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 cataloged NGC 6543, is every bit as inscrutable as
the J.R.R. Tolkien phantom character. Though the Cat’s Eye Nebula was
one of 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 exception.

The bull’s-eye patterns seen around planetary nebulae come as a surprise
to astronomers because they had no expectation that episodes of mass
loss at the end of stellar lives would 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 (NASA)

NOTE TO EDITORS: For additional information, please contact
Romano Corradi, Isaac Newton Group of Telescopes, La Palma, Spain,
(phone) 922-42-5461, (cell phone) 626-485736), (e-mail)
rcorradi@ing.iac.es or

Lars Lindberg Christensen, Hubble European Space Agency Information
Centre, Garching, Germany, (phone) 49-89-3200-6306, (cell phone)
49-173-3872-621, (e-mail) lars@eso.org or

Keith Noll, Hubble Heritage Team, Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218, (phone) 410-338-1828,
(fax) 410-338-4579, (e-mail) noll@stsci.edu.

Electronic images and additional information are available on