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“Impossibly” large stellar explosions find explanation

Dec. 19, 2013
Courtesy of of the Uni­vers­ity of Cal­i­for­nia San­ta Bar­ba­ra
and World Science staff

A newly iden­ti­fied type of stel­lar ex­plo­sion breaks all the old rules—and records—for such ex­plo­sions, a study has found. But as­tro­no­mers say this ul­tra-powerful type of blast rarely oc­curs in the mod­ern uni­verse.

The “su­pe­r­lu­mi­nous” out­bursts have been com­ing to sci­ent­ists’ at­ten­tion since the end of the last dec­ade, but no one knew what they were at first.

SNLS-06D4eu and its host gal­axy ap­pear here where the ar­row points—not as one of the bright ob­jects in the fore­ground, which are stars in our own gal­axy. The ex­treme dis­tance makes SNLS-06D4eu show up as just a faint dot, but its true bright­ness is be­lieved to be ex­traor­di­nary. Most of the oth­er back­ground ob­jects in the pho­to are dis­tant ga­lax­ies. (Cour­te­sy SNLS)


The new study pro­poses that the blasts are probably as­so­ci­at­ed with the forma­t­ion of a mag­ne­tar, an ex­tra­or­di­narily mag­net­ized, diz­zy­ingly fast-spin­ning neu­tron star. A neu­tron star is a type of highly con­densed star in which the en­tire weight of the sun can be packed in­to an ob­ject the size of a city.

The new find­ings came af­ter as­tro­no­mers af­fil­i­at­ed with a proj­ect called the Su­pe­r­no­va Leg­a­cy Sur­vey found two of the bright­est and most dis­tant “su­pe­r­novae,” or ex­plod­ing stars, ev­er recorded. 

The events were meas­ured to take place about 10 bil­lion light-years away; a light-year is the dis­tance light trav­els in a year. The mea­sure­ments imply that the bursts oc­curred early in cos­mic his­to­ry.

The blasts were al­so a hun­dred times more lu­mi­nous than a nor­mal su­pe­r­no­va—an ex­plod­ing, dy­ing star, as­tro­no­mers re­ported. The find­ings are pub­lished in the Dec. 20 is­sue of the As­t­ro­phys­i­cal Jour­nal.

The re­cently found su­pe­r­no­vae are es­pe­cially puz­zling be­cause the mech­an­ism that pow­ers most such blast­s—the col­lapse of a gi­ant star to a black hole or nor­mal neu­tron star—could­n’t ex­plain their ex­treme lu­mi­nos­ity. Dis­cov­ered in 2006 and 2007, the su­pe­r­no­vae were so un­usu­al that as­tro­no­mers at first could­n’t fig­ure out what they were or how far away. 

“We had no idea... even wheth­er they were su­pe­r­no­vae or wheth­er they were in our gal­axy or a dis­tant one,” said lead au­thor D. An­drew How­ell of the Uni­vers­ity of Cal­i­for­nia San­ta Bar­ba­ra, who is al­so a staff sci­ent­ist at Las Cum­bres Ob­serv­a­to­ry Glob­al Tel­e­scope Net­work. 

“I showed the ob­serva­t­ions at a con­fer­ence, and ev­eryone was baf­fled. No­body guessed they were dis­tant su­pe­r­no­vae be­cause it would have made the en­er­gies mind-bogglingly large. We thought it was im­pos­si­ble.”

One of the newly dis­cov­ered su­pe­r­no­vae, named SNLS-06D4eu, is the most dis­tant and pos­sibly the most lu­mi­nous mem­ber of the emerg­ing class of ex­plo­sions, now called supe­rlu­mi­nous su­pe­r­no­vae. A spe­cial sub­class of these, in­clud­ing the two in the stu­dy, is found to lack the el­e­ment hy­dro­gen.

The study pro­poses that the su­pe­r­no­vae are likely pow­ered by the crea­t­ion of a mag­ne­tar, an ex­tra­or­di­narily mag­net­ized neu­tron star spin­ning hun­dreds of times per sec­ond. Mag­ne­tars have mag­net­ic fields a hun­dred tril­lion times that of the Earth. While a hand­ful of these supe­rlu­mi­nous su­pe­r­no­vae have been seen, and mag­ne­tar forma­t­ion had been sug­gested as a pos­sible en­er­gy source, How­ell and his col­leagues de­scribe their work as the first to match de­tailed ob­serva­t­ions to mod­els of what such an ex­plo­sion might look like.

Co-au­thor Dan­iel Kasen from UC Berke­ley and Law­rence Berke­ley Na­t­ional Lab cre­at­ed mod­els of the su­pe­r­no­va that ex­plained the da­ta as the ex­plo­sion of a star only a few times the size of the sun and rich in car­bon and ox­y­gen. The star likely was in­i­tially much big­ger but ap­par­ently shed its out­er lay­ers long be­fore ex­plod­ing, leav­ing only a small­ish, na­ked co­re.

“What may have made this star spe­cial was an ex­tremely rap­id rota­t­ion,” Kasen said. “When it ul­ti­mately died, the col­laps­ing co­re could have spun up a mag­ne­tar like a gi­ant top. That enor­mous spin en­er­gy would then be un­leashed in a mag­net­ic fury.”

The blasts were dis­cov­ered as part of the Su­pe­r­no­va Leg­a­cy Sur­vey, a five-year pro­gram based on ob­serva­t­ions from sev­eral tele­scopes to study thou­sands of su­pe­r­no­vae. It took sub­se­quent ob­serva­t­ions of the faint host gal­axy with the Very Large Tel­e­scope in Chil­e for as­tro­no­mers to de­ter­mine the dis­tance and en­er­gy of the ex­plo­sions, and years of sub­se­quent the­o­ret­i­cal work to fig­ure out what could pro­duce such as­tound­ing en­er­gy.

The su­pe­r­no­vae are so far away that the ul­tra­vi­o­let light re­leased in the ex­plo­sion was stretched out fan­tas­tic­ally by the ex­pan­sion of the uni­verse dur­ing the time the light took to get he­re, as­tro­no­mers said. As a re­sult, it reached Earth as the vis­i­ble type of light, whose waves are highly stretched out com­pared to ul­tra­vi­o­let waves. In­i­tial ob­serva­t­ions baf­fled as­tro­no­mers be­cause they had­n’t seen su­pe­r­no­vae with such ex­treme ul­tra­vi­o­let. This gave them a rare glimpse in­to the in­ner work­ings of these su­pe­r­no­vae, the au­thors said. Supe­rlu­mi­nous su­pe­r­no­vae are so hot that the peak of their light out­put is ul­tra­vi­o­let, an en­er­get­ic form of light. Be­cause the Earth’s atmosphere blocks ul­tra­vi­o­let, such ob­jects had nev­er been fully ob­served.

The su­pe­r­no­vae ex­plod­ed when the uni­verse was only four bil­lion years old, less than a third of its pre­s­ent age and “be­fore the sun even ex­ist­ed,” How­ell said. The supe­rlu­mi­nous su­pe­r­no­vae are rare, oc­cur­ring pe­rhaps once for ev­ery 10,000 nor­mal su­pe­r­no­vae, he added, and seem to ex­plode pref­er­en­tially in more prim­i­tive ga­lax­ies com­mon in the early uni­verse.

“These are the di­no­saurs of su­pe­r­no­vae,” How­ell said. “They are all but ex­tinct to­day, but they were more com­mon in the early uni­verse. Luckily we can use our tele­scopes to look back in time and study their fos­sil light. We hope to find many more.”


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A newly identified type of stellar explosion breaks all the known rules—and records—for such outbursts, a study has found. But this type of ultra-powerful blast rarely occurs in the modern universe, astronomers said. Astronomers said a type of outburst dubbed “superluminous” has been coming to their attention since the end of the last decade, but no one knew quite what the explosions were at first. The new study proposes that the blasts are probably associated with the formation of a magnetar, an extraordinarily magnetized, dizzyingly fast-spinning neutron star. A neutron star is a type of highly condensed star in which the entire weight of the sun can be packed into an object the size of a city. The new findings came after astronomers affiliated with a project called the Supernova Legacy Survey found two of the brightest and most distant “supernovae,” or exploding stars, ever recorded. The events were measured to take place about 10 billion light-years away; a light-year is the distance light travels in a year. The measurements imply that the bursts occurred early in cosmic history. The blasts were also a hundred times more luminous than a normal supernova, astronomers reported. The findings are published in the Dec. 20 issue of the Astrophysical Journal. The recently found supernovae are especially puzzling because the mechanism that powers most such blasts — the collapse of a giant star to a black hole or normal neutron star — couldn’t explain their extreme luminosity. Discovered in 2006 and 2007, the supernovae were so unusual that astronomers at first couldn’t figure out what they were or how far away. “We had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one,” said lead author D. Andrew Howell of the University of California Santa Barbara, who is also a staff scientist at Las Cumbres Observatory Global Telescope Network. “I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible.” One of the newly discovered supernovae, named SNLS-06D4eu, is the most distant and possibly the most luminous member of the emerging class of explosions, now called superluminous supernovae. A special subclass of these, including the two in the study, is found to lack the element hydrogen. The study proposes that the supernovae are likely powered by the creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second. Magnetars have magnetic fields a hundred trillion times that of the Earth. While a handful of these superluminous supernovae have been seen, and magnetar formation had been suggested as a possible energy source, Howell and his colleagues describe their work as the first to match detailed observations to models of what such an explosion might look like. Co-author Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab created models of the supernova that explained the data as the explosion of a star only a few times the size of the sun and rich in carbon and oxygen. The star likely was initially much bigger but apparently shed its outer layers long before exploding, leaving only a smallish, naked core. “What may have made this star special was an extremely rapid rotation,” Kasen said. “When it ultimately died, the collapsing core could have spun up a magnetar like a giant top. That enormous spin energy would then be unleashed in a magnetic fury.” The blasts were discovered as part of the Supernova Legacy Survey, a five-year program based on observations from several telescopes to study thousands of supernovae. It took subsequent observations of the faint host galaxy with the Very Large Telescope in Chile for astronomers to determine the distance and energy of the explosions, and years of subsequent theoretical work to figure out what could produce such astounding energy. The supernovae are so far away that the ultraviolet light released in the explosion was stretched out fantastically by the expansion of the universe during the time the light took to get here, astronomers said. As a result, it reached Earth as the visible type of light, whose waves are highly stretched out compared to ultraviolet waves. Initial observations baffled astronomers because they hadn’t seen supernovae with such extreme ultraviolet. This gave them a rare glimpse into the inner workings of these supernovae, the authors said. Superluminous supernovae are so hot that the peak of their light output is ultraviolet, an energetic form of light. Because the Earth’s atmosphere blocks ultraviolet, such objects had never been fully observed. The supernovae exploded when the universe was only four billion years old, less than a third of its present age and “before the sun even existed,” Howell explained. Such superluminous supernovae are rare, occurring perhaps once for every 10,000 normal supernovae, he added, and seem to explode preferentially in more primitive galaxies common in the early universe. “These are the dinosaurs of supernovae,” Howell said. “They are all but extinct today, but they were more common in the early universe. Luckily we can use our telescopes to look back in time and study their fossil light. We hope to find many more.”