A Planetary Nebula is a transient phase of stellar evolution that exists for a short time following the death of a sunlike star having one to eight solar masses. With the depletion of its nuclear fuel as the swollen red giant phase is completed, the stars core can no longer hold on to the gases making up its stellar atmosphere and the star passively ejects the envelope out into space. The planetary nebula phase is a transient phase by standards of astronomical time, lasting only a few tens of thousands of years. During that phase the gases which once composed the layers of the star will form a variety of concentric rings, shells and halos which shine in brilliant colors specific to their chemical constituents.

NGC 6543 is a middle aged planetary nebula having been around for some 18,000 years.
The former core of the dead star is now a compact cinder, a white dwarf emitting powerful winds and radiation that ionize the various layers of the previously ejected stellar envelope. A gradient of ages defines each gaseous layer of the nebula. The outermost halo, ejected some 60,000 years ago in the fierce stellar winds of the dying star now shines faintly. The innermost bright layer is only 1000 years old while each subsequent layer is about 1500 years older than its inner neighbor. Over the next 10,000 to 20,000 years the nebula will disperse into the interstellar medium enriching the medium with light and heavy elements, the building blocks of future generations of stars.

NGC 6543 has the distinction of being the first planetary nebula to have its spectrum taken. In 1864 Sir William Huggins found three bright emission lines, a blue line identified as hydrogen and two mysterious green lines never seen before. At the time astronomers believed the mysterious lines were due to an unknown element which they named "nebulium". It wasn't until 1926 that the origin of the green lines was identified as oxygen and not an exotic new element. It was found that the green emission lines were due to the unusual behavior of oxygen that can only occur under incredibly low pressure environments such as a vacuum chamber on earth or in the relative vacuum of space.

Because they do not occur naturally on earth the two strong emission lines at 5007 and 4959 angstroms are known as the "forbidden lines" of doubly ionized oxygen. Those lines produce the classic bluish-green tint (teal) commonly seen in planetary nebulae. In order for oxygen to become doubly ionized (two electrons ejected from the oxygen atom, designated OIII) requires temperatures around 100,000 degrees Kelvin which astounded astronomers of that time as this was almost twenty times greater than the temperature of the sun. Today we know that those temperatures are easily reached by the hot central stars of planetary nebulae. The temperature of the central stars of planetary nebulae can range from 30,000 to 150,000 degrees making them the hottest stars in the universe