nce a second or so, somewhere in the universe, a
star blows itself to smithereens, blossoming momentarily to a
brilliance greater than a billion suns.
Nobody understands how these events, among the most violent in
nature, actually happen. But, until recently, that didn't much
matter unless you were a practitioner of the arcane and messy branch
of science known as nuclear astrophysics.
Lately, however, supernovas have become signal events in the life
of the cosmos, as told by modern science.
Using a particular species of supernova, Type 1a, as cosmic
distance markers, astronomers have concluded that a mysterious "dark
energy" is wrenching space apart, a discovery that has thrown
physics and cosmology into an uproar.
As a result, the fate of the universe - or at least our knowledge
of it - is at stake, and understanding supernovas has become
essential.
Astronomers are busy on many fronts trying to figure out the
details of these explosions - scanning the skies to harvest more of
them in the act, peering at the remains of ancient supernovas to
seek a clue to their demise, harnessing networks of supercomputers
to calculate moment by moment reactions in the heart of hell.
This has resulted recently in a kind of two-steps-forward,
one-step-back progress, encouraging astronomers that they are on the
right track, generally, with their theories, but at the same time
underscoring complexities and baffling puzzles when it comes to
pinning down the details of what happens in the explosions.
Last month members of an international team of astronomers led by
Dr. Pilar Ruiz-Lapuente of the University of Barcelona announced
that they had found a star speeding away from the site of a
supernova blast seen in 1572 by the astronomer Tycho Brahe. This
supernova, which appeared as a "new star" in the constellation
Cassiopeia, was one of the earliest studied by astronomers, and
helped shatter the Aristotelian notion that the heavens above the
Moon were immutable.
The newly discovered star, presumably the companion of the star
that exploded, supports a long-held notion that such explosions
happen in double star systems when one star accumulating matter from
the other reaches a critical mass and goes off like a bomb.
Meanwhile, members of a group of astrophysicists using a network
of powerful supercomputers to simulate supernova explosions say they
have succeeded for the first time in showing how such a star could
blow up.
Over the course of 300 hours of calculation at the University of
Chicago's Center for Astrophysical Thermonuclear Flashes, otherwise
known as the Flash center, they watched bubbles of thermonuclear
fury rise from the depths of the star like a deadly jellyfish and
then sweep around the surface and collide in an apocalyptic
detonation that Dr. Donald Lamb, a Chicago astrophysicist, called
"totally bizarre and novel."
If true, the Chicago results could help explain not only how
stars explode, but why the explosions are almost but not exactly
alike, allowing astronomers to better calibrate their measurements
of dark energy.
Many supernova experts said, however, that such computer
simulations were more of a good start than a final answer. Dr. J.
Craig Wheeler of the University of Texas called the Flash center
work "a courageous calculation," but added that many details needed
to be filled in. "I don't think this is the end of the story," he
said. The story of Type 1a supernovas, experts have long agreed,
begins with a dense cinder known as a white dwarf, composed of
carbon and oxygen, which is how moderate-size stars like the Sun,
having exhausted their thermonuclear fuels of hydrogen and helium,
end their lives.
If it happens to be part of a double star system, the white dwarf
can accumulate matter from its companion until it approaches a
limit, known as the Chandrasekhar mass - about 1.4 times the mass of
the Sun.
At that point, so the story goes, the pressure and density in the
previously dead star will be great enough to reignite the star and
thermonuclear reactions will ripple upward, transmuting the carbon
and oxygen into heavier and heavier elements, ripping the white
dwarf apart while its companion goes flying off.
Until recently, however, there was little evidence of this. Two
white dwarfs could collide, for example, and blow up. In that case
there would be no survivor.
Tycho Brahe's supernova has now offered new evidence for the
former model, of the white dwarf bomb.
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