NGC 6543


In Draco

See the original nebular spectrum.

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6543 NGC 6543
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, that 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 nebula to have its spectrum examined, 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 a planetary nebula in Draco. 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.


NGC 6543

Above is the simple drawing of the spectrum of NGC 6543 as seen through the visual spectroscope of Sir William Huggins in 1864. It shows the two forbidden oxygen lines at 5007 and 4959 Angstroms just to the right of center and to the far left the H-Beta line of hydrogen, tagged "H" at the bottom. The tag "F" at the top of H-Beta is a Fraunhofer designation applied before any celestial (specifically solar) spectrum lines were identified. An expected magnesium line (Fraunhofer "b") does not show up. The forbidden lines were not identified as doubly ionized oxygen until 1928, and were at the time thought to be from nitrogen (hence the tag "N" at the bottom). See them in the spectra of NGC 7009 and NGC 2440. The observations of the emission lines proved for the first time that planetary nebulae were gaseous. A similar observation of the Orion Nebula followed in 1872, showing that diffuse nebulae were gaseous as well.

Image at left 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). Quote and spectrum from The Scientific Papers of Sir William Huggins, London: William Wesley and Sons, 1909.