Multi-star collision may have caused largest supernova

[Androsphinx]

From the New Scientist

A mystery over what caused the brightest supernova ever observed finally appears to have been solved. Two astronomers in the Netherlands say the explosion was the result of a cosmic pile-up: dozens of massive stars crashing into each other, producing a monstrous heavyweight star that eventually exploded, leaving a giant black hole in its wake.

Supernova 2006gy burst into view in September 2006 in a distant galaxy, 240 million light years away. The blast was 100 times more powerful than a normal supernova, suggesting the exploding star weighed in at more than a hundred times the mass of the Sun.

But astronomers found a puzzling detail in their observations: the supernova debris contained large amounts of hydrogen, which they would not have expected for such a massive star: It should have shed its outer hydrogen layers at an earlier stage.

Although several possible explanations have been put forward to explain the massive blast – including the formation of a quark star and the production of huge quantities of antimatter – no single theory could easily explain all of the observations.
Multi-star pile-up

Now, in the journal Nature, Simon Portegies Zwart and Edward van den Heuvel of the University of Amsterdam say 2006gy may have been the result of a multiple-star collision in a dense stellar cluster.

They say dozens of stars – some of them hydrogen-rich – collided to form a giant weighing in at over 100 Suns. Unable to support its own weight, the colossus blew itself to smithereens in an explosion that outshone its home galaxy.

Computer simulations reveal that multiple collisions are quite likely in very dense star clusters. Our own galaxy, the Milky Way, contains two such superdense clusters (the Arches cluster and the Quintuplet cluster), close to its centre. Indeed, supernova 2006gy also occurred close to the core of its host galaxy.

If Portegies Zwart and van den Heuvel are right, the dense cluster of stars should become visible once the supernova has faded sufficiently. This should happen a few years from now, they say.
Multiple explosions

There may be another explanation for the brightness of the supernova, however. In the same issue of Nature, Stan Woosley of the University of California at Santa Cruz and his colleagues show how multiple explosions in a single, very massive star could account for 2006gy’s behaviour.

In this model, every explosion produces an expanding shell of material. When new ejecta catches up and collides with an older shell, so much energy is released that the result will look like an over-luminous supernova.

"One could, I suppose, make our massive star by merging smaller ones," Woosley told New Scientist, "but that was not part of our model and does not seem necessary."

According to Woosley’s calculations, the star may not yet have collapsed into a black hole. A new explosion might happen in about 10 years or so, he says.

I have both the Nature articles as PDFs, if anyone wants them