A team of astronomers headed by Frank Winkler of Middlebury College has combined precise digital observations with simple mathematics to estimate the apparent brightness of an exploding star whose light reached Earth nearly a thousand years ago, when it produced a display that was probably the brightest stellar event witnessed in recorded human history.

The image on the left shows the brightest region of the expanding shell around supernova remnant 1006 A.D. as it appeared in 1998. The long filamentary structure extending across the image is glowing hydrogen gas that has recently been excited by the shock wave from the supernova. In the image at right, this image has been subtracted from an earlier one to show the outward motion of the shock wave over 11 years. The white trace shows its position in 1987, and the dark trace shows it in 1998. Credit: Middlebury College/NOAO/AURA/NSF

On May 1, 1006 A.D., a spectacularly bright star appeared suddenly in the southern sky in the constellation Lupus (the wolf), to the south of Scorpio. Observers in China, Japan, Egypt, Iraq, Italy, and Switzerland recorded observations of the star, which remained visible for several months before becoming lost in the glare of daylight. While all agree that the star was spectacularly bright, it has not been clear until now just how bright.

Modern astronomers have long concluded that the 1006 A.D. display resulted from a supernova, a distant star that ended its life in a spectacular explosion. Yet as bright as it appeared in the 11th century, the remains of the supernova are all but invisible today.

Through a series of observations with telescopes at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, Winkler and his team, including Middlebury College undergraduate student Gaurav Gupta (now a graduate student at Cornell University) and Knox Long from the Space Telescope Science Institute in Baltimore, found a faint shell of glowing hydrogen surrounding the site where the star exploded. The glowing shell, about the diameter of the full Moon as seen from Earth, is produced by the shock wave from the original explosion as it propagates outward through the extremely tenuous gas of interstellar space.

The astronomers used imaging observations spanning a period of 11 years to measure how fast the brightest filaments in the shell are expanding. Other recent spectral observations of these same filaments can be used to determine the absolute value of the shock wave's speed. This speed turns out to be 2,900 kilometers per second (over 6 million miles an hour), or almost 1 percent of the speed of light.

Knowing both the rate at which the distant shell appears to be expanding and the corresponding true velocity, the astronomers used simple geometry to calculate a precise distance from Earth to the shell. The result, 7,100 light-years, must also be the distance to the star that exploded. (This means that while the light from the supernova first reached Earth in 1006 A.D., the actual explosion took place 7,100 years earlier.)

Although there are several different types of supernovae, the one that occurred in 1006 was almost certainly what is known as a "Type Ia," the same type that several other teams are using to measure the apparently accelerating expansion of the Universe. These are spectacularly luminous events: for a few weeks a Type Ia supernova glows as bright as five billion suns. Furthermore, all Ia's have virtually the same luminosity--just as all 100-Watt light bulbs produce the same amount of light.

The supernovae that astronomers are using to study the distant universe are located in other galaxies at vast distances, and their light is so feeble by the time it reaches Earth that large telescopes are needed just to detect them. But the 1006 supernova was located "right next door," in relative terms, in a fairly nearby part of the Milky Way galaxy.

"By knowing this distance and the standard luminosity of Ia supernovae, we can calculate, in retrospect, just how bright the star must have appeared to 11th century observers," Winkler explains. "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. And all that light would have been concentrated in a single star, which must have been twinkling like crazy."

The most explicit historical record of the 1006 star's brightness comes from the Egyptian physician and astrologer Ali bin Ridwan, who in fact compared the spectacle both with Venus and with the Moon. "It's taken a long time to interpret what he meant," Winkler comments, "but now I think we've finally got it right."

To visualize how bright the 1006 supernova appeared, find the planet Jupiter, high in the southeast and the brightest object now visible in the evening sky.

"If you compare Jupiter with the three stars that make up the belt of Orion, a bit farther west in the sky, the planet is obviously much brighter than any of the belt stars," Winkler says. "At its peak, the supernova of 1006 would have appeared about as much brighter compared to Jupiter now, as Jupiter is in comparison with the faintest of the stars in Orion's belt."

"There's no doubt that it would have been a truly dazzling sight," Winkler concludes. "In the spring of 1006, people could probably have read manuscripts at midnight by its light."

An article describing these results was published in the March 1, 2003, issue of The Astrophysical Journal.

CTIO is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation. Astrophysics research at Middlebury College, Middlebury, VT, is also supported by the National Science Foundation.