Scientists have identified a type of supernova, or exploding star, that produces unusually large amounts of calcium — enough perhaps to explain the abundance of that element in the universe and in our bones.
Perhaps more significant for astronomers, these calcium-rich exploding stars — eight have been identified so far — may also represent a new class of supernova, according to a study published Thursday in the journal Nature.
"If it's not a new genus, it's at least a new species of supernova," said Alex Filippenko, a professor of astronomy at UC Berkeley and one of the study's 28 coauthors. "This is not just a variation on a theme."
Astronomers believe that supernovae — the violent explosions that accompany the deaths of stars — have two origins. Young, massive stars can explode, leaving behind dense neutron stars or black holes (known as "core collapse" supernovae); or smaller, older white dwarf stars can become unstable, setting off a thermonuclear explosion that obliterates them.
But the supernova studied by Filippenko and colleagues, SN 2005E, didn't seem to fit either category.
It generated too little material to line up with the features of the core-collapse type of supernova and appeared, in any case, in an older galaxy where core-collapse supernovae are not usually found.
But it didn't look exactly like a white-dwarf supernova, either. Its mass and chemistry were off. Though it seemed to have originated from a white dwarf, "the white dwarf blew up in a weird way," Filippenko said — possibly exploding only partially. "That's a different beast," he said.
Not all scientists agree with Filippenko's interpretation. A second paper in the same issue of Nature examined another of the eight calcium-rich supernovae, SN 2005cz, and concluded that it is not something new. Instead, the multinational team proposed that it is a core-collapse supernova, albeit one with different features.
Koji Kawabata of the Hiroshima Astrophysical Science Center in Japan, a coauthor of that second paper, said in an e-mail that he was not "so strongly" against the other team's model but scientists must make more observations before they can know.
"The situation is still kind of muddy," Filippenko acknowledged. "It's a difference of opinion."
Understanding how these supernovae were formed could help researchers better describe the chemical evolution of the universe, said David Branch, a professor of physics and astronomy at the University of Oklahoma who wrote an editorial that accompanied the studies.
And, he added, the finding is almost certain to inspire astronomers to ramp up their efforts to detect and understand this variety.
Scientists have known for decades that supernovae created the stuff our universe is made of, including the carbon in our cells, the iron in our blood and — yes — the calcium in our bones. This is part of the reason researchers study supernovae.
"The explosions are important because they produce the heavy elements that are critical for life as we know it," Filippenko said. "We're made of star stuff."
But just because astronomers know that these elements come from supernovae doesn't mean they can explain why certain elements are present in the universe in high amounts. Calcium is a case in point.
"The total amount [of calcium] we see [in the universe] is quite high," Filippenko said, "more than can be explained with conventional supernovae. This new class could explain the large amount of calcium."
Nearly half of the material expelled from SN 2005E was calcium — five to 10 times as much, in terms of mass, than calcium produced by typical supernovae of any variety, according to the paper co-written by Filippenko. The researchers said that it would take only one or two of these calcium-rich supernovae every 100 years to generate all the calcium present in life on earth.