BY DR EMILY BALDWIN
Posted: 23 March, 2009
A star one hundred solar masses and one million times brighter than our Sun before it exploded, should not have self-destructed so early in its life, according to the fundamental theories of stellar evolution.
A prerequisite for a supernova blast is that the star has evolved a massive iron core of nuclear fusion “ash”. But according to the laws of stellar evolution, the star in question should not have been mature enough to have reached this stage. Because of its extreme brightness, astronomers suspect that the progenitor to SN 2005gl belongs to a class of stars called Luminous Blue Variables (LBVs), which shed much of their mass through violent stellar winds. At that point it develops a large iron core and ultimately explodes as a core-collapse supernova.
SN 2005gl and was observed in the barred-spiral galaxy NGC 266 on 5 October 2005. Pre-explosion pictures from the Hubble archive, taken in 1997, reveal the progenitor as a very luminous point source with an absolute visual magnitude of -10.3. Image: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel).
“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,” says Douglas Leonard from San Diego State University.
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,” says supernova expert Mario Livio of the Space Telescope Science Institute. “We should continue to keep an eye on Eta Carinae - it may surprise us yet again.”
Some astronomers speculate that the progenitor to SN 2005gl was actually a pair of stars. A binary system that merged would have stoked nuclear reactions to cause the extreme brightening, making it look more luminous and less evolved than it really is. “This also leaves open the question that there may be other mechanisms for triggering supernova explosions,” says Avishay Gal-Yam of the Weizmann Institute of Science, Rehovot, Israel. “We may be missing something very basic in understanding how a superluminous star goes through mass loss.”
Furthermore, the observations revealed that only a small part of the star’s mass was flung off in the explosion, with most of the material drawn into the collapsing core that has probably now become a black hole of around 10 to 15 solar masses.
Astronomers Gal-Yam and Leonard located the progenitor in archival Hubble images of NGC 266 taken in 1997, easily identifiable thanks to its luminosity. The team then used the Keck telescope to precisely locate the supernova on the outer arm of the galaxy, and a follow-up observation with Hubble in 2007 unequivocally showed that the superluminous star was gone.
Another case of curious supernova progenitors was the blue supergiant progenitor to SN 1987A. In this case it was thought that the progenitor star was once a red supergiant and at a later stage evolved back to blue supergiant status, leading to a major reworking of supernova theory. The progenitor star observed by Gal-Yam is too massive to have gone through such an oscillation to the red giant stage, so yet another new explanation is required, he says.