Have you ever driven through fog?

I’m going to guess, for the sake of this post, that you have. And if you haven’t…well, I think the image above should give you a pretty good idea of what it’s like. Low visibility—you can’t really see anything.

Imagine a fog bank that sort of thickens and thins out as you drive through it. In some areas, it’s so thick you can hardly hope to see the cars around you. In others, you can see for miles ahead.

That’s actually a pretty good description of the interstellar medium…or, at least, the cool clouds between the stars.

Like fog, the interstellar medium isn’t nearly hot enough to produce its own light. If you drove through a fog bank in the dark without your headlights on, you probably wouldn’t even notice the fog at all.

When you turn your headlights on to travel through the fog, their light scatters through the water vapor in the air and sets the area of the cloud directly ahead of you aglow. Anytime an oncoming car passes by, its headlights light up the fog even more.

The space between the stars is made up of fine grains of dust and low-density gas, and this gas is a lot like fog. It’s thinner in some parts of the galaxy and thicker in others. In many places, it’s thin enough to easily see stars through it.

In other places…not so much.

When you look at a dark dust cloud like this, you can imagine that you’re peering through fog in the direction of a distant highway. You know the headlights are there, passing by, but you can’t see them. The fog is too thick.

Sometimes, however, there are stars nearby enough and bright enough to light up the cloud—just like headlights on a foggy road.

The Pleiades are a star cluster located in a denser region of the interstellar medium—a place where the galactic fog is thick enough that each star casts a sort of halo of light on the cool gas surrounding it.

What’s kind of cool, though, is the reason these halos are blue. They actually have their own special name—reflection nebulae—and appear blue for the same reason our sky is blue.

Normally, you see color because the object is that color, in a sense. The green pigment in plants’ leaves absorbs most colors of visible light. Because the plant is opaque, the remaining green light can’t pass through it, and it reflects off the plant and reaches your eyes. Your visual receptors tell your brain that the plant is green.

But the gases of a reflection nebula aren’t actually blue. They’re more likely clear, the way the air around you is clear and you can see the wall in front of you (or your outdoor surroundings, as the case may be).

Look up at the sky, however, and you’ll notice it’s blue. That’s because blue light, due to a unique quirk of light physics, scatters more easily than other colors. It bounces through the atmosphere like kids in a bouncy house.

Before long, blue photons are entering your eyes from all directions. Your visual receptors register the sky as blue.

That’s what happens when you see a reflection nebula: blue light from the stars near it scatters among the gas particles and changes trajectory enough to reach Earth.

We have to remember, however, that reflection nebulae are only the visible evidence of cool clouds in interstellar space. They’re the part of the fog bank that gets lit up by passing headlights. It doesn’t mean they’re the only part of the fog bank.

Thankfully, we can still study those invisible bits of the fog bank. You might be surprised, but even fog banks on Earth emit a tiny bit of heat—because they’re not at absolute zero, the lowest possible temperature. Theoretically, nothing is at absolute zero.

Cool clouds in space are the same. They emit just a bit of infrared radiation, which human touch receptors are evolved to translate to the brain as heat. And telescopes can collect and study that infrared radiation.

We get something like this:

I’m not going to bore you with the details of what exactly this graph is showing. The basic idea is that the top half is an absorption spectrum—a graph of the wavelength of radiation vs. its intensity.

And what does that mean? Long story short, this is what a cloud of cool hydrogen looks like in an astronomy lab.

The main takeaway here, I think, is that we’re looking at radiation reaching us from a cool cloud deep in the galaxy. Because we can’t send a probe out to take a sample yet, this is our only way of studying it.

What we find from studying these cool clouds is that they’re made up mostly of neutral hydrogen gas.

What do I mean by “neutral”?

Hydrogen in the core of a star will always be ionized—will be stripped of its electron. That’s because the cores of stars are so hot and so dense that collisions between hydrogen atoms happen often, knocking their electrons away.

Cool clouds are extremely cool and not very dense at all. Collisions will be few and far between. So this hydrogen gas will always be neutral, meaning not ionized.

According to the naming convention for ions and neutral atoms, these clouds are also called “HI clouds”—that’s the atomic symbol for hydrogen, H, with the Roman numeral “I.” (A cloud of ionized hydrogen would be called an HII cloud.)

HI clouds, however, are not the only component of the interstellar medium. And we’ll visit those other components in my next few posts.