Download Lecture 31 - 2 The Death of Stars: Stellar Recycling Phase 3 -

Lecture 31 - 2
The Death of Stars: Stellar Recycling
23 March, 2012
(Read Chapter 13 as background for lecture notes.)
Note: The Red Giant phase applies to all stars regardless of mass. It is Phase 5 where mass will make a dramatic difference in how a
star behaves. Figure 12-19 in your book gives a pictorial representation of how low mass and high mass stars evolve from birth through
death. However, your text then decides to present a full discussion of Phases 3-4-5 for primarily solar mass stars in Chapter 13, while
leaving all discussion of what happens in the most massive stars to Chapters 14. We will keep to the practice of discussing each phase
separately, and within the discussion of each phase, we will include what is happening to stars of all masses.
Remember the following axiom: Low-mass stars like the Sun die slowly with a whimper, while
high mass (> 8Msun) stars die quickly and violently.
Phase 3 -- The Early Death Phase (Red Giants)
I. Timescales
Beginning -- end of 4H→He fusion in the core; all H has been converted into He
End
-- end of second expansion of stars outer layers marked by the Helium flash
Duration -- approximately 10% of a stars main sequence lifetime, i.e.
10 Msun
1 Msun
0.1 Msun
7
9
10 yrs
10 yrs
(never observed !)
II. Leaving the main sequence
Once all of the Hydrogen has been converted to Helium in the star’s core, thermonuclear reactions stop and
for the moment there is no internal energy output that can counteract the always present inward pull of the star’s
gravity. The pure Helium core now behaves separately from the surrounding layers of the star which still
contain Hydrogen, and thus can still sustain the H→He fusion reaction if and when the temperature in these
layers exceeds 10 million degrees Kelvin.
III. Core collapse, Envelope expansion
• Helium core shrinks by a factor of ~30 to a size approximately that of the Earth
• Hydrogen “burning” (i.e. H→He+energy release) continues in a thin shell just outside collapsing He core.
• total luminosity of star increases by 100 to 1000 due to renewed H burning + heating of collapsing core.
• outer envelope “puffs up” to a diameter of ~ 1AU for a 1 Msun star and ~5AU for a 10 Msun star. Planets out
to these radii are completely vaporized while planets further out are vaporized except for rocky cores.
• despite higher luminosity, the much larger surface area actually results in a decreased surface temperature
for the giant star (i.e. it becomes redder than it was when it was on the main sequence).
• Location on the H-R diagram of 1 to 10 Msun stars that have entered the red giant stage is subdivided into the
luminosity classes I, II, III, IV as shown in the H-R diagram handouts
IV. Renewed Core Burning
When the collapsing He core temperature reaches 100 million degrees new fusion reactions begin to produce
energy which is sufficient to halt the collapse. Two basic reactions produce Carbon and Oxygen:
3 He → C + energy (the “triple alpha process”)
C + He → O + energy
These reactions continue for a time approximately equal to 10% of the main sequence lifetime until all of the
Helium has been converted into Carbon and Oxygen. The end of the red giant phase is when this C+O core
again starts to collapse under its own gravity. For a 1Msun star, He burning begins quickly in a thin shell
surrounding the collapsing C+O core. This rapid He burning shell ignites the Helium Shell Flash, which can
actually be a series of intense outbursts of energy causing te star periodically to increase in luminosity by up to
a factor of 10,000. In the case of a 1 Msun star, the stars outer layers to swell to 5AU in radius in a series of
pulsations resulting in the expulsion of successive outer layers of the star and eventually a complete
detachment of all of the material outside the C+O core. In the case of much more massive stars, they also expel
their outer layers in periodic pulsations, but these eventually become violent due to a series of new fusion
reactions that are triggered in the stars collapsing C+O core.