February 20, 2016 at 05:46AM

Today I Learned: 1) Ever seen pictures of the sun? Did they look like a perfectly round, featureless disc? If not, then you weren't seeing visible light pictures. Turns out you can figure out a lot about the sun by looking at it in different wavelengths, largely but not entirely because different layers of the sun are different temperatures and therefore emit and absorb different wavelengths. Almost every wavelength looks way cooler than visible light (except microwave, the microwave pictures are pretty featureless). Relatedly, the surface of the sun is the coldest part of the sun -- not only is it cooler than the interior, which isn't surprising, but it's also cooler than the plasma around it, which is surprising. It has something to do with how plasma escapes the sun. Turns out the sun is full of tangled up, high-energy magnetic lines. You could think of it like a million-kilometer-diameter ball of rubber bands, except those rubber bands are constantly roiling past each other and are thicker than our planet. These sometimes break the surface, spewing extremely high-energy plasma out into space -- that's what we call a solar flare. That's also what populates the "atmosphere", I guess you could call it, of the sun, which is VERY BIG and VERY HOT. It's best viewed with UV and X-ray telescopes, which is kind of absurd. Anyway, the point is that with different wavelengths of light, you can see different layers of the sun, down to just below the surface. There is also a technique to see what goes on *under* the surface, called helioseismology. Basically, all of that intense roiling I mentioned causes a lot of earthquake-like vibrations in the sun (http://ift.tt/1HLU2uh) which can be tracked by telescope. With some presumably very fancy math, you can track individual seismic events as they propagate through the outer third of the usn, just like you can track vibrations in the Earth when there's an earthquake. By tracking deflections of those vibrations, you can reconstruct what's going on under the surface, at least a third of the way in. Wait, you ask, why only a third of the way in? Well, apparently the outer third of the sun is the "convctive layer". Inside it's way hotter and the matter in there... doesn't convect vibrations. They *should*, in theory, be able to transmit *gravitational waves*, but that's currently impossible to detect because we can't see the inner two-thirds of the sun. Solar astronomy is awesome. 2) More Sol facts! The sun takes up approximately 1 arcsecond of the sky. Someone should check me on this, though, I haven't actually double-checked the math. 3) Another Sol fact! Light emitted by matter at the sun's core can't go to the surface. It will run into stuff and either get reflected or get absorbed and re-emitted (which I *think* are fundamentally the same thing, but my QM isn't strong enough to say that for sure). In this way, the light bounce around randomly, eventually random-walking its way to the surface. This process, from the core of the sun to the surface, takes order-of-magnitude 100,000 years. Yeah. Bonus fact) In astronomy, "metal" commonly means "anything heavier than helium". I guess when you're talking about stars, it really doesn't matter what chemistry an element has at STP.