The planetary nebula NGC 6543 is well known and has been the subject of numerous studies. It is referred to as the Cat's Eye Nebula, the Sunflower Nebula, the Snail Nebula, Caldwell 6, PN G096.4+29.9, and PK 96+29.1. It is located about 3,000 light-years from Earth in the constellation of Draco. Its diameter is about 3 light-years. The angular diameter is about 5.8 arcminutes, although the bright elliptical nucleus measures only about 20 arcseconds. The apparent magnitude is 8.1–9.8, and it is visible as a fuzzy spot in small telescopes. The age estimate depends on the distance to the central star and the estimated expansion rate, which is different in each sector. This "kinematic age" assumes a constant velocity, which is not very realistic. The "spectroscopic age" is based on the characteristics of the central star according to stellar evolution models. To describe the different parts of the nebula in more detail: 1) The kinematic age of the core (radius 0.25 light-years, velocity 16 km/s) would be 2300 years. However, velocities of 38 km/s have been measured at some points in the core, which would reduce the age to about 1000 years. 2) The age of the central ring region, assuming the existence of at least 11 rings and an interval between pulses of 1500 years, would be at least 16,000 years. However, this assumes a constant interval between pulses (1500 years), which may not be true. 3) The age of the outer halo, assuming a distance of 2-3 light-years and the same velocity of 16 km/s, would be between 40,000 and 50,000 years.

The central star, of magnitude 11.5, is evolving into a white dwarf. It is very hot (80,000 K), with a mass similar to that of the Sun, although originally it was 5 or 6 times larger. Its radius is 0.65 times that of the Sun, but it is 10,000 times more luminous. It is losing mass rapidly, with stellar winds of about 2,000 km/s and intense UV radiation. It is composed of hydrogen and helium, but also carbon and oxygen (helium fusion products). With these characteristics, it has been classified as an sdO star (subdwarf O-type). Here “O” (specifically “O7”) refers to the Morgan-Keenan classification, based on temperature, where O is the hottest type, and 7 is the value on a scale of 0-9, from hottest to coolest. However, this star is not a massive main sequence “O” star, but a very hot, low-mass (subdwarf) star, with a lot of hydrogen loss and a high helium content, hence the name sdO. It also has a broad O emission (“O” for Oxygen), so it can be classified as a Wolf-Rayet (W) star, with low mass (brackets), a high oxygen (O) emission, and moderately evolved (4 on a scale of 0-9). In short, a [WO4] star. The star is a point source of X-rays, while the oval core also emits X-rays, although diffusely. This implies that it is the fast stellar wind that drives the expansion of the central shell. Due to some theoretical inconsistencies, the central star has been assumed to be a double star, but no evidence of a possible companion has been found, not even with the SHT.

Formation and structure of NGC 6543. The intense stellar winds and strong UV radiation from the dying star initially surrounded the nebula with ionized gas. This gas is cooler and has a slower expansion rate than the gases emitted later. This produced very complex shapes, with shock fronts, gas bubbles, knots, and radial filaments. The resulting structure is spectacular, and three parts can be defined:

1) The bright, ellipsoidal central core, with a small angular size (20") but a key morphology for understanding the formation of the nebula. In 1992, L.F. Miranda and J. Solf conducted a detailed spectrographic study [1]. What I have undestood from their reading has been exciting, allowing me to recognize and largely understand what I saw in my photograph. According to this article, the structure is a thick, expanding ellipsoid with an inner and outer layer, whose characteristics could be best explained by a binary star system. Two concentric ellipses and a perpendicular ring are formed. The inner layer forms a prolate ellipsoid (rugby ball) with an inclination of 25°. A dense outer ring perpendicular to the inner ellipse (105°) is interpreted as an equatorial ring around the central star. The outer ellipsoid has the same inclination as the inner one and is also centered on the parent star. Three symmetrical filamentary structures and polar condensations are identified. Also very striking are two polar jets. The shape of the jets suggests a collimated bipolar flow, but their angle does not coincide with the polar axes of the ellipsoids. The explanation lies in a precession of the expansion axis, most likely caused by a central double star. In 1994 and 2004, the Hubble telescope obtained spectacular images of the nucleus of NGC 6543. These images do not contradict the observations of Miranda and Solf. They correct and add a great deal of information. In the 1994 image (published in 1995), the outer ring is said to be formed by two large lobes constrained by a ring of dense gas in the orbital plane of the supposed binary system. The polar condensations are evident, as are the jets displaced from the main axis of the system, which suggest a precession motion [2, 3] The image taken by the HST in 2004 is truly beautiful and precise [4]. Interesting fact: the NASA commentator calls it the cat's eye, the bull's eye, and the eye of Sauron. Many eyes. The central bubbles, nodes, and gas jets are more detailed than in the 1994 image, although they define the same structure. However, the 2004 image makes the concentric gas layers surrounding the central ellipsoid more evident. I wanted to discuss this second, outermost structure in the next section.

2) The inner rings. Outside the ellipsoidal core, rings of dust and ionized gas are visible, resulting from fluctuations in stellar activity and the interaction of the emerging gas with the external environment. At least eleven rings are observed in the 2004 HST image. These fluctuations are not explosive. In the transition between the red giant and the white dwarf, there are pulses of activity in which greater amounts of mass and radiation are emitted, creating concentric shapes. The formation of these rings in planetary nebulae seems more the rule than the exception. Corradi and his team found them in 2004 [5] in at least 35% of the studied nebulae. According to their study, they occur in the last 10,000 to 20,000 years of the evolution of the asymptotic giant branch (AGB: when a medium-mass star consumes all its hydrogen and begins the evolution to a red giant). The spacing between rings varies from <0.01 pc to 0.06 pc, and increases with time. The pulses occur every ~1500 years, and in them a mass equivalent to the sum of all the planets of the Solar System is lost. These rings formed in NGC 6543 before the central ellipsoid, which started to form in the center of the gas and dust shells about 1000 years ago for reasons that are not well understood [4]. Since then it has continued to expand, as can be seen by comparing Hubble images taken in 1994, 1997, 2000 and 2002 [6].

3) The outer halo. The structure most external to the rings, and perhaps the most characteristic of NGC 6543, are the radial filaments that make up the outer halo. It is believed that their main origin is that the dense knots of gas formed at the beginning of the star's demise (asymptotic giant branch) acted as screens around it, creating radial shadows between which intense ionization and heating occurred, giving rise to bright filaments. In addition, fast-moving winds collide with the slower-expanding outer regions. The material is fragmented and follows the direction of expansion. Other explanations include the irregularity of the outflow pulses (one of the reasons for assuming there was a second star) and the photoevaporation of the knots exposed to UV light, creating clouds of material that flow outwards, creating long, thin tails [7, 8].

Molecular hydrogen. The presence of molecular hydrogen has been detected within the nebula. In many planetary nebulae, molecular gas is found at greater distances from the star, in cooler regions where the gas is no longer ionized. However, here it is found mainly at the inner edge of the outer halo and in nodes/filaments where punctuated emissions have been detected, especially in low-ionization structures (LIS) towards the NE and SW (LIS: regions of planetary nebulae that emit along low ionization lines: NII, SII, OII). Why doesn't strong UV radiation ionize H2? There is no single explanation, but it is clear that there are niches where H2 is protected. Infrared studies have shown that within the nebula there is very cold dust, both in the central core and in the concentric layers, the outer halo, and in the low ionization regions. Molecular hydrogen forms above the dust, and in many regions there is a protective shielding effect from UV radiation, both by the dust and by regions of denser gas and zones of photodissociation with neutral hydrogen [9].

Other objects in the image. In addition to NGC 6543, the photograph contains other objects of interest:

1- On the western side of the halo, there is a larger blue condensation. It is IC 4677. It is sometimes mistakenly identified as an alternative name for the planetary nebula or even as a galaxy, but it is only a thickening of the peripheral halo. This patch was first seen by Barnard on April 24, 1900, and added as IC 4677 to the second edition of Dreyer's "Index Catalogue." Initially thought to be a galaxy, IC 4677 was added to galaxy catalogs such as the Morphological Catalog of Galaxies (MCG) and the Principal Galaxies Catalog (PGC). IC 4677 has an atypical appearance for galaxies. Rather, it appears to be a violent early outburst of gas from the central star of NGC 6543, which, along with the large halo and numerous smaller, fainter gas wisps, makes it quite likely. According to the Simbad database, IC 4677 is part of NGC 6543. The Lyon-Meudon Extragalactic Database (LEDA) also confirms this. Furthermore, it shines primarily in the O-III range, which is extremely unusual for galaxies (deepskycorner.ch [10]).

2- In the east, 9.4 min from the center of NGC 6543, NGC 6552 is visible, a barred spiral galaxy of magnitude 13.74 (in V), angular size 1.06 x 0.73 arcmin, and surface brightness 23.16 magnitude/arcsecond2 [10]. It was discovered by d'Arrest on October 6, 1866. It is a type 2 Seyfert galaxy: an active spiral galaxy with a nucleus emitting ionized gas in certain spectral lines. Type 2 implies that the emission lines are narrow because the wide ones are hidden by a torus of dust and gas around the nucleus. The emission is due to accretion onto a supermassive black hole. It emits X-rays, mid-infrared, nuclear wind, and molecular hydrogen. Its color in the photograph is greatly influenced by the processing of NGC 6543. In the linear RGB image immediately after the SPCC color calibration, the R, G, and B (16-bit) values were 46.04, 43.15, and 39.87 (slightly orange).

3- About 6.5 minutes to the west is the galaxy PGC 2689421, 0.1 arcminute across, about which I have found no information, except that it also has the charming name of NPM1G+66.0185. Its shape is elliptical, and its color is very similar to that of NGC 6552.

Processing. Summer in Spain brought heat waves and dust haze from the south. In addition to visually reviewing the images with Blink, I performed a hard selection with SubframeSelector, primarily using the number of stars as a definition marker with each filter. I reduced eleven hours of exposure, reducing the total from 80 hours 59 minutes to 69 hours 53 minutes. Photographing NGC 6543 is a challenge not only because of the weakness of its surroundings or the delicacy of its details, but because of the difficulty in making its extensive dynamic range visible. I opted to take shots with the same exposure time for RGB and a longer one for OIII and Hα, without ever reaching saturation. This way, the shots contained all the information, but the center could reach 20,000 ADUs, and some peripheral details were barely 8 or 10 ADUs above the background. I probably should have obtained different exposures for HDR. I also started with five masters: R, G, B, Hα, and OIII. I began experimenting with RGB fusion, extracting the stars after color calibration. I tried creating false exposures for HDRcomposition, working with masks + PixelMath, and performing progressive stretching, but whenever I emphasized the periphery, I saturated the center or obtained false-looking images. If I also had to add Hα and OIII later, the future looked bleak. I decided to create a single linear master and keep trying. On each master, I neutralized the background with GraXpert, applied a slight deconvolution with BXT, a light noise treatment with NXT, and separated stars and nebulae with SXT. The luminances were surprisingly close, so I created the "full master without stars" with:

R = ((R_Nebula*0.4)+(H_Nebula*0.65))*1.3

G = (G_Nebula*0.6)+(O_Nebula*0.4)

B = (B_Nebula*0.3)+(O_Nebula*0.7)

With this "full master," I struggled for several hours without success. At one point, already in the early morning, while I was trying to stretch progressively with GeneralizedHyperbolicStretch, I saw a bright light and heard a sweet voice whisper to me: “In Colour Options, change the Mode from RGB to Colour, and in Colour Mode Options change Clip Type from RGBBlend to Rescale Global, leave Colour Blend at 1, and check the Use RGB Working Space box.” I obeyed immediately. The light disappeared, and the voice said: “Alright, genius, let’s see what you do now”. Then, between brain fogs, in the Transformation – Generalized Hyperbolic values, I set D = 8.97, b = 1.15, SP = 0.000078, LP = 0, and HP = 0.01. And finally I had a balanced image in luminances, with the entire dynamic range well represented, with a lot of work ahead, but usable. I don’t know if the voice that helped me will be of use to others, but I’ve paused in the story just in case. We talk about when we use false color, but we don't mention that we always use false luminance. It's one of the main goals of processing: stretching the image to make it visible. But in NGC 6543, I feel almost guilty about the extreme delinearization.

References:

[1] Miranda, L. F. & Solf, J. Long-slit spectroscopy of the planetary nebula NGC 6543 - Collimated bipolar ejections from a precessing central source? Astronomy and Astrophysics (ISSN 0004-6361), vol. 260, no. 1-2, p. 397-410. https://article[1]s.adsabs.harvard.edu/pdf/1992A%26A...260..397M

[2] https://science.nasa.gov/asset/hubble/the-cats-eye-nebula/

[3] https://esahubble.org/images/opo9501b/#:~:text=This%20Hubble%20Space%20Telescope%20image,evolution%20of%20a%20dying%20star

[4] https://science.nasa.gov/asset/hubble/the-cats-eye-nebula-dying-star-creates-fantasy-like-sculpture-of-gas-and-dust/

[5] Rings in the haloes of planetary nebulae. R.L.M. Corradi, P. Sánchez-Blázquez,G. Mellema, C. Gianmanco, and H. E. Schwarz. Astronomy & Astrophysics, 417, 637–646 (2004). https://www.aanda.org/articles/aa/pdf/2004/14/aa0420.pdf

[6] Darren S. Reed, Bruce Balick, Arsen R. Hajian, Tracy L. Klayton, Stefano Giovanardi, Stefano Casertano, Nino Panagia, Yervant Terzian. Hubble space telescope measurements of the expansion of NGC 6543 : parallax distance and nebular evolution. The Astronomical Journal, 118 : 2430-2441, 1999 No vember. The American Astronomical Society. All rights reserved. Printed in U.S.A. https://iopscience.iop.org/article/10.1086/301091/pdf

[7] Pikel'Ner, S. B. Origin of the Fluctuations of Density and of the Radial Filaments in Planetary Nebulae. Astrophysical Letters, Vol. 15, p.91-96. https://adsabs.harvard.edu/full/1973ApL....15...91P

[8] Balick, Bruce. NGC 6543. I. Understanding the Anatomy of the Cat’s Eye. The Astronomical Journal, 127:2262–2268, 2004 April. https://iopscience.iop.org/article/10.1086/382518/pdf

[9] Stavros Akras, Denise R Gonçalves , Gerardo Ramos-Larios , Isabel Alemán. H2 emission in the low-ionization structures of the planetary nebulae NGC 7009 and NGC 6543. MNRAS 493, 3800–3810 (2020). https://academic.oup.com/mnras/article/493/3/3800/5743430

[10] https://www.deepskycorner.ch/obj/ngc6543.en.php