Physics 321
Astrophysics II:  Lecture #13
Prof. Dale E. Gary

Stellar Evolution

Protostar Collapse to Pre-Main-Sequence (PMS) Object

Post Main-Sequence Stellar Evolution
The turns in the PMS evolutionary paths in the diagram above are due to changes in the structure of the core of the star as it adjusts, fitfully, to nuclear burning in the core.  The diagonal line joining the "8"s in the figure is called the Zero-Age Main-Sequence (ZAMS), and represents the point at which equilibrium nuclear burning of H to He is taking place.  When the star arrives on the ZAMS, it has transitioned from a PMS object to a true star.  We already saw that stars evolve slightly after reaching the main sequence, when we discussed the evolution of the Sun over its lifetime.  The track is a slight motion along the main sequence, upward and to the left as the luminosity increases.

Hydrogen Core Burning
This stage of a star's life lasts for a time proportional to its mass, roughly according to the relation

where tsun = 10 billion years.  This is the total time spent on the main sequence (the main-sequence lifetime).

Hydrogen Shell Burning
As the star uses up its initial supply of H in the core, it begins to collapse again and the temperature and density in a region around the core (a shell) reaches high enough temperature to ignite H.  This causes the star to expand and leave the main sequence, rising up the Red Giant Branch (RGB).  During this time, the He in the core is inert, and for sufficiently massive stars it grow denser until it becomes electron degenerate.

Helium Core Burning
Once the star exhausts its supply of H in the shell, the star again collapses and the temperature rises rapidly.  The dense core of Helium suddenly reaches ignition temperature (Helium flash) and the track discontinuously changes to lower luminosity as the star shrinks again from its RGB stage.  Once there, it again is stable, on a "helium main-sequence", but its lifetime in burning He is much shorter than before.

Helium Shell Burning
When the He is exhausted in the core, the star begins to burn He in a shell around the Carbon core, and the track again moves to a red giant stage along the Asymptotic Giant Branch (AGB).  Because of the temperature dependence of He burning, the star is unstable during this stage and thermal pulses begin, which causes the ejection of planetary nebula (PN).

Carbon Burning
More massive stars can repeat this scenario with collapse to a stable carbon burning stage, again with ever shorter lifetime.

End of Life
Stars of various masses behave differently in detail, but ultimately they all suffer the same fate of collapse due to the available nuclear fuel being exhausted.  In the case of stars like the Sun, He burning is the last stage.  For less massive stars, He burning may not take place at all.  Once the final collapse takes place, we have only to look at the violence of the collapse and the forces involved to determine what happens next.