Post #4 and I'm diverging from the purported theme. But it's Eta Carinae, it must be discussed! Eta Carinae is the most amazing object in the sky, in my opinion. It has been the most active nearby (around 2 kiloparsecs, or 6000 light-years, distant - close only relative to, say, the Galactic Center or other galaxies) object in the past few centuries. In 1840 it brightened by a few magnitudes, and it has been discovered since then that this event was a huge explosion. But it wasn't a 'normal' explosion. Most exploding stars blow up and die in supernovae. Those are awesome events that sometimes completely annihilate a star and sometimes leave a black hole or neutron star behind. This explosion was nearly as powerful as a supernova, but the star is still there. And it turns out that there are TWO stars there. There are a number of different theories going around, but this system is incredibly difficult to observe because there's just so much stuff there. The prevailing theory is that there are two stars, one around 80 solar masses, the other around 20. Either one of these on its own would be pretty big, hot, and impressive. Two together is something pretty impressive - but actually not that uncommon. Other pairs of hot, massive stars are observed frequently. One example is WR 40, in which two 80 solar mass stars are orbiting one another. The curious thing is that most of these massive stars have very circular orbits, but in Eta Carinae the small star has a very eccentric orbit around the large star. One explanation for this - which is not widely accepted yet because it's very hard to confirm, but it's plausible - is that Eta Carinae used to be a nearly equal-mass binary system like WR40, but one of the stars blew off its outer envelope (the outer part of the star) in a huge explosion, and when it lost this mass its orbit changed. One bit of evidence in favor of this hypothesis is that the small star has a very fast wind - 3000 km/s, which is faster than any normal star's wind. Wolf Rayet stars have winds that fast, though, and they are stars with blown-off envelopes. It's going to take a lot of skilled observations to confirm this, though. Another neat thing is that the smaller star disappears on its closest approach to the large star. Why? The large star is losing mass at a very fast rate - about a Jupiter mass every year! That's more than a million times as fast as the sun is losing its mass in the Solar Wind. All of this mass loss has an interesting result - you can't see the star, you only see the outside of its wind. But the small star passes inside the 'sphere' that is producing the wind, and so it's as though the small star was going inside the large one! You don't see it at all during closest approach. I get excited about this star... I'd like to add a lot more on it, and so you might see Luminous Blue Variables as a recurring theme on this blog. I hope to post a lot of pictures and add some links to references too.