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Birth of a Supernova
by Rick Groleau
Ever since Edwin Hubble's 1929 discovery that the universe is expanding,
astronomers have known that the more distant a galaxy, the faster it's
traveling away from us. They've long wondered to what degree gravity is slowing
down that expansion rate. If gravity is weak, the universe would expand
forever; if gravity is strong enough, the universe would eventually stop
expanding and then fall back in on itself.
Well, to the surprise of almost everyone, the evidence presented by astronomers
in the NOVA program "Runaway Universe" shows that the expansion rate is not slowing down at all, but
rather accelerating.
Their discovery would not have been possible without a special cosmic yardstick—exploding stars, or supernovae—specifically the variety known as Type Ia.
Because Type Ia supernovae are visible over distances of five billion
light-years and more, astronomers can observe them and gauge the distances to
the galaxies they reside in, for the simple reason that Type Ia's shine with a
known brightness.
What makes each Type Ia burn with the same brightness? And why don't other
kinds of supernovae shine with equal brightness? Here, based on the latest
thinking, are step-by-step explanations of the two most common types of
supernovae: Type Ia, which takes place in two-star systems, and Type II, which
occurs when a massive star runs out of its fuel and blows up.
Supernova, Type Ia
Supernova, Type II
Requires the free Flash plug-in
Supernova, Type Ia
Supernova, Type II
(non-Flash version)
Rick Groleau is managing editor for NOVA Online.
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© | Updated December 2000
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