A supernova first seen in the 11th century was the brightest flash of light from a star in recorded history, U.S. astronomers have confirmed.

The finding comes from research by Dr Frank Winkler of Middlebury College in Vermont, USA, and colleagues and is published in the latest issue of The Astrophysical Journal.

"On the magnitude scale used by astronomers, it was about minus 7.5 - which puts its brightness a little less than halfway between that of Venus and that of the full Moon," said Winkler. "And all that light would have been concentrated in a single star, which must have been twinkling like crazy."

The star appeared suddenly in May 1006 AD in the southern sky in the constellation Lupus (the wolf), near Scorpio. It was seen and recorded by scholars in China, Japan, Egypt, Iraq, Italy, and Switzerland and remained visible for several months before fading in the glare of daylight.

The most detailed record of the 1006 star is by Egyptian physician and astrologer, Ali bin Ridwan who wrote about witnessing the spectacle during his youth and compare the spectacle with Venus and the Moon: "It's light illuminated the horizon and it twinkled very much," Winkler quoted Ridwan saying.

"It's taken a long time to interpret what he meant, but now I think we've finally got it right," said Winkler. "There is no doubt it would have been a truly dazzling sight. In the spring of 1006, people could probably have read manuscripts at midnight by its light."

According to Professor Joss Hawthorn, an astronomer at the Anglo Australian Observatory in Sydney, the new work is impressive.

"It's pretty rigorous stuff," he told ABC Science Online. "There's no more uncertainty left. There were wild guesses going round - some people saying it's as bright as the Moon, others saying it wasn't very bright. But [this group] got the first accurate measurement of its distance and its intrinsic brightness.

"This is the brightest flash of light from a star in recorded history. You would have easily seen this in daytime and nightime - it would have been more intense than the Moon," he said.

Although the remnants of the supernova in 1006 AD are all but invisible today, Winkler and colleagues were able to capture a faint shell of glowing hydrogen surrounding the site where the star exploded. This shell - about the diameter of the full Moon as seen from Earth - was produced by the shock wave from the original cataclysmic explosion.

Using observations from optical telescopes at the Cerro Tololo Inter-American Observatory in Chile spanning 11 years, the astronomers calculated the speed of the shockwave at 2,900 km per second, and at a distance of 7,100 light-years away. It was from these calculations that they could then work out how bright the supernova must have been at the time it exploded.

Sixteen years ago, astronomers - for the first time since telescopes were invented - observed a supernova nearby, known as SN 1987A. It ignited 170,000 light-years away, just outside our galaxy in the Large Magellanic Cloud.

What's special about SN 1006 - as it has been dubbed - is that it's one of only four supernovae ever to be seen within our own galaxy.

While supernovae at the edge of the universe are more frequent, they are very distant beacons of light that help measure the accelerating nature of our expanding universe. Those within our galaxy allow astronomers to get a close-up look at the chemistry of such events.

"We know that most heavy elements which are essential for life, are cooked up in the pressure cooker environment at the centre of the star," said Hawthorn.

It remains a mystery why there are so few supernovae within our own Milky Way galaxy had been seen with the last one seen 400 years ago, he said: "We should have seen thirty in the last 1,000 years, so we're well overdue to see another one."

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