for National Geographic News
By blending pictures of the deep universe, astronomers have found the faint light from two supernovae that are now the most distant stellar explosions known.
The supernovae happened so far away that their light takes billions of years to reach Earth.
Being able to see such faraway objects could therefore open a new window onto the lives and deaths of the first stars formed after the big bang, nearly 13 billion years ago.
"The ones that we've detected happened about 11 billion years ago, so you're getting close already," said lead author Jeff Cooke, an astronomer at the University of California, Irvine.
Due to the expansion of the universe, the record-setting supernovae are currently 18 billion light-years from Earth.
The closest observed supernova, seen in October 1604, exploded a mere 20,000 light-years away and was visible from Earth with the naked eye.
At great distances, however, even bright supernovae appear very faint. So Cooke and his team search for so-called Type IIn supernovae. These superbright explosions happen when large stars 50 to 100 times the mass of our sun start to die.
"They're so massive that they're unstable," Cooke said. As they near death, the stars shed layers of material into space, creating shells around themselves.
"When they explode, all the material comes screaming outside of the center and it hits that shell," Cooke said. This action heats up the shell, causing it to glow brightly for years afterward.
The researchers predict that a large number of Type IIn supernovae exist in the universe.
Cooke and his team located the newfound Type IIn supernovae using a new method that replicates the long exposures photographers use to capture dim light in dark surroundings.
The astronomers first compiled pictures of the same patch of sky from a five-year survey conducted using the Canada-France-Hawaii Telescope, on the summit of Mauna Kea in Hawaii.
Comparing groupings of images over time, the team found four "sudden" appearances of light. Later observations made with the W. M. Keck telescope, also on Mauna Kea, confirmed that the lights are distant star explosions.
Two of the supernovae Cooke and colleagues found are even farther away than the previous record holder, which happened nine billion years ago.
Heavy Metal Spread
Studying such early supernovae can help scientists better understand how the universe went from containing mostly hydrogen gas to being filled with a variety of materials.
Explosions of massive stars send heavy metals out into space. Eventually, the metals cool into the clouds of dust and gas that form planets around new stars.
"You'd really like to know when [the metals] were created, how they were distributed, and how they evolved over time," Cooke said. "All the rocky planets [including Earth] need these heavier metals."
Findings appear in the July 9 issue of the journal Nature.
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