Editing Appendix/Ramblings/OriginOfPlanetaryNebulae (section)

==Maoz (2016)==

Here, we consider the descriptions presented by [http://adsabs.harvard.edu/abs/1998asa..book.....R D. Maoz (2016)].

<font color="red">'''Evolution to the Red-Giant Branch:'''</font> (&sect;4.1, p. 65)  "<font color="darkgreen">Once most of the hydrogen in the core of a star has been converted into helium, the core contracts and the inner temperatures rise.  As a result, hydrogen in the less-processed regions outside the core starts to burn in a shell surrounding the core.  Stellar models consistently predict that at this stage there is a huge expansion of the outer layers of the star &hellip; This is the <b>red-giant</b> phase.  The huge expansion of the star's envelope is difficult to explain by means of some simple and intuitive argument, but it is well understood and predicted robustly by the equations of stellar structure.</font>"

<font color="red">'''Mass Loss &amp; Formation of Planetary Nebula:'''</font> (&sect;4.1, p. 67)  "<font color="darkgreen">Evolved stars undergo large mass loss, especially on the red-giant branch and on the asymptotic branch, as a result of the low gravity in their extended outer regions and the radiation pressure produced by their large luminosities.  Mass loss is particularly severe on the AGB during so-called <b>thermal pulses</b> &#8212; roughly periodic flashes of enhanced helium shell burning.</font>"

(&sect;4.1, p. 68)  "<font color="darkgreen">&hellip; the remaining outer envelopes of the star expand to the point that they are completely blown off and dispersed.  During this very brief stage <math>~(\sim 10^4~\mathrm{yr})</math>, the star may appear as a <b>planetary nebula</b>.</font>"