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.
It’s a radio telescope, the largest in the world. It’s so huge that a normal support system can’t support its weight. So it’s basically suspended between three mountaintops. It’s 300 m across, which is 1000 feet. It’s huge.
This is the kind of construction endeavor that radio astronomers must try if they want to get much detail from radio waves. The radio wavelengths of the electromagnetic spectrum are really, really weak. You need huge telescopes to collect enough.
But, as ever, astronomers face the same basic problem: money.
Huge telescopes are expensive. It’s unfortunate for astronomers, but true—just think of the cost of labor of basically burying a whole valley under a radio dish.