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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)


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, dubbed SN 2008D, 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 su­per­no­vae, 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 star 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 at birth. “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.


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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.