The remnants of two massive stars that exploded about 11 billion years ago have shattered the record for the most distant supernovas in the known universe.
The faraway explosive remnants, found using a new method, could help researchers learn more about the evolution of the universe, how the elements in it formed and how they were distributed in later generations of stars and planets.
"When stars explode, they spew matter into space. Eventually, gravity collapses the matter into a new star, which could have planets such as Earth around it," said study leader Jeff Cooke of the University of California, Irvine.
Before the discovery of these distant supernovas, which belong to a category known as Type IIn, the most distant known supernovas of the same type were 6 billion light-years away, and the most distant of any supernova type were 9 billion light-years away.
A supernova occurs when a massive star (more than eight times the mass of the sun) dies in a powerful explosion. Type IIn supernovas result from the explosive death of stars that are 50 to 100 times the mass of the sun.
These stars shed most of their material before they die, and when they finally explode the remaining material is spewed out into space, plowing through the previously expelled gas. The collisions between the gas clouds make the entire stellar remnant gleam brightly for several years after the star's demise.
To find supernovas, astronomers compare images of the same area of the sky taken at different times. A new pinprick of light that appears in one image and seems to fade over time can indicate the temporary brightening and dimming of the stellar explosion.
Cooke and his colleagues used a variation on this traditional method: They examined combined data from the Canada-France-Hawaii Telescope Legacy Survey for the same patch of sky for four different years.
By comparing the images, Cooke's team identified four very distant objects that appeared to brighten and fade over time.
The data showed that the light from the supernovas had traveled nearly 11 billion light-years to reach Earth. That means the explosions happened 11 billion years ago, and the light is just now reaching us.
"The universe is about 13.7 billion years old, so really we are seeing some of the first stars ever formed," Cooke said.
Cooke's technique is "powerful and reliable," said astronomer Alicia Soderberg of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Soderberg was not involved in the new study, which is detailed in the July 9 issue of the journal Nature.
The new method should make it possible to identify even more distant supernovas, possibly even some of the very first stars that blew apart. Other efforts already planned, such as the Large Synoptic Survey Telescope, could identify thousands of candidate supernovas.
"This new method could not have been published at a better time," Soderberg said.
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