Planetary Nebula

• The Envelope of the stars is only loosely bound because the AGB star has a large radius. During the shell flashes the envelope is subjected to bursts of photons and energy.

  With some complications glossed over, the envelope and as much as 50% of the stellar mass is detached from the star and expelled into space leaving the AGB star very hot core exposed.

• This is called a Planetary Nebula.

  

  Note that the name is a misnomer. There are several PN that can be barely resolved with the naked eye as is the case for the brighter planets. So these objects do not ``twinkle'' like the stars and they (PN and planets) got lumped together.

• The core of the former AGB star is very hot - the surface temperature can be 150,000K. It is supported against gravity by degeneracy and has a very high density of 1000kg/cm or about 1 ton per thimblefull. We will come back to this object in a little bit.

• The high temperature of the ``central star'' (it is not REALLY a star as there is no fusion energy source) means it has a Planck curve that peaks way out in the UV and produces many UV and every soft X-ray photons. These collide with the H, He, C and O atoms in the former envelope that we now call a PN. These atoms get ionized, and on recombination the drop through the energy levels giving off various lower energy photons (that add up in energy to the original UV or X-ray ionizing photon) as they head for the ground state.

  

• This is a lot like a florescent light

• What kind of spectrum do we expect to see from a Planetary Nebula?

  Lots of emission lines from the cascades

  

• The Halpha line is usually strong which gives some PN a reddish color. There is also a strong line in the green part of the spectrum that was not recognized from studies of terrestrial sources and originally attributed to a new element called ``nebulium''. This was later recognized as the emission from a ``forbidden'' transition of doubly ionized Oxygen.

  Some electron orbits are ``meta-stable'' and rather than immediately de-exciting via emission of a photon from these starts the sit in these particular excited states for a long time before dropping down and emitting a photon. On Earth, before the ``forbidden'' photon can be emitted, the atom is collisionally de-excited. It is only because the density in the PN is very low that an atom can float around long enough in the excited state for a photon-emission de-excitation to occur.

• PN often appear as a ring - why?

  It is a geometrical effect.

  

• PN play an important role in the Chemical Enrichment of the Galaxy.

  In the RGB and AGB phases of evolution the convection zone of stars extends deep into the center of the star and the freshly minted He, Carbon and Oxygen products of fusion reactions are mixed up to the surface of stars. PN envelopes are therefore composed partly of ``new'' elements.

• PN expansion velocities are around 20 km/sec and after around 30,000 years the envelopes are so thin that they become invisible. These gas has become a part of the interstellar medium to be later incorporated into new stars.

  PN return between 0.05 and 0.6 of partly enriched material back to the ISM.

• In the Galaxy there are around 30,000 PN.

• Planetary Nebulae ejection probably occurs only for stars with