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Spectrum reveals supernova surprise
Astronomers have developed a new way to determine the size of a star before it went supernova, by studying its chemistry.

The discovery will help scientists better understand the evolution of stars including our Sun.

The study was carried out by a Japanese research group led by Professor Koji Kawabata of Hiroshima University, and colleagues from the Institute for the Physics and Mathematics of the Universe.

Reporting in the journal Nature, the scientists were able to determine the mass of a star, known as progenitor, before it became a type 1b supernova.

They identified chemical signatures in the spectrum of supernova SN2005cz, including an abundance of calcium. This led them to conclude the progenitor's mass at its birth was about ten solar masses, or ten times the size of the Sun.
Strange supernovae

They say their initial observations of the event were puzzling. Firstly because it showed up in an elliptical galaxy, which usually lacks massive stars that typically become type Ib supernovae.

It was also fairly faint, reaching only 20% the brightness of other type Ib supernovae, and it faded quickly.

They then examined the supernova's spectrum using the Subaru telescope in Hawaii and discovered an emission line from oxygen - strongest in type Ib supernovae - was nearly missing. Instead, they found a very strong emission line representing calcium.

Computer models predicted the presence of calcium as a unique feature in progenitor's 10 solar masses.

These sized supernovae are believed to be main sources for some of the key elements for life on Earth, including calcium, carbon and nitrogen.

A second supernovae study in Nature reported observations of SN2005E, which look similar to SN2005cz.

However, it suggests that SN2005E is a new type of supernovae involving an explosion within the surface layer of a white-dwarf rather than a core collapse.
Rare events

Anglo Australian Observatory astronomer, Dr Stuart Ryder says only a half dozen or so supernovae have been identified as coming from 10 solar mass progenitor stars.

"That's been a problem because compared to other stars, the ratio of supernovae events of this size don't match the ratio of stars of this mass," he says.

"The work also helps astronomers determine the lower limit at which a star will go supernovae at the end of its life".

"Current theories tell us anything smaller than eight solar masses would simply puff off its outer layers and become a white dwarf.

"Only stars more massive can achieve core collapse and go supernova."

Ryder says "scientists are still looking for an eight solar mass supernova.

"Anything smaller would really throw astronomers into consternation causing our theories a lot of problems."

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