They've found many stars between about 10 times the mass of the sun and 30 times the mass of the sun have exploded, and they also have many supernovae where no star was seen beforehand. That doesn't mean the explosion came from no where, it just means the star was too faint to see from the Earth. These stars probably were "only" seven to ten times the mass of the sun.
Even more interesting, they haven't yet seen stars bigger than about 30 times the mass of the sun explode. We know these stars exist, so why don't we see them exploding?
One idea, which many astronomers at the meeting believe, is that Smartt and his team haven't looked at enough exploding stars yet. Monster stars are rare; probably only one in every 10 stars big enough to explode are bigger than 30 times the mass of the sun. And while Smartt and his team have looked at a couple dozen supernovae now, it is possible that nature is playing some trick on us. As any gambler knows, sometimes you have cold streaks, even if the odds tell you that you have to win sometimes. If this is the case, Smartt and his team will eventually find a big star exploding.
The other idea is that stars more than 30 times the mass of the sun don't make supernovae, but they collapse on themselves to make a black hole. This is the theory that Smartt was proposing, and it is a theory I've heard since I began studying astronomy. The idea is that these giant stars start to explode, but their gravity is so strong that the explosion can't break free.
We know that some stars make black holes at the end of their lives, because we can detect the presence of the black holes. But just because not even light can escape a black hole doesn't mean that we couldn't have seen an explosion. When the black hole forms in the center of a dying star, most of the star is far outside the black hole -- these black holes are less than 30 miles across, and the dying star is millions, if not tens or hundreds of millions, of miles across.
If you've ever had a bathtub filled with what looks like still water, and you open the drain to let the water out, you know that the water will start swirling around the drain. The water will not just disappear down the drain. The same would happen if you try to drain a star down the "drain" of a a black hole; the remains of the star will start to swirl. And as it swirls, it heats up due to friction, to temperatures of billions of degrees. At these temperatures, all kinds of nuclear reactions happen, and the dying star will release copious amounts of energy. You would think that at least some of that energy would escape the star to be seen as light on Earth.
Well, maybe, and maybe not. There are all kinds of complex physics to worry about, and our computers just cannot yet give a fully believeable model. So, for now, astronomers disagree on whether a star can just wink out of existence and make a black hole with no outward sign. And it will probably take Smartt and his team many dozens more supernovae before he can convince astronomers that the most massive stars are indeed disappearing without exploding, even if our computers can't tell us why.