Exploding stars offer clues to dark energy
Washington: Observation of two bright exploding stars is improving the astronomical "tape measure" used to calculate the acceleration of the expansion of the universe, say scientists.
Light from two massive stars that exploded hundreds of millions of years ago recently reached Earth, and each event was identified as a supernova, Southern Methodist University scientists said.
Ferrante, a graduate student at Southern Methodist University, Dallas, who made the initial observation.
The exploding star is in a relatively empty portion of the sky labeled "anonymous" in the faint constellation Canes Venatici. Home to a handful of galaxies, Canes Venatici is near the constellation Ursa Major , best known for the Big Dipper.
A second supernova discovered Nov. 20 exploded about 230 million years ago, said Ferrante, who made the initial observation. That exploding star is in one of the many galaxies of the Virgo constellation.
Both supernovae were spotted with the Robotic Optical Transient Search Experiment's robotic telescope ROTSE3b, which is now operated by SMU graduate students. ROTSE3b is at the McDonald Observatory in the Davis Mountains of West Texas near Fort Davis.
The supernova that exploded about 450 million years ago is officially designated Supernova 2013X. It occurred when life on Earth consisted of creatures in the seas and oceans and along coastlines. Following naming conventions for supernova, Supernova 2013X was nicknamed "Everest" by Govinda Dhungana, an SMU graduate student who participated in the discovery.
The supernova that exploded about 230 million years ago is officially designated Supernova 2012ha. The light from that explosion has been en route to Earth since the Triassic geologic period, when dinosaurs roamed the planet.
"That's fairly recent as these explosions go," Ferrante said.
Dhungana gave the nickname "Sherpa" to Supernova 2012ha.
Everest and Sherpa are two of about 200 supernovae discovered worldwide in a given year, according to the scientists.
"Everest and Sherpa aren't noteworthy for being the youngest, oldest, closest, furthest or biggest supernovae ever observed. But both, like other supernovae of their kind, are important because they provide us with information for further science," Ferrante said.
Everest and Sherpa are Type 1a supernovae, the result of white dwarf explosions, said Robert Kehoe, physics professor and leader of the SMU astronomy team in the SMU Department of Physics.
"We call these Type 1a supernovae standard candles. Since Type 1a supernovae begin from this standard process, their intrinsic brightness is very similar. So they become a device by which scientists can measure cosmic distance. From Earth, we measure the light intensity of the exploded star. As star distances from Earth increase, their brilliance diminishes," Ferrante said.
Like other Type 1a supernovae, Everest and Sherpa provide scientists with a tiny piece to the puzzle of one of the greatest mysteries of the universe: What is dark energy?
Every Type 1a supernova provides astronomers with indirect information about dark energy, which makes up 73 percent of the mass-energy in the universe. It's theorized that dark energy explains the accelerating expansion of our universe at various epochs after the Big Bang.
"Every exploding star observed allows astronomers to more precisely calibrate the increasing speed at which our universe is expanding," Ferrante said.
"The older the explosion, the farther away, the closer it was to the Big Bang and the better it helps us understand dark energy," he added.