Planetary Nebula Formation

After the ejection of the envelope of the AGB star (through thermal pulsing or some other mechanism), the bare core of the AGB star is revealed. The temperature of the core is over 100,000 Kelvins initially and is an intense source of Ultraviolet radiation and a high-speed stream of particles (stellar wind). Both of these energy outflows affect the formation of the planetary nebula.

• The wind moves away from the star at speeds of up to several thousand kilometers per second and easily overtake the ejected envelope of the AGB star. The wind compresses the ejected envelope of the star into a thin dense shell.
• The UV radiation strongly ionizes the gas in the shell.

  

  However, we note that planetary nebulae appear as shells on the sky. Why is this so? Let's see.

  

  The observer is to the right in the above figure. The figure shows a cut through the equatorial plane the shell ejected from the star, that is, the figure is actually a spherical shell surrounding the hot core. Now consider the three lines of sight through the shell (1, 2, and 3). Paths 1 and 2 are long and a large amount of gas contributes to emission along these lines of sight. As a result the shell will appear bright at these locations. For path 2, the amount of gas which contributes to the light is small ===> the shell will appear faint at this point.

  The above implies that the center of the disk of the planetary nebula will be faint while the outer edges of the disk will be the brightest. A nice example of this is the Ring Nebula in Lyra. A not so nice example of this is the Rosette Nebula.