M57 (The Ring Nebula) (NGC 6720)

Distance: 1630 light years

Right Ascension: 18 : 53.6 (hours : minutes)
Declination: +33 : 02 (degrees : minutes)

text copyright Robert Gendler

One of the best examples of a planetary nebula is the Ring Nebula which represents the gaseous remains of a sunlike star which has entered its final stages of evolution. The intense radiation from the stellar remnant ionizes the stars previously ejected gases. The inner shell glows green from ionized oxygen (OII and OIII) and nitrogen while hydrogen in the outer shell glows red. The stellar object in the center is a 15th magnitude planet sized "white dwarf". Now over 100,000 degrees the stellar remnant or white dwarf produces heat not by nuclear fusion but by its incredible density of a metric ton/cubic cm.

The planetary nebula stage of an intermediate mass star lasts only 10 to 30 thousand years, an astronomical instant in the overall life of a star. Eventually the ejected envelope disperses into the interstellar medium enriching it with both light and heavier elements originally created deep within the nuclear furnace of the now dead star. The outer shell of M57 expands at a rate of about 1 arc seconds per century. Astronomers have used its expansion rate to calculate the age of M57 which is estimated at 6000 to 8000 years old. Although the brightest part of M57 extends 1.4 arc minutes, deep observations of the nebula mostly in h-alpha light reveal an outer halo extending 3.5 arc minutes.

M57 is one of the brightest and largest of all planetary nebulae and has been studied extensively at multiple wavelengths. Although its shape appears spherical (as its name suggests) its true structure is complex, consisting of a bright nebula core with multiple rings, a bright bilobed inner halo, and a faint outer halo. It seems that many more PN's initially thought to be ring shaped or spherical are now believed to have a bipolar shape. M57 is no exception as it is believed to possess a bipolar shape seen end on. The bright nebula core representing the most conspicuous part of the nebula at optical wavelengths is composed of ionized gases with a stratified emission structure of ionized helium, hydrogen, nitrogen, oxygen, (HeII, H-alpha, NII, OIII, OII). The inner halo is a bilobed structure of ionized winds showing a brightened rim that is optically evident. The outer halo is a dim structure that consists of a limb brightened inner subshell and a diffuse faint outer subshell. The outer halo represents the ionized residue of stellar winds ejected during the stars red giant phase some 10,000 to 100,000 years earlier.

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.