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 Digital renderings captured from a video animation show a representation of a supernova in the act of exploding.Scientists viewing images of the galaxy NGC2770 using NASA's Swift satellite on Jan. 9, 2008, observed Supernova 2008D burst onto the scene, giving scientists the unique opportunity to witness the birth of a supernova.
Image courtesy of Princeton University, Gemini Observatory, and NASA.
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Exploding Star, Caught on Tape
Observation of Sudden Supernova's Birth is a First
May 21, 2008
By Jason Socrates Bardi
ISNS Contributor
Princeton, NJ -- Call it fantastic timing. Early this year, a group of astronomers led by Princeton University's Alicia Soderberg were using NASA's Swift satellite to observe a new supernova—one of those spectacular explosions that mark the end of a massive star's life.
This supernova was in a galaxy some 100 million light years away. It was relatively unremarkable, Soderberg admits. But then something extraordinary happened. On January 9, in what some astronomers are calling a remarkable stroke of good luck, another star in their field of view went supernova.
"We actually watched the star explode," says Soderberg, who was in Michigan, talking to an audience of fellow scientists about her research when the call about the supernova came from her colleague. This set off a week of scrambling to get astronomers across the globe to point telescopes at the supernova to confirm and better study the phenomenon.
Astronomers have never before seen a star at the first moments of its explosive death. Usually, astronomers miss the earliest flash of a supernova because the explosion is only visible to orbiting x-ray detectors on platforms like Swift. In the latest issue of Nature, Soderberg and her colleagues describe how the supernova's initial burst lasted a few minutes and then faded away. Its power was remarkable. In 10 minutes, the exploding star expelled the about the same amount of energy as the sun puts out in 82,000 years.
"It's incredibly serendipitous," says Harvard astrophysics professor Josh Grindlay, a supernova expert who was not involved in the research. "This almost certainly provides a whole new way of detecting supernovae."
Though astronomers have known about supernovas for hundreds of years, the events are rare, only seen about once a century in any given galaxy. They are only visible to the eye or to ordinary telescopes a few weeks after the initial burst, when the supernova begins to shine brightly—sometimes becoming one of the brightest objects in the evening sky.
Supernovae are remarkable events not only for such displays of power but because they culminate a natural process of stellar renewal—sort of like cosmological compost. As famed physicist Hans Bethe said in 1967, upon winning his Nobel Prize, “Stars have a life cycle much like animals. They get born, they grow, they go through a definite internal development, and finally they die, to give back the material of which they are made so that new stars may live.”
Stars are formed when clouds of mostly hydrogen collapse into a plasma orb like our own sun, and for billions of years, stars are fueled by the fusion of hydrogen to form helium in their cores. Eventually the hydrogen gets used up, and the star begins to fuse helium. When the helium is gone, the star continues fueling itself by fusing heavier and heavier elements, but this process terminates as iron is formed. Iron cannot fuel the star's energy production because fusing iron takes more energy than it produces.
As iron accumulates in the star's core and the fuel runs out, a cataclysmic event awaits. Running out of fuel doesn't simply shut a star to off. During a star's life, nuclear fusion provides a constant source of pressure that keeps the star from collapsing in on itself under its own weight. When the fuel disappears, the star collapses. If the star is massive enough (it must be much more massive than the sun), this collapse creates a supernova.
What causes a supernova is that the star's core collapses into a tiny, incredibly dense orb. But the rest of the material in the star collapses as well, and when material from the outer layers of the star falls upon this dense core, it bounces off. This forms a shock wave that races out to the star's edge, and breaks out, creating the enormous burst of X rays like the one that Soderberg and her colleagues captured on tape.
The explosion also creates heavy elements and spreads these elements throughout space. The heavy elements in the universe, including those on Earth, originated long ago in supernova explosions. Some of this matter is radioactive, and its decay over time creates the brightly visible display we associate with supernovae.
The accidental discovery of the new supernova in January is significant, says Soderberg, because it demonstrates that the first light of exploding stars is these x-ray bursts. They are like early warning beacons heralding the sometimes luminous display that follows.
Bigger and better telescopes proposed for the future will be able to scan the skies and detect these x-ray bursts routinely from all the nearby galaxies. Grindlay, the Harvard astronomer, is the principle investigator on a candidate future NASA mission called EXIST that will scan the entire heavens every few hours and look for nearby black holes and distant gamma ray bursts. If built, the telescope should be able to detect many supernovae in their first explosive moments—perhaps hundreds a year.
"If you can look for events going off this way," says Grindlay, "you have a powerful trigger telling you where to look and when." This should help other astronomers who are looking for neutrinos and gravity waves—other phenomena created by the exploding stars.
"The future is bright," says Soderberg.
Jason Socrates Bardi is a senior science writer for the Inside Science News Service.
This story is provided for media use by the Inside Science News Service, which is supported by the American Institute of Physics, a not-for-profit publisher of scientific journals. Please credit ISNS. Contact: Jim Dawson, news editor, at jdawson@aip.org.
Alicia Soderberg, Carnegie-Princeton Fellow and Hubble Postdoctoral Fellow, Princeton University
This animation (quicktime, 1.5MB) shows an artist's rendering of the shock wave discovered by Princeton University's Alicia Soderberg and a team of scientists. A supernova is born when the core of a massive star (the blue orb) runs out of nuclear fuel and collapses under its own gravity to form an ultradense object known as a neutron star. The shock wave erupts and ripples through the star, emitting X-rays (seen here as bright white light). The remnants of the explosion cool (the white light gets smaller), and then the visual light from the supernova glows (seen as yellow clouds). The fading white dot in the middle of the animation represents a newly born neutron star.
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