Using cameras aboard an orbiting NASA satellite, Princeton astronomer Alicia Soderberg has captured a never-before-seen cosmic phenomenon - a burst of X rays marking the start of a supernova.

These exploding stars can light up surrounding space with the power of a billion suns. But no one had ever seen the initial flash of a supernova, thought to last a few minutes before a more steady glow takes over.

An astronomer could train a camera on a given galaxy for a century or more and get nothing.

With a stroke of serendipity, however, Soderberg had the right cameras pointed to the right place at the right time.

Supernovae are more than just cosmic firecrackers. They and other dying stars spew out heavy elements that infused the planets of our solar system and made life possible.

The calcium in our bones, the iron in our blood, and the carbon that pervades all life were formed during the death throes of faraway stars. And the explosions spread these elements through the universe like so much cosmic compost.

Catching this event was improbable.

"This was the earliest we've ever seen a supernova," said Roger Chevalier, an astronomer at the University of Virginia.

The closest they had come before was a dying star called 1987A, which was caught hours after exploding.

Soderberg said this new finding could help astronomers better understand what kinds of stars explode, the physics of the explosions, and how they influence the evolution of the universe.

Previously known supernovae were seen only later, usually as an afterglow of radioactive gases that persist for a few months.

Scientists working out the physics of these explosions had long predicted an initial burst of X rays called breakout light. But with supernovae occurring only once a century or so in a given galaxy, there was no way to know where to look.

Adding to the challenge was the fact that Earth's atmosphere deflects X rays, so this blast could only be detected using one of two satellites.

"After my colloquium, I rushed and looked at the data," she said, "and obviously there was something really bright in the image that wasn't there two days before."

The X rays had overwhelmed the detector, she said. "It's as if you took a digital camera and pointed it at the sun and took a picture."

Against all odds, she realized that another supernova was exploding in this same distant galaxy that had produced one just a month before.

Later, the Hubble Space Telescope would be aimed at the supernova, whose brightness in other wavelengths started to increase over the next few days.

Before stars explode, they start to collapse, said astronomer Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics. When a star runs out of fuel, it loses the gas pressure that had pushed outwards to balance the inward force of gravity.

If it's large enough, the star's core collapses so hard as to crush its very atoms, creating a superdense "neutron star" in about a second.

During a star's death throes, Kirshner said, nuclear reactions glue together small elements to make carbon, oxygen, and other elements up to iron on the periodic table. Heavier elements such as gold and uranium start to form in the heat of the blast.

So all the carbon, oxygen, and heavier elements on Earth were made in other stars that exploded long before the sun was born, Kirshner said.

Our sun will die, too, but without so much fanfare. "There's no explosion for us," Kirshner said. Instead, in about five billion years, the sun will swell to become a red-giant, which will kill all life, he said, after heating the seas, "and boiling all the lobsters."

To explode like a supernova, a star has to be 10 or 20 times the mass of our sun, Kirshner said.

This latest supernova can't be seen by the naked eye because it's so far away. There's the potential for a good show in our own galaxy, but we haven't had one for a long time.

Because there's so much dust in our galaxy, we can't see all the supernovae that have exploded in recent centuries, said Kirshner, though Renaissance astronomers Tycho Brahe and Johannes Kepler reported seeing them.

Chinese astronomers described some very bright ones, and observers in various places reported something in 1006 that outshone the full moon. In 1987, a supernova in a neighboring galaxy was visible from the Southern Hemisphere.

"If it hadn't happened in that hour, she would have missed it," he said.

On the other hand, he said, Soderberg was quick to recognize what she had found and act on it. "Sometimes if you're really energetic, you manufacture your own luck."