Cosmic Shrapnel Holds History

At the edge of the universe, two stars the size of Earth, made of million-degree diamond and oxygen, smash together in an explosion that outshines their entire galaxy.

The shrapnel from explosions like that one are thought to have made everything we know on Earth—including us—and could provide clues to the ultimate fate of the universe.

And last week, a team led by Harvard astronomers announced they had seen such shrapnel.

What the team observed was a stellar explosion, called a supernova, that was caused by the merger and collision of two white dwarf stars—the shriveled-up remnants of burnt-out stars.

Typically, these gigantic explosions are thought to involve only one white dwarf, and astronomers have inferred from previous studies of white dwarf supernovae that the rate of expansion of the universe is accelerating.

“This one might be an example of another way to make this kind of explosion, where two white dwarfs come together and merge,” said astronomy professor Robert P. Kirshner ’70, one of the authors of the study.

Kirshner added that one of the clues that tipped the team off to the unusual double nature of the explosion, named Supernova 2006gz, was a large quantity of carbon that they detected. Computer models predict high carbon levels when two white dwarfs merge.

“Maybe this is a kind of fluffy mess where the white dwarfs have merged,” Kirshner said.

To observe supernovae, astronomers analyze spectra from a computerized telescope, rather than observing the explosions directly.

Harvard graduate student Malcolm S. Hicken, who led the study, said that the explosion was also extremely luminous for an unusually long time.

“It stayed bright a lot longer than the allowable range that is used,” Hicken said. “This would be a weird object even if we didn’t have this early spectra [carbon evidence].”

Astronomers believe that nearly all of the elements as heavy or heavier than iron are created in supernova explosions like these. That includes Earth and all the iron found on it.

“Supernovae are very important in the big picture because they create all the heavy elements. They produce the iron in your blood [and] in the core of the Earth,” Kirshner said.

“We personally are made out of stardust,” he added.

Supernova expert Alexei V. Filippenko, a professor at the University of California, Berkeley who played a central role in discovering that the expansion of the universe is accelerating, said he was intrigued by the team’s findings.

“Spectroscopically, it was certainly a very interesting supernova that deserves to be followed up,” he said.