Astronomers using the Hubble Space Telescope have found two distant supernovae - exploding stars - that provide new clues about the accelerating Universe and its mysterious "dark energy".
By Dr David Whitehouse
BBC News Online science editor
The supernovae are approximately 4.7 and 7.6 billion light-years from Earth, deep in the constellation of Ursa Major, the Great Bear.
Before the supernova was found...
Two views of the same patch of sky taken several years apart revealed the supernovae, among the most distant ever seen and certainly the most detailed.
The later images were taken by the Advanced Camera for Surveys (ACS) that has delighted astronomers since it was installed on Hubble in March 2002.
"This beautifully demonstrates that the ACS is a 'supernova machine' for probing the early Universe," says Holland Ford, of Johns Hopkins University, US, who headed the team that developed the ACS.
More distant supernovae have been identified but they do not have the same level of detailed information obtained by Hubble for the new pair.
The objects are so-called Type 1a supernovae which are white dwarf stars that have been drawing mass from a companion star. The white dwarf siphons off mass until it reaches a critical mass known as the Chandrasekhar limit.
"Hitting this limit causes a deflagration that consumes the star in about five seconds," says Ford. "A thermonuclear burning wave, burning oxygen and carbon and higher elements, goes through the star."
As a result, the star explodes and shines as brightly as several billion stars for several days, enabling astronomers to see it across the Universe.
Astronomers Zlatan Tsvetanov of Johns Hopkins and Dan Magee of the University of California-Santa Cruz compared earlier Hubble images of the same patch of sky with those obtained by the new ACS images to find the supernovae.
It was information from studies of Type 1a that a few years ago indicated that the galaxies appeared to be moving away from each other at an ever-increasing rate. Astronomers attribute this accelerating expansion to a mysterious "dark energy" believed to permeate the Universe.
Looking farther back in space and time, researchers have some indication that gravity was at one point holding back the acceleration of the expansion of the Universe. They have very little data on this period of transition.
"We're trying to fill in a blank region where the Universe's rate of expansion switched from decelerating due to gravity to accelerating growth driven by dark energy," says John Blakeslee of Johns Hopkins.
"That's a real challenge, but the ACS is making it very straightforward to find distant supernovae and get detailed information about them," he adds.
"Continued studies of supernovae will allow us to uncover the full history of the universal expansion," says Blakeslee.
"The sharper images, wider viewing area, and keener sensitivity of ACS should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble's previous main imaging camera."