Info in a Rainbow

milky way.jpg

What do you see in this image?

If you’re from a larger city and haven’t had the opportunity to venture into a place like the desert, you might not know what you’re looking at. That’s the Milky Way, our name for our galaxy.

Inside this galaxy are billions of stars, including our own. Galileo Galilei was the first to discover that it was really many tiny points of light, not just a cloud-like haze across the dark night sky.

We can’t see our galaxy from outside, but we can learn a lot about it by looking out at it from within. It’s difficult. It’s like trying to learn about a building if you can never step outside one of its rooms.

But we can do it, with the help of the spectrograph.

The spectrograph is an astronomer’s most valuable tool. It has only one job—to make plain white light into a rainbow.

Now how could that be important, you ask?

Well, consider something for me.

star cluster.jpg

This is the star cluster Westerlund 1. In this image, it looks like little more than a bunch of little points of light. Most have a bluish tinge, but a few are redder. This doesn’t tell us much.

The stars above are different. Something has to cause that difference. Why are some bluer and some redder? There’s no way we can know that, unless we can learn more about light. We can’t touch these stars, after all.

So what is there to learn about light? How can it reach our eyes from distant stars as different colors?

You know the answer already. You see this often, after rainstorms.


A rainbow is nothing but visible light reaching our eyes as separate colors. But here’s the question. How does this happen? How does the light that makes our sky blue turn into the colors of the rainbow?

Here’s the secret: visible light, also known as white light, is made up of all the colors of the rainbow.

It’s part of the electromagnetic spectrum, which I’ve written about before. It might help to take a quick look at that post—the electromagnetic spectrum contains all we can ever know about anything in astronomy. It’s that important.

But here’s a quick little diagram to give you an idea of what we’re dealing with.

electromagnetic spectrum

The spectrograph can tell us about any part of the electromagnetic spectrum. But for this post, we’re going to focus on visible light, the colors of the rainbow.

Before Isaac Newton’s time, no one recognized that a rainbow was just white light spread out into its composite colors. That probably explains all the Saint Patrick’s Day myths surrounding it. But Newton was a scientist, and he dared to experiment.

Newton went so far as to poke a hole in his bedroom window shutter. Yeah, he defaced his own property for science. Now that’s what I call dedication.

Naturally, sunlight shone through this hole, producing a thin beam of sunlight. Newton wanted to see what would happen if he put a prism in the beam. And what happened was a rainbow.

So what’s a prism, you ask?

isaac newton prism.jpg

Well, it’s that triangular thing the light is passing through. I’m not exactly clear on what goes on in a prism, but it basically bends light. The secret to a prism is actually a quirk of light—each color of light bends differently.

Light enters a prism heading parallel. But what happens when each part of light is bent so that they each head in a slightly different direction?

You get the colors of light spread out in a rainbow. All those colors didn’t magically appear. They already exist in white light. It’s the same as combining all the colors of paint to make black or dark, dark brown. All the colors of light together make white.

But when they’re bent and separated out, they become individual colors.


Now we can study light a bit more. It’s the difference between trying to study a rock face, and cracking it open to see what’s inside. We get more information from studying the parts of something, rather than just looking at the whole.

Now, astronomers don’t use prisms to separate light out into a spectrum. They use what we call a grating.

You’ve seen gratings before if you’ve seen a DVD or CD. Have you ever noticed that when you tilt a CD at different angles, the patterns of color on its surface change a bit?

cd grating.jpg

A CD has thousands of grooves etched on its surface, radiating out from its center. And that’s exactly what a grating is. The grooves make the different colors of light—the different wavelengths—bounce off the grating in different directions.

And that way, we get a spectrum.

Once you have a spectrum, you have everything—but until you have it, you have nothing. Without one, stars are just points of light traveling through your telescope. They’re incredible to look at, but you know almost nothing about them.

But get a spectrum, and you have the power to know.

Spectra (plural for spectrum) are so important that there’s a whole field of science devoted to them, called spectroscopy.

But a spectrum on its own doesn’t tell you much. The power of a spectrum lies in its spectral lines.

We’ll talk about these a lot more in later posts. They’re especially important in finding out what something in space is made of. But here’s the basic idea.

spectral lines.jpg

Here, you’re looking at a spectrum. It shows all the visible wavelengths of light. The numbers at the bottom are measurements of the wavelength in nanometers. How small are nanometers? Well, let’s just say, they’re nothing you deal with normally.

If a millimeter is basically the smallest unit of measurement you’re used to working with, then a nanometer is much smaller.

So what’s up with the thin black lines?

Those are called absorption lines, but we’ll talk about that another time. What matters here is that each element on the periodic table—each of the microscopic building blocks of the universe—show up on a spectrum as one of these lines.

And each element will always show up in the exact same places on the spectrum.

Well, almost always. If they’re shifted, something sciency happened, and we’ll look into what that could mean later.

The uses of spectra are far-reaching. If splitting light from any object in the sky into a rainbow shows us this spectrum with these lines, we know what building blocks of the universe are involved. We can tell what stars are made of.

We can tell what a lot of stuff’s made of.

True, not everything emits light. Dust clouds in space don’t. But we still have ways to use spectra there, and we’ll talk about all that later.

Next time, we’re finally going to dive into radio telescopes!

Questions? Or just want to talk?

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