"Long before it's in the papers"
September 26, 2008

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Finally, a supernova seen at birth

May 21, 2008
Courtesy National Radio Astronomy Observatory
and World Science staff

A stroke of luck has let as­tro­no­mers see the birth of a stel­lar ex­plo­sion known as a su­per­no­va, which oc­curs when a mas­sive star runs out of fu­el.

“For years, we have dreamed of see­ing a star just as it was ex­plod­ing,” said Ali­cia Soder­berg of Prince­ton Uni­ver­s­ity, lead­er of a team re­search­ing the find­ing. The images cap­tured will be “the Ro­set­ta Stone of su­per­no­va stud­ies for years to come,” she added.

On Jan. 9, the Swift sa­tel­lite caught a bright X-ray burst from an ex­plod­ing star (above, see up­per-right cir­cle). A few days lat­er, SN 2008D ap­peared in vis­i­ble light (be­low; see up­per right cir­cle again). (Cour­te­sy NA­SA/Swift Sci­ence Team/Ste­fan Imm­ler)

A stroke of luck has let astronomers see the birth of a stellar explosion known as a supernova, which occurs when a massive star runs out of fuel. “For years, we have dreamed of seeing a star just as it was exploding,” said Alicia Soderberg of Princeton University, leader of a team researching the finding. “This… is going to be the Rosetta Stone of supernova studies for years to come.” Although thousands of the explosions have been seen, all others were discovered while already underway. This is the first one caught at the outset—a breakthrough that could help unravel lingering mysteries about how such blasts really work, researchers said. Theorists had predicted for four decades that a bright burst of X-rays should appear as a supernova’s shock wave—the outward-moving zone of violent disturbance associated with an explosion—first emerges. But in order to see this burst, scientists faced the conundrum of knowing in advance where to point their telescopes to catch a supernova in the act of exploding. On Jan. 9, luck intervened. Soderberg and colleagues were making a scheduled observation of the galaxy NGC 2770, 88 million light-years from Earth, using the X-ray telescope on NASA’s Swift satellite. That’s when a bright burst of X-rays came from one of the galaxy’s spiral arms. Soderberg led a 38-person international team that quickly began studying the new object. To determine whether they had really seen the predicted early burst of supernova X-rays, they had to eliminate alternative explanations and show the object behaved like a normal supernova. They scrutinized it with Swift’s gamma-ray instrument, NASA’s Chandra X-ray Observatory, and the Hubble Space Telescope as well as several ground-based telescopes. The object was found to evolve like other supernovae. “That initial X-ray burst thus is the earliest observation ever of an exploding star,” Soderberg said. Stars much more massive than our Sun die out as supernovas, as they run out of fuel for the thermonuclear reactions that power them. With no more energy being released at the star’s core, the core collapses. Further collapse of the star is thought to cause a violent rebound that blasts most of the stars’s material into space, leaving a superdense neutron star or black hole. The details of this scenario aren’t well understood, though, largely because of the lack of observations of supernovas in production. “We think that every core-collapse supernova will show an X-ray burst like this one. If so, with the right instruments, we should be able to discover and study several hundred of them every year. Astronomical instruments planned for the future should then allow us to finally unravel the mystery of how these explosions occur,” Soderberg said. The scientists are reporting their findings in the May 22 issue of the research journal Nature. The findings provide “valuable support for the prevalent theories of supernova progenitors,” wrote University of Virginia astronomer Roger Chevalier, who wasn’t involved with the research, in an accompanying commentary.