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The supernova as seen in ultraviolet images and X-ray images. ((NASA/Associated Press))

In a stroke of cosmic luck, U.S. astronomers for the first time witnessed the start of one of the universe's most fiery events: the end of a star's life as it exploded into a supernova.

On Jan. 9, astronomers used a NASA X-ray satellite to spy on a star already well into its death throes, when another star in the same galaxy started to explode. The outburst was 100 billion times brighter than Earth's sun. The scientists were able to get several ground-based telescopes to join in the early viewing and the first results were published in Thursday's issue of the journal Nature.

"It's like winning the astronomy lottery," said lead author Alicia Soderberg, an astrophysics researcher at Princeton University in New Jersey. "We caught the whole thing from start to finish on tape."

Another scientist, University of California at Berkeley astronomy professor Alex Filippenko, called it a "very special moment, because this is the birth, in a sense, of the death of a star."

And what a death blast it is.

"As much energy is released in one second by the death of a star as by all of the other stars you can see in the visible universe," Filippenko said.

Less than one per cent of the stars in the universe will die in a supernova, said Filippenko, who has written a separate paper awaiting publication. Most stars, including our sun, will get stronger and then slowly fade into white dwarfs, what he likes to call "retired stars," and which produce little energy.

The first explosion of this supernova can be seen only in the X-ray wavelength. It was spotted by NASA's Swift satellite, which looks at X-rays, and which happened to be focused on the right region, Soderberg said. The blast was so bright it flooded the satellite's instrument, giving it a picture akin to "pointing your digital camera at the sun," she said.

The chances of two simultaneous supernova explosions so close to each other is maybe 1 in 10,000, she said. The odds of looking at them at the right time with the right telescope are, well, astronomical.

Add to that the serendipity of the Berkeley team's viewing the same region with an optical light telescope. It took pictures of the star about three hours before it exploded.

This new glimpse of a supernova seems to confirm, with few surprises, decades-old theories on how stars explode and die, scientists said.

That makes the findings "a cool thing," but not one that fundamentally changes astrophysics, said University of California, Santa Cruz astrophysicist Stan Woosley, who was not part of the research.

The galaxy with the dual explosions is a run-of-the-mill cluster of stars, not too close and not too far from the Milky Way in cosmic terms, Ms. Soderberg said. The galaxy, NGC2770, is about 100 million light years away. One light year is 9.5 trillion kilometres.

The star that exploded – only about 10 million years old – was the same size in diameter as the sun, but about 10 to 20 times more dense.

The death of this star went through stages, with the core getting heavier in successive nuclear reactions and atomic particles being shed out toward the cosmos, Filippenko said. It started out in its normal life with hydrogen being converted to helium, which is what is happening in our sun. The helium then converts to oxygen and carbon, and into heavier and heavier elements until it turns into iron.

That is when the star core becomes so heavy it collapses in on itself, and the supernova starts with a shock wave of particles piercing the shell of the star,

That is the image Soderberg's team captured from the X-rays generated by the supernova explosion.