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National Optical Astronomy Observatory
Tucson, Arizona

For More Information:
Douglas Isbell, Public Information Officer
National Optical Astronomy Observatory
Phone: 520/318-8214
E-mail: disbell{at}noao.edu

Phil Benoit, Director of Public Affairs
Middlebury College
Phone: 802/443-5198
E-mail: benoit{at}middlebury.edu

FOR IMMEDIATE RELEASE: March 5, 2003

RELEASE NO: NOAO 03-04

Astronomers Peg Brightness of History's Brightest Star
======================================================

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. 

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 concluded, "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. A still image and a short 
movie showing the movement of the expanding shell observed around the 
supernova of 1006 A.D. is available.

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.

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