A team led by a former UC Berkeley postdoctoral student discovered a special type of supernova that occurs when a white dwarf star explodes.

According to Ariel Goobar, the former campus postdoctoral student and professor at Stockholm University who led the discovery, iPTF16geu is a special kind of Type Ia, or “standard candle,” supernova, which is a category of explosions that has been used over the past few decades to map the expansion history of the universe.

Campus adjunct professor of astronomy Peter Nugent ran the pipeline that produced the images of the supernovae. He said supernova iPTF16geu exploded in a galaxy about four billion light-years away. He added that the supernova was perfectly aligned with another galaxy within the team’s line of sight, allowing it to magnify four images of the supernova.

“This allowed us to find the supernovae, which are normally way too far away for a survey,” Nugent said. “This one we found … was due to the magnification caused by what we call gravitational lensing.”

Based on the “serendipitous discovery” of supernova iPTF16geu, Nugent and UC Berkeley graduate student Danny Goldstein then devised a new method to identify such gravitationally lensed standard candle supernovae. The newly devised method generalizes the properties of the standard candle supernova discovery in order to search for similar events in future data sets, according to Goobar.

“The idea is that by using ground based telescopes with cameras with very wide fields of view, we can monitor hundreds of thousands of galaxies, thus maximizing our chances to find these rare chance alignments,” Goobar said in an email.

Goldstein added that researchers often look for supernovae that are brighter than standardly accepted brightness levels because then they know that the standard candle supernova is likely being highly gravitationally lensed and magnified.

Alex Filippenko, a UC Berkeley astronomy professor who was not involved with the study, said the new technique developed by Nugent and Goldstein is a promising and exciting way to identify gravitationally lensed standard candle supernovae.

Filippenko said in an email that the discovery of the standard candle supernova itself marked only the second time a “multiply imaged, gravitationally lensed supernova” has ever been found.

Nugent and Goldstein emphasized that both the discovery of the new supernova and the technique devised to identify similar events were incredible tools for cosmology.

Goldstein said the discovery of supernova iPTF16geu marks the beginning of a “new era” of cosmology, as supernovae are suited to resolve questions about the values of cosmological parameters.

“Berkeley is really pushing this (research) forward, and there is a lot of effort going on here to find more of these things,” Goldstein said.