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.

Analyze the spectrum of any nebula—that’s right, any nebula—and you’ll discover that for the most part, they’re all made of the same stuff.

Wait a second…what’s a spectrum, again?

One of these.

Spectra are astronomers’ best way of understanding the universe. We can’t reach out and physically touch the cosmos, or bring back samples to study in labs. We have to rely on our knowledge of the way light interacts with matter—the “stuff” in the universe.

And, thankfully, light does interact with matter. A lot. Seriously—the only reason you can see anything is because light bounces off of it and hits your eyes.

The difference between studying stars and studying nebulae is a lot like the difference between studying why a lightbulb shines, and why the desk surface it illuminates looks brown.

When we study stars, we study the light—well, radiation, if we want to see the whole picture—that they emit. Visible light separated out into its components takes the form of a rainbow, or the continuous spectrum above. It’s only a small part of the total radiation the star emits.

But when we study nebulae, the spectra we get aren’t always continuous. Hot, ionized emission nebulae give us the emission spectrum above, the result of only emitting a few specific wavelengths of radiation.

Reflection nebulae are the result of light from nearby stars scattering among the dust particles and reflecting back to us, so we see whatever wavelengths are reflected—most often blue.

And dark nebulae aren’t the result of any reflection or emission—they’re what happens when you stand in front of a light source, and someone sees your silhouette. But it’s still possible to study the spectra of dark nebulae, because they do emit some low-energy wavelengths of light (radio waves).

Anyway…those little lines you see in the spectra above are basically the footprints of specific types of matter—known as elements. Some that you might have heard of before are hydrogen, helium, oxygen, nitrogen, carbon…

I’m guessing you get the picture.

We can tell from the spectra of nebulae that they’re mainly made of hydrogen—about 70%, in fact. Helium is also present, about 28%. And the remaining 2% is other stuff (the most abundant being oxygen, nitrogen, and carbon).

What’s interesting about the elements that make up nebulae is that they’re the same ones found in abundance within our own solar system. Why? Because nebulae are the precursors to solar systems.

I’ll show you in later posts how solar systems form out of nebulae. So naturally, nebulae will have all the parts to make a solar system.

The overabundance of hydrogen and, to an extent, helium, also makes sense. The sun makes up 99.9% of our solar system’s total mass. Crazy, right? And it’s mostly composed of hydrogen, with helium as a close second. The same is true for many other solar systems.

Another interesting thing about nebulae is their typical density. We’re used to a specific range of density here on Earth; the ground beneath our feet averages at about 5.51 g/cm³ (grams per cubic centimeter).

5.51g is just a little heavier than a sheet of paper. So if you crumpled up a sheet of paper and squeezed it into a cube 1 centimeter on all sides, you’d have the density of the Earth’s surface.

The air, on the other hand, is something like 0.0012 g/cm³ (at sea level, at least). That sounds like a lot less than the ground, and it’s true. But it’s still a whole lot more dense than a nebula.

See, in the air, you would expect about 3 x 10²⁰ molecules to be floating about in that cubic centimeter. That’s 3,000,000,000,000,000,000,000 molecules—three million trillion. But in a nebula, you’re more likely to find about one.

That’s right. Just one molecule for every cubic centimeter of space.

And that low density leads to some interesting consequences.

On Earth, we often play around with different gases in chemistry labs. We try to learn more about the footprints they leave on spectra, so that when we see them in practice, we know what we’re looking at.

There are some footprints we just never see. It’s because these “footprints,” as I’ve been calling them, are caused by the electrons within atoms.

Electrons orbit atoms at specific distances from the inner nucleus, called orbits. They also have a tendency to fall inward, and they release radiation as they do—in the form of the photon you see in this image.

But there are certain orbits, called metastable levels, where electrons sort of get “stuck,” and it could take them as long as an hour to get free and fall to a lower level.

On Earth, their atom is more likely to collide with another atom before that happens, effectively knocking the electron out of its “stuck” state.

But in a nebulae, where the heck is that other atom gonna come from? The first atom would have to drift a whole cubic centimeter away, and it’s just not moving that fast.

So the “stuck” electron has the time to emit a photon specific to that orbit before it falls inward again. That photon leaves a footprint on the nebula’s spectrum that we simply would never see on Earth, because collisions are far more common.

These rare footprints are called forbidden lines, because they show up as those dark or bright lines on the absorption or emission spectra and are so rare as to be practically “forbidden” on Earth.

Now, remember what I was saying up above about solar systems forming out of nebulae, and nebulae having all the parts to make a solar system?

Well, some of those “parts” include the ingredients for life.

That’s right. We’ve recognized the unique signature of organic compounds in the spectra of nebulae.

I’m not talking about living, breathing bacteria, by the way. I just mean that we’ve found molecules that have a carbon chain structure, which is pretty much a trademark unique to life.

Nebulae are also made up of a great deal of what we call interstellar dust. If you thought the dust grains floating through the air in your house were small, take a look at those in a nebula—they’re really tiny.

How tiny? Well, less than a single millimeter. Make that a thousandth of a millimeter.

Anyway, I think that’s enough on nebulae for now. Next up, I’ll talk about extinction and reddening, which—in the case of nebulae—refers to what happens when a nebula blocks the light from stars behind it.