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
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. A still image and a short animation showing the
movement of the expanding shell observed around the supernova of 1006 A.D.
is available at:

http://www.noao.edu/outreach/press/pr03/pr0304.html

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.