Krause noticed odd infrared patterns at the edge of image, and follow-up observations found the patterns had changed shape.
Soon the researchers realized they were observing the release of infrared energy from the expanding shockwave of the 300-year-old supernova.
The team got telescope time to determine the spectra of the reflected supernova light and found it matched previously observed Type IIb supernovas.
"The signature of the supernova flash illuminated this dust cloud," Krause said.
"The light is reflected back, and by analyzing this echo you can study the original light outburst. Like with a time machine, we see the 300-year-delayed echo."
"This is an important paper about a very important object," said Doug Welch, a professor in the McMaster University department of astronomy and physics in Hamilton, Ontario. He was not involved in the new research.
Welch, who helped pioneer the use of light echoes, presented findings at the 2007 meeting of the American Astronomical Society describing the light echoes of Cass A and Tycho Brahe's 1572 supernova.
"What type of supernova caused Cass A has been an enduring mystery for astronomers since it was detected in radio, and [Krause's team has] acquired the key piece of information that allows us to understand it," Welch said.
Using light echoes to study such supernovas is an important new tool, researchers say, because supernovas reveal insights into how particles are dispersed within galaxies and space and how new stars form.
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