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Scientists have discovered a glowing bubble of hot gas and an
unexpected X-ray bright central star within the planetary nebula
known as the Cat’s Eye using NASA’s Chandra X-ray
Observatory. The new results, presented today at the American
Astronomical Society meeting, provide insight into the ways that
stars like our Sun end their lives.

Scientists believe they are witnessing the expulsion of material
from a star that is in the last stages of its existence as a normal
star. Material shed by the star is flying away at a speed of about 4
million miles per hour, and the star itself is expected to collapse
to become a white dwarf star in a few million years.

The X-ray data from the Cat’s Eye Nebula, also known as NGC
6543, clearly show a bright central star surrounded by a cloud of
multimillion-degree gas. By comparing the Chandra data with
those from the Hubble Space Telescope, researchers are able to
see where the hotter, X-ray emitting gas appears in relation to the
cooler material seen in optical wavelengths by Hubble.

“Despite the complex optical appearance of the nebula, the X-ray
emission illustrates unambiguously that the hot gas in the central
bubble is driving the expansion of the optical nebula,” said
You-Hua Chu of the University of Illinois and lead author of the
paper submitted to the Astrophysical Journal. “The Chandra data
will help us to better understand how stars similar to our Sun
produce planetary nebulae and evolve into white dwarfs as they
grow old.”

With Chandra, astronomers measured the temperature of the
central bubble of X-ray emitting material, and this presents a new
puzzle. Though still incredibly energetic and hot enough to emit
X-rays, this hot gas is cooler than scientists would have expected
from the stellar wind that has come to stagnation from the initial
high speed of 4 million miles per hour.

At first, the researchers thought that the cooler, outer shell might
have mixed with the energetic material closer to the central star
to create this “lukewarm” area. However, this theory apparently
does not apply for NGC 6543. Chu and her colleagues found that
the chemical abundances within the hot gas were like those in
the wind from the star, and different from the cooler outer
material. These results indicate that mixing is not occurring, and
that the cooling between the inner and outer shells of material is
due to some other process.

The intensity of the X-rays from the central star was also
unexpected. The star itself has a surface temperature of about
60,000 degrees, whereas the X-ray measurement indicates a
temperature of a few million degrees.

“We could be seeing shock waves in the fast stellar wind itself,”
said Martin Guerrero of the University of Illinois, lead author on a
companion paper that describes the central star. “This is the first
time we see such X-ray emission from the central star of a
planetary nebula.”

A planetary nebula (so called because it looks like a planet when
viewed with a small telescope) is formed when a dying red giant
star puffs off its outer layer, leaving behind a hot core that will
eventually collapse to form a dense star called a white dwarf. A
fast wind emanating from the hot core rams into the ejected
atmosphere, pushes it outward, and creates the graceful
filamentary structures seen with optical telescopes.

With Chandra, it is now possible to see the high-pressure hot
bubble inside these filaments and study how the nebula is formed
in more detail. The Cat’s Eye Nebula, which is about 3,000 light
years from Earth, was formed about a thousand years ago.

Other members of the research team include Robert Gruendl,
and James Kaler (University of Illinois), and Rosa Williams
(National Research Council). NGC 6543 was observed with the
Advanced CCD Imaging Spectrometer (ACIS) on May 10-11,
1999, for a total exposure time of 46,000 seconds.

The ACIS X-ray camera was developed for NASA by
Pennsylvania State University and MIT. NASA’s Marshall Space
Flight Center in Huntsville, Ala., manages the Chandra program.
TRW, Inc., Redondo Beach, Ca., is the prime contractor for the
spacecraft. The Smithsonian’s Chandra X-ray Center controls
science and flight operations from Cambridge, Mass.

The color composite of optical and X-ray images was made by
Zoltan G. Levay (Space Telescope Science Institute). The optical
images were taken by J.P. Harrington and K.J. Borkowski
(University of Maryland) with the Hubble Space Telescope.

View this just released image from the Chandra X-ray Observatory and the Hubble Space Telescope:
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