NGC 6543, also known as the Cat's Eye Nebula, is a spectacular planetary nebula located in the constellation of Draco. It is one of the most well-known objects of its kind in the northern hemisphere.

Additionally, this one of the brightest planetary nebulae in the sky holds significant historical importance. The nebula was discovered in 1786 by William Herschel, and its name comes from its characteristic ring with a bright, central region that resembles a cat's eye.

While the "first" planetary nebula to which William Herschel applied the (now known to be incorrect) term was the Saturn Nebula (NGC 7009), it was in the Cat's Eye Nebula that he first located the central star. Herschel believed that he was witnessing the birth of a star and that the nebula was condensing to form a new sun. Although his speculation was merely a mistaken research hypothesis, it must be acknowledged that his intuition was quite good - he suspected that these objects had something to do with the life cycles of stars.

According to sources, the brightness of the outer ring of the nebula is variable. This is due to the emission of light from the gas excited by the hot central star. The inner disk is elliptical and elongated. The star at the center of the nebula is a remnant of an explosion from a star similar to the Sun. The Cat's Eye Nebula also possesses characteristic structures called "spikes," which are streaks of ionized gas on the periphery of the ring. Their origin is still the subject of scientific research. Here are a few more facts from the sources:

• Discovered on February 15, 1786, by William Herschel.
• Located approximately 3,500 light-years away from Earth.
• It has, as mentioned, uneven surface brightness - its total observed brightness is about magnitude 8.9, while its central star has a brightness of magnitude 11.1.
• Its observed diameter, including the halo, is approximately 5 arc minutes, and its actual diameter is about 5 light-years.
• The nebula is expanding at a velocity of about 16.4 km/s.
• It is situated almost perfectly at the ecliptic pole, which means its coordinates undergo very little change due to precession.
• Observations indicate that the central star has been shedding its mass by ejecting it during explosions that occur every 1,500 years. Each eruption expels a mass greater than the combined mass of all the planets in the Solar System.
• The faint halo surrounding the central part of the nebula consists of gas and dust from material ejected by the star during previous periods of stellar activity.
• The age of the nebula itself is estimated to be around 10,000 years, while the age of the outer fibrous shell is estimated to be between 50,000 and 90,000 years.

It has become a tradition for me to include a spectroscopic measurement image - a spectrum - along with its analysis for each planetary nebula. It's done purely for illustrative and informational purposes, but it's always satisfying to add something substantive to a plain picture In the case of this nebula, it holds the distinction of being the first planetary nebula whose spectrum was analyzed by William Huggins in 1864. Huggins discovered hydrogen emission lines but also "mysterious lines" ranging from green to azure, which were attributed to an unknown element called "nebulium." These lines are always among the strongest emissions and were eventually identified as emissions from doubly ionized oxygen by I.S. Bowen in 1928.

Modern studies of this nebula, including professional spectroscopic measurements, shed more light on this object and explained intriguing mechanisms occurring within it. In 1987, Professor Balick and his team concluded that density gradients in NGC 6543 are sustained by shocks generated by stellar winds. The inner part of the planetary nebula resembles a hot "bubble," propelled by winds from the central star of the nebula (CSPN) and expanding within the originally ejected envelope. Additionally, there are entire networks of filaments sustained by the pressure differences between the expanding internal gases and the cooler, dispersing outer shell.

Nebula:
TS ONTC 200/1000, Pro Uranus-C (IMX 585), 90% of frames from 18000 x 0.3 sec, 95% of frames from 1000 x 5 sec; 88 x 300 sec [February 26, 2023; February 28, 2023; April 26, 2023; May 8-10, 2023]
Spectrum:
SW 200/1000, ASI 294 MC Pro, SA 200 filter - 14 x 20 sec [May 2, 2022]