengths of light from the photosphere that the chromosphere blocks?
We’d get a very strange image of the sun…one that looks more like a household carpet under a microscope than the atmosphere of a star.
This is called a filtergram—an image made using only the wavelengths of light that the chromosphere blocks. That’s why it doesn’t look like the sun. All the light that reaches our eyes has been eliminated from this image, so we can see things we don’t usually see.
Those little hairy tendrils we can see curling up from the solar surface are called spicules. Viewed from the sun’s profile, they look a bit like a burning prairie.
What we’re seeing here isn’t even close to being flames, though. The sun isn’t burning the same way a fire does. We’re seeing jets of cooler gas rising up into hotter regions of the chromosphere.
Anyway…I think that’s plenty on the chromosphere for now. Next up, let’s take a closer look at the sun’s outer atmosphere, above the chromosphere: the corona.