The Medusa Nebula, previously thought to represent a supernova remnant was found by astronomers in the early 1980's to actually be a very old Planetary Nebula. Also known as Abell 21 or PK205+14.5, the Medusa Nebula spans approximately 4 light years across. The burned out stellar cinder known as a white dwarf can be seen glowing blue (by virtue of its emitted heat) in the center of the nebula complex.
Planetary nebulae (PN's) are formed in the final stage of the lifetime of stars which begin their lives having a total mass of one to eight solar masses (suns). In the final stage prior to the formation of a PN the evolved star is known as an asymptotic giant branch (AGB) star. The AGB star ultimately expels its outer envelope which eventually becomes ionized by the hot remnant star or "white dwarf". The precursors to PN's are low to intermediate mass stars.
The road to a planetary nebula begins with a main sequence star having one to eight solar masses. A star of this mass spends most of its life on the main sequence fusing its hydrogen fuel to helium deep within its core. While the star is on the main sequence an equilibrium is struck between energy production which tends to expand the star and gravity which wants to contract it. The more massive a star is the shorter its life. Stars with the mass of the sun can last on the main sequence for about 9 billion years while very high mass stars may last a million years or less.
Towards the end of the stars life, the hydrogen fuel becomes depleted allowing gravity to pull ahead forcing the core to contract. Contraction of the core causes a rise in core temperature but also a somewhat paradoxical expansion of the outer layers which cools the surface of the star. The stars luminosity increases as it bloats in size. At this stage the star is known as a red giant. In contrast to the main sequence star the red giant is characterized by hotter core and cooler surface temperatures. Eventually the hydrogen becomes completely depleted, Core contraction becomes unchecked and temperatures rise in the core to about 300 million degrees which triggers the onset of helium fusion. The star has found a new energy source however it won't last very long.
The onset of helium burning raises the surface temperature of the star again and increases its luminosity moving the star in a path of the HR diagram (Hertzsprung-Russell Diagram) almost aligned with its previous "red giant" track, hence the name Asymptotic Giant Branch star. When our sun reaches the AGB phase its radius will extend past the earth's orbit literally swallowing our planet up. AGB stars become unstable and their instability causes them to pulsate erratically. During the pulsations the star can lose half its mass, much of it converted to dust which eventually surrounds the star in a shell so thick that light becomes blocked and the star disappears from the visual sky. Studies of M57 have detected a substantial dust content of about 1/1000th of a solar mass mostly concentrated in the knots visible in the outer region of the bright nebula core. During this phase the star emits almost all of its energy in the infrared, becoming a strong infrared source.
Ultimately the AGB star sheds so much of its mass (all but 0.6 solar masses) that the hot stellar interior becomes exposed. The temperature of the core is so hot at this point (50,000 to 150,000 degrees Kelvin) that ultraviolet radiation ionizes the gaseous shell and surrounding dust forming the planetary nebula stage. The hot stellar interior of a planetary nebula collapses into an extremely compact object called a white dwarf. Within the white dwarf every electron is compressed as close to the nucleus as possible producing an extraordinary massive object having a density of one metric ton/cubic centimeter. For most stars this is the final stage as forces known as electron degeneracy pressure prevent further collapse.