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  • Star Death Seen From Start
May 2008
GREENBELT, Md., May 22, 2008 -- Images captured by NASA's Swift satellite have allowed an international team of astronomers to observe, for the first time, a star just as it started to explode. Thousands of previous observations of such star explosions, or supernovae, conducted over the last 100 years occurred only once the blasts were well underway.

Galaxies typically host a supernova only once or twice every 100 years, making it nearly impossible to record one as it actually explodes, but in this case Swift was already monitoring another supernova in the galaxy when the second one exploded. Seeing one just moments after the violent event began is a major breakthrough that points the way to unraveling longstanding mysteries about how such explosions really work, astronomers said.supernovabefore.jpg
NASA’s Swift satellite took this image of supernova SN 2007uy in a galaxy 90 million light-years from Earth before the star SN 2008D (above and to the right) exploded. (Images: NASA/Swift Science Team/Stefan Immler)
"For years we have dreamed of seeing a star just as it was exploding, but actually finding one is a once in a lifetime event," said team leader Alicia Soderberg, a Hubble and Carnegie-Princeton Fellow at Princeton University in Princeton, N.J. "This newly born supernova is going to be the Rosetta stone of supernova studies for years to come."

A typical supernova occurs when the core of a massive star runs out of nuclear fuel and collapses under its own gravity to form an ultradense object known as a neutron star. The newborn neutron star compresses and then rebounds, triggering a shock wave that plows through the star's gaseous outer layers and blows the star to smithereens. Astronomers thought for nearly four decades that this shock "break-out" will produce bright x-ray emission lasting a few minutes.

But until this discovery, astronomers have never observed this signal. Instead, they have observed supernovae brightening days or weeks later, when the expanding shell of debris is energized by the decay of radioactive elements forged in the explosion.

"Seeing the shock break-out in x-rays can give a direct view of the exploding star in the last minutes of its life and also provide a signpost to which astronomers can quickly point their telescopes to watch the explosion unfold," said Edo Berger, a Carnegie-Princeton Fellow at Princeton.

Soderberg's discovery of the first shock breakout has been attributed to luck and Swift's multiple instruments, which observe celestial objects in gamma rays, x-rays, and ultraviolet light. On Jan. 9, 2008, Soderberg and Berger were using Swift to observe a two-week-old supernova known as SN 2007uy in the spiral galaxy NGC 2770, located 90 million light-years from Earth in the constellation Lynx, when they spotted an extremely bright five-minute x-ray outburst in NGC 2770 and quickly recognized that the x-rays were coming from another location in the same galaxy.supernovaAfter.jpg
Bright x-ray burst from exploding star SN 2008D.
"It was a gift of nature for Swift to be observing that patch of sky when the supernova exploded. But thanks to Swift's flexibility, we have been able to trace its evolution in detail every day since," said Swift lead scientist Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt.

In a paper appearing in the May 22 issue of Nature, Soderberg and 38 colleagues in the US, UK, Canada, China, Israel, South Africa, Taiwan, Germany and the Netherlands show that the energy and pattern of the x-ray outburst is consistent with a shock wave bursting through the surface of the progenitor star. This marks the birth of the supernova now known as SN 2008D.

Due to the significance of the x-ray outburst, Soderberg immediately mounted an international observing campaign to study SN 2008D. Observations were made with major telescopes such as the Hubble Space Telescope, the Chandra X-ray Observatory, the Very Large Array in New Mexico, the Gemini North telescope in Hawaii, the Keck I telescope in Hawaii, the 200-in. and 60-in. telescopes at the Palomar Observatory in California, and the 3.5-m telescope at the Apache Point Observatory in New Mexico.

The combined observations helped Soderberg and her colleagues pin down the energy of the initial x-ray outburst, which will help theorists better understand supernovae. The observations also show that SN 2008D is an ordinary Type Ibc supernova, which occurs when a massive, compact star explodes. The star was thought to have originally been more than 30 times the mass of the sun, but with a radius less than or equal to that of the sun. Radio and x-ray observations also ruled out that the blast was a gamma-ray burst, a rare type of stellar explosion.

"This was a typical supernova," said Swift team member Stefan Immler of Goddard. "The significance is not the explosion itself, but the fact that we were able to see the star blow up in real time, which gives us unprecedented insight into the explosion process."

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