The Veil Nebula, a delicate network of glowing gaseous
filaments in the northern constellation of Cygnus the Swan,
has long been a favorite of both amateur and professional
astronomers. Part of a much larger nebula known as the
Cygnus Loop, the Veil is comprised of the leftovers of a
star that exploded between 5,000 and 8,000 years ago. For
at least half a century, scientists have probed the Cygnus
Loop with various techniques, trying to understand its
physical characteristics as a model for comprehending a
whole class of similar objects that cannot be observed in
such detail. Even with intensive study, however, many of
the Loop's basic parameters, such as its distance and the
density of its gaseous filaments, have been poorly
understood.
But a creative new observation of a star situated
behind the Veil Nebula may alter the way scientists think
about this supernova remnant. At an American Astronomical
Society meeting in Denver last week, astronomers from Johns
Hopkins provided confirmation that the Cygnus Loop is
closer to Earth than many have thought. These new findings,
obtained largely using the
Far Ultraviolet
Spectroscopic Explorer satellite, have a major impact
on the derived properties of this important, prototypical
object.
"The key to this result was in finding a background
star with sufficient far ultraviolet output to be observed
with FUSE," said William P. Blair, a research professor in
Physics and
Astronomy and the principal author of the study.
To identify the candidate star, Blair and his
colleagues used an image of the Veil Nebula taken with the
Ultraviolet Imaging Telescope on a 1995 space shuttle
flight. In the far ultraviolet, most stars visible at
optical wavelengths simply fade away.
"It was a real stroke of luck to find an
ultraviolet-bright star located behind the Veil," Blair
said.
Late last year, Blair and his colleagues trained FUSE
— a NASA satellite designed at and operated by a team
at Johns Hopkins — on the star, known only as
KPD2055+311. The resulting spectrum of the star in
ultraviolet light shows many absorption lines, or dips, in
light intensity. Some of these dips arise in the star's
atmosphere or from cold molecular gas in the interstellar
space along the way to the star. But some absorptions,
attributable to very hot gas, must arise from the
absorption within the Veil Nebula itself.
"Indeed, this confirmed for us that the star is behind
the Cygnus Loop," Blair said.
Part two of the story involves the star itself.
Blair's group used the observed properties of the star and
a model of the star's emission to calculate a distance of
1,860 light-years to KPD2055+311. (A light-year is the
distance light travels in one year, about 6 trillion
miles.) Until a few years ago, astronomers placed the
Cygnus Loop more than 2,500 light-years distant.
In 1999, astronomers using the
Hubble Space
Telescope revised that estimate to 40 percent closer,
or 1,470 light-years. Nevertheless, uncertainties and
assumptions used in making that estimate left some
researchers unpersuaded. Because the star's distance is
well determined and it is located behind the Cygnus Loop,
it places an upper boundary on the distance and provides an
independent confirmation of the shorter distance scale.
According to Blair, the shorter distance makes "a
tremendous difference" in the calculated size, age, energy
and average expansion velocity of the supernova remnant
compared with previous estimates.
"Since we want to use the Cygnus Loop to scale to
similar objects, it is important to have an accurate
starting point," Blair said. "This observation goes a long
way toward improving our understanding of this important
object."
For more information about this result, including
electronic images, go to
fuse.pha.jhu.edu/~wpb/cyglpstar.html.
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