NGC 6543
THE CAT'S EYE NEBULA
One of the brightest planetary
nebulae in the sky, NGC 6543 is of great historical importance.
The "first planetary," the one to which William Herschel first
applied the term, is the Saturn Nebula,
NGC 7009 in Aquarius. But it was in the
Cat's Eye, NGC 6543 in Draco, in which
he found the first known central star. Herschel thought that he
had discovered star birth, that
the nebula was condensing to form the star. While he had it
backwards (the nebula actually ejected by a dying star), his
brilliance was in that he understood that these objects had
something to do with stellar life cycles. The Cat's Eye was also
the first planetary examined by spectroscope, by William
Huggins in 1864. Listen to his words from his 1897 memoir, which
reflect Herschel's ideas and reveal his thrill at his great
discovery:
"On the evening of August 29, 1864, I directed the telescope...to
a planetary nebula in Draco [NGC 6543]. The reader may be able to
picture to himself...the feeling of excited suspense, mingled with
a degree of awe, with which, after a few moments of hesitation, I
put my eye to the spectroscope. Was I not about to look into a
secret place of creation?
"I looked into the spectroscope. No such spectrum as I expected!
A single bright line only! ...
The light of the nebula was monochromatic, and so, unlike any other
light I had yet subjected to prismatic examination, could not be
extended out to form a complete spectrum...A little closer looking
showed two other bright lines on the side towards the blue. The
riddle of the nebulae was solved. The answer, which had come to us
in the light itself, read: Not an aggregation of stars, but a
luminous gas" [emissions being characteristic of hot gases under
low pressure].
Huggins had discovered an emission line of hydrogen in the blue
part of the spectrum and two "mystery lines" in the green that were
later thought to come from an unknown element called "nebulium."
Among the strongest emissions in planetary nebulae, the "nebulium"
lines were finally found by I. S. Bowen in 1928 to be emissions of
doubly-ionized oxygen
On the left is Curtis's drawing. Made from several photographic
images, gives a sense of the object's visual appearance through the
telescope. On the right is the Hubble Space Telescope's view,
which shows vastly greater detail with intricate interlocking rings
and a stunning bipolar flow quite like the ansae seen in NGC 7009.
With a decently known distance of 3000 light years (determined from
the expansion of the nebula coupled with the expansion velocity of
around 20 kilometers per second), the main body of the nebula is
some 0.4 light years across, while the twin flows stretch out to
about twice that distance. (One source, however, suggests 5000
light years from the same data, showing how tricky distance
measures really are.) Surrounding the Cat's Eye is a huge shell
from earlier stellar winds with a diameter of close to 4.5 light
years, somewhat larger than the distance from the Sun to Alpha
Centauri. Concentric rings reveal episodic mass loss. The ionic
excitation is relatively low, as there is no ionized helium. With
a temperature of about 50,000 Kelvin, the 11th magnitude central
star is still heating with a total luminosity of around 1000 times
that of the Sun.
Left: Image by H. D. Curtis from Publications of the Lick
Observatory, Volume 13, Part III, 1918. Right: NASA, ESA, HEIC,
and the Hubble Heritage Team (STScI/AURA).
