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.
But what exactly does that mean?Continue reading