Space.com (November 5, 2009)
"A new type of supernova – the explosive death of a star – has been discovered in which helium detonates on the surface of a white dwarf star.
"The exploding star, dubbed SN2002bj, was first observed seven years ago in the galaxy NGC 1821 by amateur astronomers, but was misclassified as a Type II supernova.
"There are two general types of supernovas: In a Type I, a star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites. In a Type II, a star runs out of nuclear fuel and collapses under its own gravity.
"But SN2002bj had a different signature than any of the variations known in these two types. In particular, it brightened and dimmed over the course of less than 27 days, whereas most supernovas brighten and dim over three or four months...."
So far, the model which matches best with observations is a binary system: two white dwarfs, one of them mostly helium, close enough so that helium from one gets pulled off and falls on the surface of the other. When enough helium builds up, a fusion reactions starts in the substance, releasing about a thousand times as much energy as a normal nova - but quite a lot less than comes from a collapsing-star supernova.
HypernovaI'm pretty sure that SN2002bj and the way it works will be discussed for quite a while - and new observations may mean that the white-dwarf-binary model has to be modified, or abandoned entirely. Or, not.
So far, SN2002bj is the only one of its kind known. My guess is that more will turn up, as other galaxies are studied in greater detail. And, with more examples, a clearer picture of how that sort of supernova works can be made.
Then there are hypernovae. There are a few probable hypernova remnants known - so eventually astronomers, stellar physicists, and cosmologists will get a handle on why (stars?) occasionally release such unreasonable amounts of energy, so fast. (More, at space.com, Jun 30, 2005, November 8, 2000, March 7, 2000)
More about supernovae at: