emission spectrum

What are Planetary Nebulae?

/ Emma / 3 Comments

Meet the planetary nebula, one of the universe’s most gorgeous phenomena.

If you’ve ever looked through a telescope, you may have seen one of these before. Through a small telescope, one might look like a little planet—hence the name. But make no mistake, these nebulae have nothing to do with planets, and everything to do with stars.

Up until now, we’ve covered how stars form, evolve, and eventually meet their end. They form out of a giant molecular cloud, or GMC. Eventually one cloud fragments and the cores condense into multiple stars, forming a star cluster.

The star then evolves across the main sequence, runs out of hydrogen fuel, expands into a giant, and begins to fuse helium in its core, which causes the star to contract a little and get hotter.

Then, as the star runs out of helium fuel in its core, it expands into a giant a second time. This is the last time a medium-mass star will expand. It’s also the end of the line for the fuel in its core, since it can’t get hot enough to fuse carbon.

At this point, the star is so big that gravity at the surface is too weak to hold onto its atmosphere, especially in the face of the superwind of radiation pressure from the still-collapsing core.

The result is a planetary nebula…but what exactly is a planetary nebula? What is it made of? Why does it look the way it does?

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From Cold Cloud to Hot Protostar

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Paradoxically, stars begin in the galaxy’s coolest places: the dense giant molecular clouds (or GMCs).

This is not quite the paradox it seems, as in the beginning, stars require little else but gravity to form. And that’s really quite lucky, because one thing they do need is regions of high density, and high density is unlikely to occur where temperatures are high.

And so stars begin in perhaps the most surprising of ways: as a high-density bundle of very cool gases within an equally cool interstellar cloud.

But they do heat up eventually. How?

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Interstellar Spectra

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I often refer to what we call the interstellar medium as the galaxy’s “backstage,” and I do that for a reason: for the most part, we can’t see it.

The backstage of any theater isn’t part of the show. You, as part of the audience, never see it. But you see evidence of it, when new props appear as the play progresses through scene after scene and the actors interact with their backstage.

The same thing happens with the interstellar medium. It’s not the hidden area behind the stars of the galaxy. (Ha, get it? Stars?) In fact, more often than not it’s actually the one hiding stars from view. But we can’t see it…unless we study how stars interact with it.

One way to do that is to look at reflection nebulae—evidence of the light from bright young stars reflecting off the dust of the nebula. That qualifies as interaction.

And in the case of emission nebulae, hot O-type stars ionize the hydrogen gas of the nebula. I’d say that’s interaction, too.

Even dark nebulae can technically be seen, since we see them as shadowy clouds silhouetted against background nebulae or stars.

But sometimes, it’s not that simple. Sometimes, we have to rely on the galaxy’s props to guess at what must be stored backstage. And that means studying stellar spectra.

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What is a Nebula Made of?

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What you see here is the Trifid Nebula, a vast cloud of gas and dust in space.

In my last post, we explored why it looks the way it does. We discovered that the pink hues of emission nebulae are caused when extremely hot nearby stars “excite” the gas of the nebula itself to emit its own light, which our eyes perceive as pink.

The haze of blue to the right, on the other hand, is the result of light from hot young stars nearby getting scattered among the nebula’s dust particles. It looks blue for the same reason the sky looks blue. We call nebulae like this reflection nebulae.

And the black wisps of dark nebulae are hardly as ominous as they look; they’re simply ordinary clouds of gas and dust, ordinary nebulae, that we can only see because they’re silhouetted by brighter objects in the background.

But nebulae, for all their different names, are actually a heck of a lot more similar than you might think.

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What is a Nebula?

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What’s a nebula?

Well…you’re looking at one.

Okay, okay, I know. You want to know what that actually is. You want to know why it’s there. You want to know why there are colors in space…and why you’ve never noticed such a thing in your own night sky before.

Nebulae are the stuff between the stars. They’re the galaxy’s backstage. They’re the only visible evidence of a vast expanse of gas and dust between the stars, completely invisible to the human eye, called the interstellar medium.

Nebulae are the sites of star birth. Planets form from the dusty particles present in these glowing space clouds. They’re the galaxy’s way of replenishing itself. And they’re pretty cool to look at, too.

But how come they look the way they do?

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Our Sun: The Chromosphere

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sun layers

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.

Well…almost never.

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The Atomic Spectrum

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Astronomers know that if white light passes through a prism and is bent, it’s separated out into its component colors—the colors of the rainbow.

Astronomers also know that when light interacts with atoms, the building blocks of the universe, the atoms absorb photons of light and reemit them—but in a different direction.

Put these two bits of knowledge together, and astronomers now have everything they need to understand spectra (the plural for spectrum).

A spectrum is something I’ve covered in previous posts. In astronomy, it means the wavelengths of electromagnetic radiation spread out so we can analyze them individually. And it’s an astronomer’s most valuable tool.

So, what exactly is a spectrum, and how can we use it to analyze radiation from space and learn more about the universe?

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