"Light Echo" Helps Solve Supernova Mystery

Graeme Stemp-Morlock
for National Geographic News
May 29, 2008

Astronomers have used an interstellar "mirror" to solve the longstanding mystery over what kind of supernova created Cassiopeia A, one of the brightest radio objects in the sky.

Cass A, as the object is often called, is the expanding remains of a stellar explosion about 9,000 light-years away that is believed to have occurred around A.D. 1680. Until now no one has been able to pinpoint the exact nature of the blast.

So scientists analyzed a "light echo" reflected by interstellar dust to conclude that Cass A arose from a Type IIb supernova—an extremely rare stellar explosion that occurs when a red giant star exhausts the hydrogen in its outer layer, causing the star's helium core to collapse.

"We are able to conclude what type of supernova it was, which is certainly the most important result scientifically," said Oliver Krause, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany.

"It brings together the wealth of observation about the remnant with the explosion."

The results appear in this week's issue of the journal Science.

Mirror to a Star's Soul

In the past 2,000 years, humans have seen six stars go supernova in the Milky Way. But only one was observed with modern-day astronomical equipment, limiting the amount of data scientists have about the phenomena.

Krause and his coworkers used the fledgling technique of light echoes to rewind time and look at those explosions again. (Related: "Centuries-Old Supernova "Echoes" Detected" [March 20, 2008].)

When stars explode, light erupts in all directions, with only a small fraction directly in Earth's line of sight.

But interstellar dust can act as a mirror, reflecting light that went shooting off in other directions back to Earth hundreds of years later.

Krause's group found infrared signatures from Cass A in an early Spitzer Space Telescope image. The object was one of the first Spitzer captured when it was testing out its optics in 2003.

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