Twenty-one million years ago in the Pinwheel Galaxy, a neighbor of our own Milky Way, an old star exploded and began to blaze like a billion suns. Now the blaze of light is visible on Earth, and astronomers are calling it the supernova of a generation.

A robotic telescope at the Lawrence Berkeley National Laboratory detected the exploding star on Aug. 24. Every night, astronomers spot several exploding stars across the universe. But this one was very close — in cosmic terms — and spotted soon after its light reached Earth.

An automated sky survey that searches for new nighttime objects, along with a telescope at California's Palomar Observatory, first flagged the supernova. Soon, telescopes around the world swiveled to take a peek.

"This is not an every-year event,'' said Robert Kirshner, a veteran Harvard University supernova hunter. "This is an every-decade-or-four event.''

Astrophysicist Peter Nugent said the blaze will continue to brighten until Wednesday or Thursday, visible just above the handle of the Big Dipper to backyard astronomers with binoculars.

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