NASA’s Hubble Space Telescope has identified a star a million times brighter than the sun that exploded as a supernova in 2005 — well before it should have, according to current theories of stellar evolution.
The doomed star, estimated at about 100 times our sun’s mass, was not mature enough, according to theory, to have evolved a massive iron core of nuclear fusion ash. This is the supposed prerequisite for a core implosion that triggers a supernova blast.
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“This might mean that we are fundamentally wrong about the evolution of massive stars, and that theories need revising,” says Avishay Gal-Yam of the Weizmann Institute of Science, in Rehovot, Israel. The finding appears in the online version of Nature Magazine.
The explosion, called supernova SN 2005gl, was seen in the barred-spiral galaxy NGC 266 on October 5, 2005. NGC 266 is about 200 million light years away, in the constellation Pisces.
The progenitor was so bright that it probably belonged to a class of stars called Luminous Blue Variables (LBVs), “because no other type of star is as intrinsically brilliant,” says Gal-Yam. But there’s a wrinkle: as an LBV-class star evolves, it sheds much of its mass through a violent stellar wind. Only at that point does it develop a large iron core and ultimately explodes as a core-collapse supernova.
“The progenitor identification shows that, at least in some cases, massive stars explode before losing most of their hydrogen envelope, suggesting that the evolution of the core and the evolution of the envelope are less coupled than previously thought, a finding which may require a revision of stellar evolution theory,” co-author Douglas Leonard, from California’s San Diego State University, said in a press release.
One possibility is that the progenitor to SN 2005gl was really a pair of stars — a binary system — that merged. This would have stoked nuclear reactions to brighten the star enormously, making it look more luminous and less evolved than it really was.
“This also leaves open the question that there may be other mechanisms for triggering supernova explosions,” says Gal-Yam. “We may be missing something very basic in understanding how a superluminous star goes through mass loss.”
Gal-Yam and Leonard located the progenitor in archival images of NGC 266 taken in 1997. They then used the Keck telescope to precisely locate the supernova on the outer arm of the galaxy. A follow-up observation with Hubble in 2007 unequivocally showed that the superluminous star was gone.
Extremely massive and luminous stars topping 100 solar masses, such as Eta Carinae in our own Milky Way Galaxy, are expected to lose their entire hydrogen envelopes prior to their ultimate explosions as supernovae.
“These observations demonstrate that many details in the evolution and fate of LBVs remain a mystery,” said Mario Livio, of the Space Telescope Science Institute in Baltimore. “We should continue to keep an eye on Eta Carinae – it may surprise us yet again.”
MOSAIC CAPTIONS: [Top Center] 2005 ground-based image of the supernova; [Bottom Left] 1997 Hubble archival visible-light image of the region of the galaxy where the supernova exploded, with white circle marking the progenitor star; [Bottom Center] Near-infrared light photo of the supernova explosion taken on Nov. 11, 2005, with the Keck telescope, with the blast centered on the position of the progenitor; [Bottom Right] Visible-light Hubble follow-up image taken on September 26, 2007. The progenitor star is gone. Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel)