Editing Appendix/Ramblings/OriginOfPlanetaryNebulae (section)

==Hansen &amp; Kawaler (1994)==

Here, we consider the descriptions presented by [<b>[[Appendix/References#HK94|<font color="red">HK94</font>]]</b>].

<font color="red">'''Evolution to the Red-Giant Branch &amp; the SC Limit:'''</font> (&sect;2.3, pp. 53-55)  "<font color="darkgreen">Following exhaustion of hydrogen in the core, the growing helium remnant is surrounded by an active shell of burning hydrogen, which supplies the power for the star.  The core itself, however, has no energy source of its own (except some input from contraction) and hence it tends to be isothermal since no temperature gradients are required.  It is at this point &hellip; that the structure of the star begins to change radically and the main sequence phase ends. &hellip; evolution to &hellip; the red giant branch (RGB) is characterized by a continual expansion and reddening of the star to lower [surface] temperatures. </font>" 

"<font color="darkgreen">In an early 1942 study Sch&ouml;nberg and Chandrasekhar demonstrated that when an isothermal helium core is built up to a mass corresponding to about 10% of the initial hydrogen mass of the star, it is no longer possible to maintain quasi-hydrostatic equilibrium for the core of the model star if pressure support is due to an ideal gas.  Other studies, including evolutionary calculations, support this by showing that the core contracts and heats rapidly &hellip; The envelope &hellip; however, responds by expanding rapidly &hellip;  This signals the end of the main sequence phase of evolution for the star &hellip;</font>"

<font color="red">'''Stellar Pulsation:'''</font>


<font color="red">'''Mass Loss &amp; Formation of Planetary Nebula:'''</font> (&sect;2.4.1, pp. 60-61)  "<font color="darkgreen">The ignition of helium under electron degenerate conditions &hellip;</font>" occurs via "<font color="darkgreen">&hellip; an explosive runaway or ''helium flash''. &hellip; The major evidence that leads us to believe that the helium flash is not explosive enough to disrupt the star in a serious fashion is that stars in the post-helium flash stage are observed and they constitute the horizontal branch of globular clusters, for example.</font>"

(&sect;2.4.1, p. 62)  "<font color="darkgreen">The critical point about the</font> [observationally determined] <font color="darkgreen">masses &hellip; for RR Lyrae stars, and therefore HB stars, is that they are less than the masses of their progenitor main sequence stars.  If these results are correct, then mass must have been lost during the time elapsed between the main sequence and the HB stages.  Such mass loss from red giants is observed, but the physical mechanism is not well understood.</font>"