News in Science

Stellar explosion redefines cosmic measure

Super candle The spectacular death of a star has given astronomers a rare opportunity to recalibrate cosmic distance scales across the universe.

The type 1a supernova, reported in the Astrophysical journal, increases the precision of measurements beyond the Milky Way and improves our understanding of these cataclysmic events.

The supernova called SN 2012fr, occurred last October in the galaxy NGC 1365 and sparked tremendous interest, according to the study's lead author Dr Michael Childress of the Australian National University.

"This galaxy was already known to contain a type of star called a cepheid variable," says Childress.

Both type 1a supernovae and cepheids are used as cosmic distance markers, or standard candles.

Astronomers can calculate exactly how bright they are -- the fainter they appear the further away they must be.

"But these things operate on different distance scales," says Childress.

"The cepheid variable method, we can use up to maybe 100 million light years at most. But supernovae can be seen very far into the past, billions and billions of light years."

On the rare occasions when both occur in the one galaxy, it allows astronomers to cross calibrate, providing them with two independent distance measurements of the same galaxy.

"So many people were observing it, that we had a huge volume of data," says Childress.

Detailed observations

The astronomers were able to study SN 2012fr in greater detail than almost any supernova ever discovered.

"We observed it every night for the first three weeks after it exploded, so we were able to watch in very fine detail how it evolved."

Childress and colleagues detected unusual layering in the material ejected during the explosion.

They found two distinct layers of silicon; a thick outer layer much larger than anything previously observed, which faded away over a week and a half, and a deeper thin layer that hardly changed at all.

"We have in the past seen this double layering, but we haven't ever seen it in such exquisite detail," says Childress.

"In previous examples most people have only been able to have a single observation ... so they had no information as to how thick this layer was."

"The difference between this and other supernovae is that we just have so much more information on this one."

According to Childress, the most popular idea to explain the observation is the double detonation theory.

"You have a surface layer of helium that ignites. That explosion on the surface sends a shock wave through the star and ignites the material in the core of the star causing the entire thing to explode," he says.

The authors are planning to search for more supernovae to study in similar detail, to determine how common this variation is.