Yesterday — March 10, 2024 — was New Moon, which means we are less than one lunation (a full cycle of lunar phases) from the total solar eclipse on April 8! Are you ready?

I’ll be traveling a bit more before then to give talks about the event, including Akron OH, Boston, MA, and Rochester NY, but in the end it looks like I’ll be home in Virginia for the big day, watching things online. That’s OK; I’m just glad I got to see the one in 2017! There’s an eclipse somewhere on Earth twice a year or so, and I expect to get plenty more chances to see one (if you’re a museum, library, science center, university, or wealthy eccentric, I’m available to give lectures!). I hope you get the chance to see this one though. It should be amazing.

Every now and again I see a bit of science communication that’s so cool I marvel that anyone thought of it. 

The European Southern Observatory put out just such a video. It’s extremely clever, and one thing I love about it is that it shows more than their description tells.

First, a little background. I talk about spectra a lot, because they’re so massively useful. In a nutshell, it means breaking up incoming light from a source into individual wavelengths (think of them as colors, which is actually true). A prism (or a raindrop) does this for sunlight, creating a rainbow. But the colors overlap, and it’s hard to distinguish them in any detail because it’s a very low resolution spectrum.

We can use a grating (glass or metal etched with extremely fine parallel lines in it) to split light as well, and this can do much better than a drop of water. Some gratings can show you thousands or even hundreds of thousands of individual colors — very high resolution.

Why is this important? One example is that stars emit different amounts of light at different wavelengths in a characteristic way. If you plot brightness versus wavelength you get a shape that can ID what kind of star you’re seeing. When you dig down more finely into that spectrum you can really tell a lot about the star, even things like its mass and in some cases its age.

But there’s more. As I talk about in the episode of Crash Course Astronomy about light, different atoms emit very specific wavelengths when they get excited (that is, they’re given energy, usually by absorbing light or getting bumped by another atom). We know these wavelengths for most atoms to extremely fine precision, so you know when you’re seeing hydrogen, say, or sulfur.

[This is not the video that’s the topic of this article; just a by-the-way that’s helpful.]

Gas clouds in galaxies are filled with excited atoms, and glow at these well-known wavelengths. If you spread the light out from a galaxy into a nice spectrum you’ll see it gets brighter at those colors.

OK? Now, there’s a camera on the Very Large Telescope called MUSE, for Multi Unit Spectroscopic Explorer. It breaks up the light at every single point on the detector into a spectrum! So if you have a galaxy, you can map the whole thing out in all those different wavelengths to see what elements it has in it, and where they are.

And that brings us to the Very Cool Video I was talking about. Astronomers used MUSE to look at the lovely face-on spiral galaxy NGC 3456 about 200 million light-years from Earth, and got a spectrum at every point on that galaxy, so at any given wavelength you can see how bright all the different parts of the galaxy are.

What’s so clever, though, is that they then stacked all these images like a deck of cards, and animated it so that you see the galaxy as they slice across the spectrum. It’s hard to describe, but obvious if you watch it. Ready?

Cooool. See how the brightness flickers, changing at different colors? For example, the video slows down around 16 seconds in, right around 650 nanometers, where hydrogen glows strongly. The galaxy gets much brighter (they adjusted the contrast to make this a little tough to see though, but suddenly the salt-and-pepper noise in the background goes away and the sky around the galaxy gets really dark, because the galaxy gets so bright). They even label that point (called H-alpha) along the top of the back edge of the graph. Another spot earlier on emphasizes the light from oxygen (called [OIII]), as well as another wavelength at which hydrogen glows (H-beta).

I love this! It shows you how spectra work, without having to resort to complicated graphs that might be difficult to interpret.

Mind you, that wasn’t exactly the point of this video. In this case astronomers were curious specifically about what kind of elements are in the region of space around a supernova (called SN2018ie), an exploding star, in that galaxy. By taking a spectrum of the galaxy they can isolate those elements’ light and see what’s going on around the supernova, which is to the upper right of the galaxy center in that ragged looking spiral arm near the top of the frame (see page 8 of this paper).

However, it’s a pretty cool lesson in how spectra work, too. I’d love to see this done again with the contrast held constant so you can see how faint the galaxy is at most colors, then blammo!, it gets superbright at the ones where abundant atoms blast out light. Still, this is a pretty useful demo.

I used to work on a spectrograph, and didn’t know that much about them at first. I learned quickly, though, and found them to be fascinating, even though the complexity and finicky nature of a space-based instrument sometimes drove my blood pressure up. They’re fantastically important though; it’s been said that spectroscopy transformed astronomy from guesswork into physics. It’s true. Once we had spectra, we began to understand the stars, and from there, the Universe.

Pretty good for what’s really just a very, very detailed rainbow.

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