His idea is that an extremely massive star can explode not once, like a normal supernova,
but again and again. When Woosley and two colleagues did detailed calculations for
their model, the results matched the observations on SN 2006gy.
The researchers describe their model in a paper to be published in the November 15
issue of the journal Nature. Woosley's co-authors are Sergei Blinnikov and Alexander
Heger of Los Alamos National Laboratory. Blinnikov is a visiting researcher at UCSC
from the Institute of Theoretical and Experimental Physics in Moscow.
SN 2006gy was a "stupendously bright supernova", says Woosley - "and we think we
have the leading model to explain it."
Astronomers usually think of a supernova as the death of a star, he added, "but in
this case the same star can blow up half a dozen times."
The first explosion throws off the star's outer shell. This produces a not-very-bright
display. The second explosion puts another supernova's worth of energy into a second
shell. This expands at very high speed until it collides with the first shell. When
this happens it produces an extraordinarilybrilliantdisplay.
In this model far more of the kineticenergy of the explosion is converted into light
than in a normal supernova - which converts only 1% of its kineticenergy into light.
"The two shells collide out at a distance such that the full kineticenergy is converted
into light," Woosley said. "So it is up to 100 times more luminous than an ordinary
This new mechanism needs an extremely massive star. It would have to be 90 to 130
times the mass of the Sun. As a star this huge nears the end of its life, the temperature
in the core gets extremely hot.
"As the corecontracts it goes deeper into instability until it collapses and begins
to burn fuel explosively," Woosley said. "The star then expands violently."
For stars between 90 and 130 times the mass of our sun, this cycle can happen several
times. "It hits this instability, violently expands, then radiates and contracts
until it gets hotter and hits the instability again. It keeps going until it loses
enough mass to be stable again."
Stars large enough for this to happen are very rare in our own galaxy. But they may
have been more common in the early universe. "Until recently, we would have said
such stars don't exist," Woosley says.
"But any mechanism that could explain this event requires a very large mass."
Other researchers had suggested pair instabilityas a possible mechanism for some
supernovae. But the idea of repeated explosions is new. The scientists are calling
it pulsational pair instability.
According to Woosley, the new mechanism can produce a variety of explosions.
"You could have anywhere from two to six explosions, and they could be weak or strong,"
he said. "A lot of variety is possible."
It gets even more complicated, he added. What is left behind at the end of all the
collapsing and expanding is still a huge star - about 40 times the mass of our sun.
"This continues to evolve and eventually makes an iron core and collapses, so you
can end up with a gamma-ray burst.
"The possibilities are very exciting."
More help with words
What's it all about?
A supernova is the explosion of a large star. How much brighter than a normal supernova
was SN 2006gy?