March 10, 2003
Contact: Sarah Ray
802-443-5794
sray@middlebury.edu
Posted: March 10, 2003
MIDDLEBURY,
VT - 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 1,000 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 kinds 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, every Ia has 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 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/pr04/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 is also supported by the National Science
Foundation.
Additional
contact:
Douglas Isbell
Public Information Officer
National Optical Astronomy Observatory
Phone: 520/318-8214
E-mail: disbell@noao.edu