Unearthing the grave of Cassiopeia A
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 30, 2008

By decoding ghostly echoes of light traveling away from the remains of supernova Cassiopeia A, scientists have pieced together what the star looked like in life and how it met its demise, the first time the life history of a supernova remnant in our Galaxy has been resurrected.

Wide field image of the Cassiopeia A supernova remnant. It is a colour composite of mid-infrared by Spitzer (red), visible by Hubble (green), and X-ray by Chandra (blue). Image: NASA/JPL-Caltech/ O. Krause (Steward Observatory).

Cassiopeia A is one of the most explored objects in our sky and the subject of more than 1,000 scientific papers. It is the burnt-out corpse of a massive star that met its demise in a fiery supernova about 11,300 years ago. Until recently, it was the youngest supernova remnant in the Milky Way Galaxy (the new record holder, G1.9+0.3, was recently discovered using NASA's Chandra X-ray Observatory and other ground-based telescopes, see our news story ‘Astronomers find youngest supernova remnant in the Milky Way’). Light from the explosion of Cassiopeia A would have swept past the Earth about 300 years ago, and astronomers have long believed that this light would never be seen again. However, in 2005, Oliver Krause and colleagues at the Max Planck Institute for Astronomy in Germany discovered a glimmer of light still bouncing around clouds surrounding the remnant. Using the Spitzer Space Telescope's infrared eyes, they conducted a post-mortem study of the exploded star and found so-called infrared echoes, which occur when a flash of light from the supernova blasts through stellar clouds, heating them up and causing them to glow at infrared wavelengths.

In the new study, the scientists used the Japanese Subaru telescope on Mauna Kea, and other ground based telescopes, to hone in on faint visible-light echoes, which occur when visible light from the supernova scatters off local dust pockets. Unlike infrared echoes, they are direct signals from the graves of exploded stars, bearing all the information about the nature of the original blast.

"Cassiopeia A lies in our cosmic backyard and offers the sharpest view of what is left hundreds of years after a supernova explosion," says Krause. "The echoes of light we found around Cassiopeia A provide us with a time machine to go back and see its past."

Colour composite optical image of the echo region. The faint white features in the middle of the image are the light echoes. Image: Subaru Telescope.

Subaru's spectrometer instrument was used to break the light apart and reveal signatures of the atoms present when Cassiopeia A exploded. The resulting spectrum of light revealed hydrogen and helium, telltale signs that Cassiopeia A was once a huge red supergiant star whose core collapsed in a rare supernova referred to as Type IIb. The result solves one of the longstanding mysteries of Cassiopeia A’s previous life.

"This is an exciting result," said Alex Filippenko of the University of California. "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."

The findings also offer insight into another mystery shrouding Cassiopeia A: the puzzling fact that almost no one saw the supernova event when it occurred in 1681. "Type IIb supernovas fade quickly," says George Rieke of the University of Arizona. "This, plus a few cloudy nights, might explain the historical enigma around Cassiopeia A."

The discovery of the origin and type of Cassiopeia A will put all previous observations and theories of the supernova remnant into proper context. Other recent observations of light echoes have also been able to identify the origins of supernova outside our Galaxy, demonstrating the power of this technique for conjuring up the ghosts of long-dead stars.