So when is the supermassive star Eta Carinae going to blow? No one knows. But at 100 - 150 times the Sun's mass, it doesn't have much time left. And the presence of lots of nitrogen in the gas surrounding it is a bad sign: that means that the star was making heavy elements in its core, then belching them up into space. By the time a star like Eta Car is making nitrogen, it doesn't have long left to go. And when it goes, it'll go.
The image above is from NASA's Chandra X-ray observatory. The blue part is an optical image from Hubble, and shows the bipolar lobes of gas ejected when Eta Car had a coughing fit back in the 1840s. That's 20 octillion tons of gas (20,000,000,000,000,000,000,000,000,000) it ejected at about a million miles per hour, in case you're not getting enough awesome in your diet.
The yellowish ring is gas the star ejected long ago getting slammed by more recently blown out matter. The gas gets heated to a few million degrees by the impact, and gives off X-rays. By examining the energies of the X-rays, astronomers have found the ring is rich in nitrogen, and that's the bad thing. Stars like the Sun fuse hydrogen into helium in their cores. If the star is really massive, then it can fuse helium into carbon once the hydrogen in the core is used up. Then it can fuse carbon into neon, and up the periodic table until it hits iron. Stars can't fuse iron in their cores, which takes away the source of support for the zillions of tons of gas in the star. It collapses, and then complicated stuff happens (forgive me, I just wrote a long chapter for my next book about this, and I don't wanna repeat myself right now... but I wrote a description of how this works on my Bitesize pages), and then the outer part of the star explodes. The inner part collapses down into a neutron star or a black hole.
Usually, a star like Eta Car can go through its hydrogen in a few million years. But each element fuses more quickly than the last, and by the time neon or oxygen is fusing, the star has literally centuries left at most. The presence of nitrogen in the gas means there was time to make that much of it, get it to the surface, expel it, and let it expand for a while.
So Eta Car is ticking bomb. It could go off tonight, or in the year 3000 (did Futurama ever cover this?), but it won't be much longer than that.
Note that the lobes appear to be tilted away from us by about 40 degrees or so. That's a good thing. When stars like Eta Carinae explode, they tend to shoot of beams of energy and matter that, at its distance of 7500 light years, could kill every living thing on Earth. But since it's pointed away from us, all we'll get is a spectacular light show. If you're keeping score at home, gamma-ray burst aimed at you = bad, pretty supernova with no accompanying high energy radiation = good.
I just wrote a lot about Eta for my upcoming book, so if you like this kind of stuff you'll love that chapter. It gave me the creeps, so I'm glad I'm done with it. Now I'm off to write about black holes devouring the Earth...