[Over the past few weeks, I've collected a metric ton of cool pictures to post, but somehow have never gotten around to actually posting them. Sometimes I was too busy, sometimes too lazy, sometimes they just fell by the wayside... but I decided my computer's desktop was getting cluttered, and I'll never clean it up without some sort of incentive. I've therefore made a pact with myself to post one of the pictures with an abbreviated description every day until they're gone, thus cleaning up my desktop, showing you neat and/or beautiful pictures, and making me feel better about my work habits. Enjoy.]
Sometimes, what you see depends on how you see it.
For example, take the Sun. Imagine it as a ball of ionized gas 1.4 million kilometers in diameter, churning and roiling, with intense magnetic fields piercing its surface and causing vast eruptions of material 150,000 km across.
OK, you don't have to imagine that, since a) that's the way the Sun really is, and 2) I can show you a picture of it! Like this one, from astrophotographer Ted Wolfe:
That shows the Sun as it was on November 25, 2011. That towering arc is plasma -- gas that has had one or more electrons stripped from its atoms -- being guided by the crazy strong and complex magnetic fields looping into and out of the Sun's surface. This picture is interesting, since this loop of plasma is nearly aligned with our line-of-sight. One foot of it is on the near side of the Sun, and it arcs over across the Sun's limb to the other side!
What's funny is that when you get one of these on the face of the Sun it's called a filament. When it's seen projected against the black of space, it's called a prominence. This terminology is a holdover from a long time ago, but we still use it. To be fair, the terminology comes up because usually filaments look very different than prominences. If you use a regular filter (one that just blocks light) to take pictures of the Sun, filaments look dark; they are cooler than the Sun's surface and absorb the light from coming up from beneath (much like sunspots). But they're still plenty hot, and look bright when seen against the black of space. You can see examples of this here and here.
This picture from Ted Wolfe is different. He used a special Hα filter, which doesn't just darken the Sun but picks out a very specific color of light (in the red part of the spectrum) and isolates that, letting it through while blocking everything else. Warm hydrogen (like in the filament) emits that color, so if you use that filter the loop of plasma looks pretty much the same against the Sun as it does against the sky. It's a bit of a trick, but is useful in showing that filaments and prominences are just two different views of the same structure.
So what do we call this thing in Ted's picture? We see it against the Sun and against space! A filanence? A prominent? Beats me. But it's pretty cool either way.
Credits: Tom Wolfe, used by permission.