Stars are hot. Space is cold. We’re all familiar with that, right?
Technically, it’s more complicated than that. Space isn’t completely frigid—absolute zero, the temperature at which there is no heat whatsoever, is purely theoretical and not thought to exist in the universe. But it is pretty darn cold.
In general, though, stars are pretty darn hot. Some special types of stars reach up to 200,000 K—that’s 359,540.33℉. Our own sun is about 5,778 K, which much cooler, but still almost ten thousand degrees Fahrenheit.
As a rule, we can think of stars as being much hotter than the space in between…except in the case of coronal gas.
Meet the Pillars of Creation, a photograph taken by the Hubble Telescope in 1995. These apparent “pillars” of dust and gas are what we call molecular clouds. And this region of clouds in space is aptly named: it’s where stars are created.
Technically, there are two types of molecular clouds—molecular clouds and giant molecular clouds, or GMCs—but I’ll get into that in a second.
Molecular clouds are deep within the interstellar medium. In case you don’t remember the ISM from my “recent” posts (sorry about that), it’s the stuff between the stars. It’s the galaxy’s backstage. Space is in fact not a perfect vacuum—it’s full of the ISM.
So what’s going on with molecular clouds like the Pillars of Creation?
Stars are like headlights in a fog bank that’s impossibly thick in some places, and so thin as to be transparent in others. Sometimes, we get lucky enough for starlight to light up the fog. Other times, stars shine straight through it.
That “fog” is the interstellar medium. I’ve covered it in several posts already. We’ve gone over nebulae, the visible evidence of the stuff between the stars. I’ve talked about ways to study the interstellar medium. And I’ve introduced you to cool clouds, the clouds of mostly neutral hydrogen gas.
Now I want to introduce you to the intercloud medium. It’s different from cool HI clouds in that it’s ionized, rather than neutral.
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.