|The Carina Nebula in the constellation Carina, the Keel.|
Like the Orion Nebula, the Carina Nebula is a star-forming region, a stellar hatchery if you will. It contains some of the largest and most massive stars known to exist in our galaxy. One of these stars is known as Eta Carinae, and it is actually visible in this image. Below, I have an crop of the center of the image, with a few prominent objects marked in it.
The large circle on the right is a star cluster known as Trumpler 14, the structure in the middle is a dark dusty nebula called the Keyhole Nebula (silhouetted against the glowing hydrogen behind), and the star in the small circle on the left is Eta Carina.
Eta Carina is pretty mysterious as stars go, and there is much we still have yet to learn about it. For starters, it is probably more than one star. Currently it is thought to be two stars, one with a mass of ~100 times that of the Sun, and one with about 30 solar masses. It's hard to say, because back in 1841 it put out a huge cloud of gas and dust that makes it pretty much impossible to see the star itself. This corresponded with a major increase in its luminosity (the amount of energy it puts out). In fact, just two years later, in 1843, Eta Carinae underwent what is known as a supernova impostor event. Basically, it put out as much light as a normal supernova would, but didn't blow up.
How is this possible, you ask? Well, remember how I said Eta Carinae was one of the most massive stars in the Milky Way. Normally, a star only about 8 times more massive than the Sun can go supernova. Eta Carinae, of course, is much, much more massive than that, so it can easily fling out huge amounts of material and still survive. Eta Carinae (or at least the larger component of it) is so large, in fact, that it is in serious danger of blowing itself apart just from its normal energy generation. In a normal star, energy generated by fusing hydrogen to helium is just sufficient to counteract the force of gravity trying to collapse the star. As more mass is added, the force of gravity increases, which increases the rate of energy generation, keeping the star in hydrostatic equilibrium. However, there comes a point where the energy output of the core becomes so great that it can actually start to drive off the outer layers of the star.
This point is known as the Eddington Limit, after Sir Arthur Eddington who first proposed it. It is roughly 32,000 \(\times\) the mass of the star in solar masses, so our Sun would have to be putting out about 32,000 times more energy than it currently does before it would begin to disintegrate under its own power. It doesn't do this because its mass is not sufficient to crush its core hard enough to generate that amount of power. Eta Carina, however, may be big enough to do so (or at least get really, really, close). It's hard to tell, because we don't know for sure just how luminous it is. This theory may help explain why it put out such a huge burst of material back in the 19th century. Or it may be some other, completely unrelated mechanism. The bottom line is, we just don't know yet. That's all just part of the wonder and excitement of studying the universe!