DYING STAR CREATES
FANTASY-LIKE SCULPTURE OF GAS AND DUST
In this detailed view from NASA's Hubble Space
Telescope, the so-called Cat's Eye Nebula looks
like the penetrating eye of the disembodied sorcerer
Sauron from the film adaptation of "The Lord
of the Rings."
The nebula, formally catalogued NGC 6543, is every
bit as inscrutable as the J.R.R. Tolkien phantom
character. Though the Cat's Eye Nebula was the first
planetary nebula to be discovered, it is one of
the most complex such nebulae seen in space. A planetary
nebula forms when Sun-like stars gently eject their
outer gaseous layers that form bright nebulae with
amazing and confounding shapes.
In 1994, Hubble first revealed NGC 6543's surprisingly
intricate structures, including concentric gas shells,
jets of high-speed gas, and unusual shock-induced
knots of gas.
As if the Cat's Eye itself isn't spectacular enough,
this new image taken with Hubble's Advanced Camera
for Surveys (ACS) reveals the full beauty of a bull's
eye pattern of eleven or even more concentric rings,
or shells, around the Cat's Eye. Each 'ring' is
actually the edge of a spherical bubble seen projected
onto the sky -- that's why it appears bright along
its outer edge.
Observations suggest the star ejected its mass
in a series of pulses at 1,500-year intervals. These
convulsions created dust shells, each of which contain
as much mass as all of the planets in our solar
system combined (still only one percent of the Sun's
mass). These concentric shells make a layered, onion-skin
structure around the dying star. The view from Hubble
is like seeing an onion cut in half, where each
skin layer is discernible.
Until recently, it was thought that such shells
around planetary nebulae were a rare phenomenon.
However, Romano Corradi (Isaac Newton Group of Telescopes,
Spain) and collaborators, in a paper published in
the European journal Astronomy and Astrophysics
in April 2004, have instead shown that the formation
of these rings is likely to be the rule rather than
The bull's-eye patterns seen around planetary nebulae
come as a surprise to astronomers because they had
no expectation of episodes of mass loss at the end
of stellar lives that repeat every 1,500 years.
Several explanations have been proposed, including
cycles of magnetic activity somewhat similar to
our own Sun's sunspot cycle, the action of companion
stars orbiting around the dying star, and stellar
pulsations. Another school of thought is that the
material is ejected smoothly from the star, and
the rings are created later on due to formation
of waves in the outflowing material. It will take
further observations and more theoretical studies
to decide between these and other possible explanations.
Approximately 1,000 years ago the pattern of mass
loss suddenly changed, and the Cat's Eye Nebula
started forming inside the dusty shells. It has
been expanding ever since, as discernible in comparing
Hubble images taken in 1994, 1997, 2000, and 2002.
The puzzle is what caused this dramatic change?
Many aspects of the process that leads a star to
lose its gaseous envelope are still poorly known,
and the study of planetary nebulae is one of the
few ways to recover information about these last
few thousand years in the life of a Sun-like star.
Credit: NASA, ESA, HEIC, and The Hubble Heritage
Team (STScI/AURA) Acknowledgment: R. Corradi (Isaac
Newton Group of Telescopes, Spain) and Z. Tsvetanov