Extinction and Reddening of Starlight


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Take a wild guess: What do you think this image is showing you?

If you said it looks like a giant black hole in space, I don’t blame you. I also don’t blame you if you thought it looks like a giant outer space blob…and the funny thing is, that’s actually closer to the truth.

This isn’t a hole in space. We can’t see any stars in this region, but not because there aren’t any. In fact, there are just as many there as there are flanking the giant space blob.

What you’re seeing is evidence of the vast interstellar medium, the galaxy’s backstage. The interstellar medium is the stuff between the stars, often invisible since it’s not hot enough to produce its own light.

Sometimes we can see it as a pale blue reflection nebula, or a bright pink emission nebula. But in this case, we’re looking at a dark nebula—visible only because it blocks the light from stars beyond it. It appears to be a hole in space.

It’s closer to being an outer space blob. But what exactly is it?

This dark nebula, known as Barnard 68 (or “The Black Cloud”—unimaginative but descriptive enough), is a cloud of gas and dust in space. It’s a site of star formation. Hiding deep in these clouds of dust, newborn stars are just beginning to burn.

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A huge part of astronomy is about naming things. We need some way to describe these incredible sights we see in the night sky, some way to communicate our discoveries with the rest of the world.

So when we see a dark nebula that’s blocking light from stars beyond, we say that interstellar extinction is happening.

Wait a second…what exactly is that?

Interstellar extinction is simple enough. When a species on Earth goes extinct, it means there aren’t any left. It’s gone. Wiped out. Never to be seen again.

Extinct starlight is never seen, because it never reaches Earth. It’s blocked by the nebula.

Well, in a broader sense, it’s actually blocked by the interstellar medium in general. Nebulae and the interstellar medium aren’t actually different things. They’re the only visible evidence of the interstellar medium.

Here’s an example. Imagine that you’re having a vacation on the beach, and you find some rocks that you arrange in cool patterns and scatter over the sand. Imagine that the rocks are the stars, and the sand is the interstellar medium.

See, it’s misleading to think of “space” as empty. Space is more of a concept—an area that’s filled with the interstellar medium like sand in a sandbox, and sprinkled with stars.

Anyway. A lot of starlight passes through the interstellar medium, obviously, or we wouldn’t see stars in the sky. But not all of it makes it through, and we call that extinction.

But that’s not all that happens to that starlight.

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There’s something else called interstellar reddening. But before we dive too far into that, let’s take a quick refresher look at an astronomer’s most valuable tool: the electromagnetic spectrum.

electromagnetic spectrum

The electromagnetic spectrum is a spectrum of all radiation in the universe. Visible light, heat, the ultraviolet rays that cause sunburns, microwaves in household microwaves, radio waves transmitted by radio stations…it’s all part of the electromagnetic spectrum.

Now, if you’ve been around for all the other posts where I’ve rehashed the electromagnetic spectrum, don’t worry. I’m not going to go into too much detail here. But I do want you to take a look at that little squiggly line at the top of the diagram.

That’s showing the radiation’s wavelength.

You might remember from previous posts that wavelength is what defines radiation. How do we know if any one photon of light is infrared or ultraviolet or radio or gamma? We check its wavelength.

When I say “wavelength,” I quite literally mean the length of a wave.

Light travels in a shape that looks a whole lot like an ocean wave, with crests and troughs. The distance between two crests, or two troughs, or…really any two similar points on a wave, is the wavelength.

There’s a quirk of light physics that says shorter wavelengths are more easily scattered than longer wavelengths.

Now, why is this? No clue. I’ll research it sometime in the future, and I’ll be sure to write a post on it.

For now…the sky is blue because blue is a shorter wavelength than red. Blue photons scatter easily and bounce around in the atmosphere until they’re entering your eyes no matter where you look.

Make sense?

Red light is responsible for sunsets. While your visual receptors are getting bombarded by blue photons, red ones are passing you right by, because their longer wavelength means they can penetrate Earth’s atmosphere better.

They manage to travel all the way to your horizon…and observers standing near Earth’s terminator, on the edge of the sun’s light, will see the reddish hues of sunset.

So it should be no surprise that red light finds it easier to penetrate a dark nebula than blue light.

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Well. See how the stars on the edge of the nebula look kind of dimmed and reddened? They don’t just happen to be red stars. They’re producing blue light, at least a little bit, but it’s not reaching our eyes because the dust cloud in the way scatters it.

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So here’s what happens. We’ve got a distant object, in this case a bright star—bright enough to shine through the outer, less dense edges of the dust cloud. We’ve got a dust cloud in the middle, and a great big eye in space on the other side.

Ha, just kidding. That’s supposed to be Earth. But some diagrams like showing the “observer” as a giant eye. It works better in telescope diagrams, when you literally do put your eye up to it. Here…I don’t know.

Anyway, back to the point. The star emits all light on the electromagnetic spectrum, but we’re focusing on visible light—the colors of the rainbow—since that’s what a human observer can see.

Both blue and red wavelengths—and every color in between—set a course for the dust cloud. But not all of them get through—shorter wavelengths, especially blue, get scattered along the way.

So the colors that reach our eyes are mostly red.

Wait a second. So what about the other wavelengths? Clearly red light manages to pass through on the outer edges of the cloud, but don’t we have some wavelength of light that can pass through the entire thing?

Yes we do.

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Meet Barnard 68 in the infrared. Infrared radiation is just a bit longer than the red wavelengths of visible light, but it’s long enough that it can penetrate a dust cloud like this one.

Keep in mind that when we “use” infrared radiation to see objects through dust clouds, what we’re actually doing is using telescopes to collect infrared radiation that they emit. Because that radiation can pass through the dust cloud, it will eventually reach Earth, and our telescopes can collect and study it—even though we can never see it.

Studying the radiation that travels our way from the farthest reaches of the universe is our only means of discovering what’s out there, at least for now. So you’d think that extinction and reddening would be handicaps.

Not necessarily.

We can compare two stars of the same spectral type—the same temperature, as revealed by their spectral lines…but that are affected differently by extinction and reddening.

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The black lines are spectral lines, in this case absorption lines. The numbers on the right are temperatures in Kelvins. All stars of the same temperature will have the same pattern of spectral lines.

So, to make a long story short, we take the spectra of a bunch of the stars behind Barnard 68, and find two that have the same temperature, but one is brighter than the other. That way we can measure the reddening that’s happening, and correct for it.

And for the rest of the stars that are hidden behind the cloud…we can study them just as well as any other star, because we know how to account for the reddening.

Anyway, enough on extinction and reddening. Next up is interstellar absorption lines…because the stellar spectra you see above are only a sneak preview of what we see thanks to the interstellar medium.

Questions? Or just want to talk?

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