PLANETARY NEBULAE

The evolution of stars of mass up to 8 - 12 times the mass of the Sun ends with the formation of a planetary nebula and the production of a white dwarf. As such, the formation of a planetary nebula is the most common form for stardeath. There are roughly 1,000 planetary nebula known in our neighborhood of the Galaxy. There are undoubtly many more that aren't observed, however, planetary nebulae are still rather rare beasts. Why is this so if planetary nebulae are indeed the most common way for a star to die? Well, because the lifetimes of planetary nebulae are so short, they live for only 50,000 or so years (which is less than 0.001 % of the Sun's lifetime), the chances of seeing this phase of stellar evolution is very small.

Planetary nebulae are characterized by a shell of material (of mass roughly 10 to 20 % of a solar mass) moving away from a hot (temperatures of 20,000 to more than 100,000 Kelvins), faint star (in their center) at a speed of 10 to 30 kilometers per second. The glowing material in the shell are various types of ionized elements. The name planetary nebulae derives from the fact that they can appear greenish in telescopes much as do Uranus and Neptune. The greenish tint is not due to methane, however, it is due to the emission from doubly ionized oxygen (OIII). The reddish tint of planetary nebulae is due to the emission of Hydrogen Balmer beta lines.

• The pre-planetary nebula object is an Asymptotic Giant Branch (AGB) star (cartoon and bullseye). The stars are composed of two distinct regions.

  • Central Region: the carbon/oxygen core (the ashes of the triple alpha process) surrounded by the two energy producing shell sources (1. the helium ===> carbon,oxygen shell; 2. the hydrogen ===> helium shell). The core has high density, 10**5 grams per cc, high temperature, 250,000,000 Kelvins, and small radius, "earth-sized"!

    Question: How much mass is contained in the core?

  • Envelope: the rest of the star is referred to as the envelope. It is a large convective region of low density which surrounds the core. It has a diameter of ~ 400,000,000 kilometers or so (uncertain--but note that the current diameter of the Sun is ~ 1,400,000 kilometers and the diameter of the Earth's orbit about the Sun is ~ 300,000,000 kilometers)

• Such AGB stars are extremely bright, ~ 1,000 to 10,000 L(Sun) but are cool, ~ 3,500 Kelvins because of their large size. Hmmm, What happens to the Solar System during this time?
• In double-shell source AGB stars, the nuclear burning shells may be unstable. They could undergo thermal pulsing. This is interesting because thermal pulsing may lead to the ejection of the envelope of the AGB star (either in a massive, but nonetheless gentle wind or through the ejection of shells of material). Either process could play a large role in the formation of planetary nebulae, e.g., the Ring nebula (M57) in Lyra shown above. For more views of mundane and bizarre planetary nebulae see the planetary nebulae gallery
• Planetary nebula fade away after 50,000 or so years because the material in the shell becomes too rarified to be seen. This leaves the bare core of the progenitor star exposed. The hot core evolves into white dwarf.