Astronomers have found "light echoes" from three ancient supernovae by detecting their faint, centuries-old light reflected in the clouds of interstellar dust. The finding, published in Nature, means astronomers will, for the first time, be able to study these important but rare events that appeared hundreds to thousands of years ago.

"This is a Rosetta Stone finding," says Doug Welch, professor of physics and astronomy at McMaster University. "Usually, we're limited to studying the current snapshot of the heavens. For the first time, we have the opportunity to study the original light from the outburst and the debris left behind centuries later in the same way that archeologists make links with ancient civilizations."

"It is almost as if the universe left a TiVo running so that we had the opportunity to view ancient celestial fireworks that we missed," says Welch.

Supernovae are of enormous importance to our understanding of the universe, he adds. "They are the ultimate source for all elements except for hydrogen and helium. The expanding remnants are the way that these heavy elements get back out into circulation and eventually get incorporated into new stars, and planets and people."

All images recorded on by a camera are "light echoes". As we observe our daylight surroundings we see everything, except the Sun itself, by reflected light. The light, which has reflected off any object around us, takes slightly longer to reach us than rays coming directly from the Sun, but since the Sun is a steady source of light, we don't notice the time delay.

A supernova is the ultimate "flash unit", explains Welch. For a month or two, it produces more light than all of the other millions of stars in a galaxy combined. Supernovae located near us are very rare. The last one visible to the naked eye in Earth's own Milky Way galaxy was in 1604. Only one naked-eye supernova in our companion galaxy, the Large Magellanic Cloud, has been observed in historical times. Discovered by Canadian observer Ian Shelton while working in Chile, it was called Supernova 1987A. Modern instruments were used to study it, but it will be centuries before the remnant itself develops sufficiently for detailed analysis.

In this latest discovery, Welch and his international team of collaborators were analyzing the images obtained at a large telescope in Chile when they happened upon the light echoes. "We were comparing images taken of the same patch of sky at different times to locate brightening or dimming stars. But we saw something we didn't expect - dim streaks of light that moved from year to year." said Welch.

When members of the team first concluded they were seeing light echoing off interstellar dust clouds, everyone assumed that they must be from supernova 1987A. A closer look revealed otherwise. Team members began plotting the motions of the streaks on the sky to determine the points from which they appeared to be emanating.

"I can still remember the instant I realized that one group of features was associated with an older supernova remnant. I think the e-mail I sent around to the other team members had more than twenty exclamation marks in it!" says Welch.

"It is sobering to think that we may have a second chance to study the light from centuries-old supernovae using modern spectroscopic instruments - we might be analyzing light from the supernovae that Kepler or Tycho saw with their own eyes in the16th century, " says Welch. Tycho's measurements of the position of the 1572 supernova revealed that it must be more distant than the Moon, contradicting teachings of the day. The 1572 and 1604 supernovae are widely recognized as turning points in the development of modern science.

The research for this study was funded by the Natural Sciences and Engineering Research Council, the U.S. National Science Foundation, the U.S. Department of Energy, and the National Nuclear Security Administration.

McMaster University, a world-renowned, research-intensive university, fosters a culture of innovation, and a commitment to discovery and learning in teaching, research and scholarship. Based in Hamilton, the University, one of only four Canadian universities to be listed on the Top 100 universities in the world, has a student population of more than 23,000, and an alumni population of more than 115,000 in 128 countries.