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.
So let’s recap, for those of you who haven’t read my posts on spectra or don’t remember them. What exactly are they?
Stellar spectra depend on the interaction of light and matter—meaning, what happens when radiation hits an atom.
What’s an atom, again?
Atoms are the building blocks of the universe. They’re quite literally what “stuff”—matter—is made of. Like letters in a written language, atoms can be combined together and remixed to create any substance you’ve ever heard of.
Atoms are made of moving parts. A cloud of electrons orbits a nucleus or protons and neutrons. And when radiation hits an atom, those electrons jump around a bit, absorbing and then reemitting the photon that hit them.
But the thing is, the photon doesn’t get emitted in the same direction.
So even if it was originally headed for Earth, destined to be picked up by our instruments and studied, it’s lost now. That pesky atom has sent it in an entirely new direction, and no human will ever see it.
But…what does this look like to us?
Well, like this.
Each of these bands of color has been taken from a different type of star. Stars emit every kind of radiation there is, but we’re just going to focus on visible light, spread out into a rainbow.
When an atom in a star’s atmosphere absorbs and reemits a photon, we lose the chance of ever seeing that photon. So even though it didn’t disappear, it’s lost to us, and it’ll never take its place on the spectrum of color from that star.
We see its absence as a little black line, darkness where there should be light.
Each slight variation in the hue of the colors of the rainbow represents a different wavelength of photon—let’s just think of them as different types. So we can tell, by looking at where the black line appears, exactly which photon is missing.
And that’s not all. Specific atoms interact with specific photons. So dark lines on a spectrum are footprints left behind by certain atoms that we can identify.
Want to track a mountain lion? Study its footprint. Want to track a specific letter in the alphabet, through history? Get to know its shape and look for it. Want to track a specific building block of the universe? Look for its spectral line.
But the spectral lines we see aren’t always from the atmospheres of stars.
Think about it. If starlight passes through its own atmosphere and travels through the interstellar medium to reach Earth, it’s bound to interact with atoms along the way. So more often than not, we’ll also see spectral lines left behind from the interstellar medium itself.
Thankfully, we can tell which these are—because the material in the interstellar medium simply wouldn’t make sense in a star. How weird would it be to discover a layer of ice inside the surface of your desk? Or, for that matter…volcanic rock?
Stars are made primarily of hydrogen and helium, but even the trace amounts of other stuff they’re made of wouldn’t be found in the interstellar medium.
Matter in the interstellar medium is much cooler, and much less dense. Unless we’re talking about an emission nebula, it’s not going to be ionized, and it’ll only emit its own radiation at very low-energy wavelengths like radio waves.
So, in short, if you see the signature of cool silicates in a spectrum, it didn’t come from a star—it came from the interstellar medium.
Another easy way to tell when it’s an interstellar absorption line, rather than one from a star, is the width. Because stars are dense and hot, collisions between atoms often blur the spectral lines.
That doesn’t happen in the interstellar medium. Interstellar absorption lines are narrower—and, in graphs, much sharper.
Here it is on a graph, for your viewing pleasure.
What you’re seeing is the equivalent of dark lines on a rainbow background. Imagine the rainbow going from left to right, just as always. A dip in the intensity means the wavelength is less intense, the same thing as a dark line.
This graph makes it abundantly clear how easy it is to tell the difference between an absorption line from a star’s atmosphere and one from the interstellar medium. The interstellar ones are much narrower and sharper.
And then, of course, I’d be remiss if I neglected to mention interstellar emission lines.
These are essentially the opposite of absorption lines. Absorption lines mean an atom has absorbed a photon and prevented us from ever seeing it. An emission line means the atom has produced that photon.
Emission spectra are produced by excited gas—meaning, a gas that’s hot enough to emit its own light. And by that, I mean the interstellar medium.
Now, it’s not going to emit its own visible light. It’s not “excited” enough for that. But it’s just barely excited enough to emit some low-energy photons…by which I mean, some radio waves.
When the interstellar medium emits a radio wave, it’s different from a star emitting a radio wave.
Stars emit all radio waves and more—all the radiation there is. They emit a continuous spectrum, which becomes an absorption spectrum by the time it reaches our eyes. Atoms in the path of the radiation absorb some of it along the way.
The interstellar medium only emits a few specific wavelengths of radio waves, so it never produces a continuous spectrum in the first place. It emits an emission spectrum.
As you can see here, an emission spectrum is basically the opposite of an absorption spectrum—we’re seeing sharp upward spikes of increasing brightness, not downward spikes of decreasing brightness.
But what’s important are those sharp lines of carbon monoxide, magnified for you. Carbon monoxide is an especially powerful emitter of radio energy, and it leaves its signature here on this emission spectrum.
So…when exactly does the interstellar medium produce an absorption spectrum, and when does it produce an emission spectrum?
Well, like many things in astronomy, it all comes down to what angle we happen to see it from.
If our line of sight is off to the side, the light from the star off to the side won’t pass through the cloud before it reaches us. We’ll see it perfectly, unhindered by the interstellar medium. We’ll see emission lines from the cloud itself.
But if our line of sight puts us right on the other side of the cloud from that star, then we’ll see its spectrum after its radiation has passed through the cloud. It’ll reach us as an absorption spectrum…with a little extra from the interstellar medium thrown in.
These spectra are extremely useful to us. We don’t have to worry about “contaminating” the starlight—we have ways to correct for what the dust cloud blocks and study the star unhindered. And so these spectra offer us a chance to study the interstellar medium.
We get to figure out what it’s really made of. And we’ve detected over 150 different molecules floating about back there.
We’re not nearly done studying the galaxy’s backstage—next up, we’ll take a closer look at what exactly is inside the interstellar medium.