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.
"This is the fastest evolving supernova we have ever
seen," said Dovi Poznanski of the Lawrence Berkeley National Laboratory.
Poznanski rediscovered the supernova while searching through old observations.
The rapid dimming of the supernova and certain signatures in
its spectrum, such as a strong helium signal, suggest that this star exploded
by a previously unknown mechanism, first proposed by Lars Bildsten of UC Santa
Barbara.
The idea involves a binary pair of white dwarfs, one of
which is primarily made of helium that is being slowly siphoned off by its
companion. When enough helium accumulates on the surface of the main white
dwarf, an explosion occurs that powers a faint and brief thermonuclear
supernova.
This process is akin to a nova, where matter, mainly
hydrogen, falls onto a star, slowly building up and then exploding, but with
less force then a full-fledged supernova. SN2002bj had about 1,000 times more
energy than a typical nova though.
Christopher Stubbs of Harvard University has jokingly dubbed
these ".Ia" supernovas, because they are one-tenth as bright and last
for one-tenth the time as a Type Ia supernova.
The new supernova is described in the Nov. 6 issue of the
journal Science.
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