Washington, May 30 : Astronomers have decoded ghostly echoes of light traveling away from the remains of a supernova, to piece together what the star looked like in life and how its met its demise, a process which is literally a "cosmic seance".
The discovery, made using primarily NASA's Spitzer Space Telescope and Japan's Subaru telescope on Mauna Kea in Hawaii, represents the first time astronomers have been able to resurrect the life history of a supernova remnant in our own galaxy.
"Cassiopeia A lies in our cosmic backyard and offers the sharpest view of what is left hundreds of years after a supernova explosion," said Oliver Krause of the Max Planck Institute for Astronomy in Germany, lead author of a paper about the discovery.
"The echoes of light we found around Cassiopeia A provide us with a time machine to go back and see its past," he added.
Cassiopeia A is he burnt-out corpse of a massive star that ended its life in a fiery supernova about 11,300 years ago.
Because Cassiopeia A is 11,000 light-years from Earth, the light from its explosion would have reached Earth, sweeping right past it, about 300 years ago.
Astronomers had thought this supernova light was never to be seen again, until 2005, when Krause and his colleagues discovered hints of it still bouncing around clouds surrounding the remnant.
Using Spitzer's infrared eyes, they found so-called infrared echoes, which occur when a flash of light from the supernova blasts through clouds, heating them up and causing them to glow in infrared.
As the light rolls outward, the infrared echoes continue to flare up and travel away from the star.
In the new study, the astronomers used Cassiopeia A's infrared echoes to hone in on faint visible-light echoes with Subaru and other ground-based telescopes.
Visible-light echoes, known simply as light echoes, occur when visible light from the supernova scatters off dust. Unlike infrared echoes, they are direct signals from the graves of exploded stars, bearing all the information about the nature of the original blast.
The astronomers then used Subaru's spectrometer instrument to break the light apart and reveal signatures of atoms present when Cassiopeia A exploded.
The resulting spectrum of light revealed hydrogen and helium, which are telltale signs that Cassiopeia A was once a huge red supergiant star whose core collapsed in a rare supernova referred to as Type IIb.
"This is an exciting result," said Alex Filippenko of the University of California, Berkeley. "Cassiopeia A has been studied extensively with many telescopes over a wide range of wavelengths. It is gratifying that we finally know what kind of star exploded so long ago," he added.