Scientists witness start of star's explosive death

AP Science Writer

   WASHINGTON (AP) -- In a stroke of cosmic luck, 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.

   "A star exploded right before my eyes," lead author Alicia Soderberg, an astrophysics researcher at Princeton University, said Wednesday in a teleconference.

   She likened it to "winning the astronomy lottery. 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 1 percent of the stars in the universe will die this way, 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 Filippenko likes to call "retired stars," which produce little energy.

   The first explosion of this supernova can only be seen in the X-ray wave length. It was spotted by NASA's Swift satellite, which looks at X-rays, and 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.

   Soderberg said that by seeing it live in X-rays, astronomers on Earth learned of the supernova about a month before they normally would.

   The chances of two simultaneous supernovae explosions so close to each other is maybe 1 in 10,000, Soderberg 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 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 decades-old theories on how stars explode and die, not providing many surprises, 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 wasn't 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, Soderberg said. The galaxy, NGC2770, is about 100 million light years away. One light year is 5.9 trillion miles.

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

   "The big stars live fast and die young," said Harvard astronomy professor Robert Kirshner. "We don't know if they leave a beautiful corpse."

   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's when the star core becomes so heavy it collapses in on itself, and the supernova starts with a shock wave of particles piercing through the shell of the star, which is what the Soderberg team captured on x-rays.

   People at home can simulate how this shockwave works, Filippenko said.

   Take a basketball and a tennis ball, get about five feet above the ground and rest the tennis ball on top of the basketball. Drop them together and the tennis ball will soar on the bounce. The basketball is the collapsing core and the tennis ball is the shockwave that was seen by astronomers, he said.

   (Copyright 2008 by The Associated Press.  All Rights Reserved.)