by Chris Peterson » Thu Apr 23, 2009 5:09 pm
rstevenson wrote:One of the things it's so hard to wrap my mind around is how violent much of what we see throughout the universe appears (and, I suppose, how violent much of it is, at least when it's beginning to happen) while at the same time it remains wispy and tenuous compared to solid things like planets.
The relationship between energy and energy density can be fairly non-intuitive at times. The atmospheric temperature up at the height of the ISS (yes, there is atmosphere up there) is well over 1000°C. But you'd freeze if you were in the shade of the ISS, simply because there aren't enough atoms and molecules to transfer much of that energy to you. Same thing with these nebulas. The gases are heated to thousands of degrees, enough to ionize. But they are so thin that they qualify as a vacuum. Consider comets; we see these apparently dense, bright comas, but they are also pretty good vacuums. We only see them because they're a million miles thick. Nebulas are light years thick. On Earth, you only need to look through a few hundred miles of air until it become pretty opaque. Yet you can see stars through nebulas- just think how thin something has to be to let starlight through a few light years!
[quote="rstevenson"]One of the things it's so hard to wrap my mind around is how violent much of what we see throughout the universe appears (and, I suppose, how violent much of it is, at least when it's beginning to happen) while at the same time it remains wispy and tenuous compared to solid things like planets.[/quote]
The relationship between energy and energy density can be fairly non-intuitive at times. The atmospheric temperature up at the height of the ISS (yes, there is atmosphere up there) is well over 1000°C. But you'd freeze if you were in the shade of the ISS, simply because there aren't enough atoms and molecules to transfer much of that energy to you. Same thing with these nebulas. The gases are heated to thousands of degrees, enough to ionize. But they are so thin that they qualify as a vacuum. Consider comets; we see these apparently dense, bright comas, but they are also pretty good vacuums. We only see them because they're a million miles thick. Nebulas are light years thick. On Earth, you only need to look through a few hundred miles of air until it become pretty opaque. Yet you can see stars through nebulas- just think how thin something has to be to let starlight through a few light years!