The     Post-Main-Sequence Evolution of the Sun and Similar Stellar-Mass Stars, Stage by Stage:

    1. The post-main-sequence phase of stars of stellar mass ∼ 1 solar mass M_☉ = 1.98855(25)*10**30 kg (e.g., the Sun) begins at ∼ 10 gigayear (Gyr) after star formation when hydrogen burning has become exhausted in the stellar core which is then mostly unburning (i.e., inert) helium-4 (He-4).

      The stellar evolution of the Sun in images is shown in the figure below (local link / general link: sun_evolution_images.html) and on the Hertzsprung-Russell (HR) diagram in the second figure (local link / general link: sun_evolution_hr.html).

    2. The stellar core has become inert, but a hydrogen burning shell will still exist around it.

    3. The stellar core and hydrogen burning shell continue contracting because of heat energy loss, but they will get hotter because of the contraction. The contraction converts gravitational potential energy into heat energy.

    4. The energy generation rate will actually increase because of the hotter hydrogen burning shell.

    5. The post-main-sequence star will then expand into a red giant with a stellar radius of order 1 astronomical unit (AU) because of the extra heating (Se-249; FK-470).

        Red giantism is something we understand in a computer simulation sense. We put the right ingredients into a computer simulation and they show that after the stellar core hydrogen burning is over post-main-sequence stars should expand into red giants. But there is no short explanation of the process in words or so yours truly believes.

    6. The star is now much brighter than when it was on the main sequence because of its much larger photosphere, but its photospheric temperature, will be lower eventually reaching ∼ 3000 K (Ze2002-341).

    7. By Wien's law the peak wavelength of it's emission will then be given by 

              wavelength_max ≅ 3000 micron-K / 3000 K  = 1 micron
      which is in the infrared.

    8. The star in the visible band (fiducial range 0.4--0.7 μm) will be red: hence the name red giant.

    9. When the helium core's temperature reaches about 100*10**6 K, the Helium-4 (He-4) will suddenly start burning to carbon and oxygen pretty much everywhere in the core within perhaps a few minutes (Ni-177; Ze2002-343).

      Because of its double proton positive charge and its stability???, He-4 requires higher temperatures and densities to burn than hydrogen.

    10. This event is the core helium flash. We never see this event because it is always buried in the core and lasts only seconds---but theoretically it must happens it seems (FK-473).

      Core helium flashes happen only to stars in the 0.4--2 or 3 solar mass range (Se-251; FK-472).

    11. After the core helium flash, the Sun will then contract and become a bit less bright oddly enough. Essentially this is because there is again nuclear burning in the core, but now it is helium burning.

    12. At this stage the Sun is called a horizontal branch star (Shu-152;. Shu-252). See the figure below (local link / general link: redgiant_hb.html).

    13. But the helium burns much more quickly than hydrogen.

      So the helium exhausts in the core and then there is an non-burning carbon-oxygen core, surrounded by a helium-burning shell surrounded by a hydrogen-burning shell.

    14. The Sun then puffs up to be a red giant again---but only for awhile.

        This second red giant phase is called the asymptotic giant branch (AGB) phase (Shu-151) and a in this phase is called an AGB star.

    15. The helium burning is unstable and of order every 100,000 years has a runaway burst of burning called a thermal pulse (AKA helium shell flashes), (FK-493; Wikipedia: AGB state; helium shell flash).

      Each thermal pulse causes a superwind that blows off much of the Sun's envelope.

    16. The escaping material forms a complex expanding nebula called a planetary nebula---the name is a total misnomer since these nebulae have nothing to do with planets.

        The name planetary nebula originated in the 18th century because the closest, most obvious ones are large enough to have a finite disk in a telescope and their greenish color made them look sort of like Uranus ⛢,♅ (FK-494; CK-329).

      For the evolution from the AGB star phase to the planetary nebula and white dwarf phases, see the figure below (local link / general link: agb_white_dwarf.html).

      Planetary nebula atoms are excited by UV radiation from the remnant star---which is very hot on the surface---and emit LINE SPECTRA.

      Green lines from doubly ionized oxygen are often particularly noticeable (Se-268).

      Planetary nebulae have huge range of behavior---which we won't go into here, but we will show some pictures.

      But we will say that the thermal pulses (AKA helium shell flashes) that throw of matter are probably usually aspherical and chaotic.

      So the ejecta are aspherical and chaotic.

      Also the ejecta from one pulse can run into the ejecta from earlier pulses and create shocks and other odd features.

      Example planetary nebulae are shown in the two figures below (local link / general link: planetary_nebula_ring.html; local link / general link: planetary_nebula_cats_eye.html).

      Planetary nebulae disperse in the interstellar medium (ISM) after a few thousands to tens of thousands of years.

      After some thermal pulses (AKA helium shell flashes) and several planetary nebula phases (say ???), the remnant Sun will have lost most of its hydrogen and helium and will NOT be able to burn what is left even though it is still very hot (FK-493--494).

      As the Sun is losing mass, what is left shrinks and heats from contraction.

      It will finally have about half its original mass and be of order the size of the Earth.

    17. The Sun is then a white dwarf: a dense compact remnant.

      The time from becoming an asymptotic giant branch (AGB) star (i.e., 2nd red giant phase star) to being a white dwarf is about 0.7 Myr (FK-493).

    18. General Caveat: The post-main-sequence evolution of stars is fairly well understood in qualitatively from a combination of observations and theory.

      But quantitative predictions are difficult. The three-dimensional hydrodynamic computations required test the limits of our modern computer codes and supercomputers.

      We can only tell the best story we can given present understanding.

    19. EOF

    Credit/Permission: © David Jeffery, 2003 / Own work.
    Local file: local link: post_main_sequence_sun.html.
    File: Star/Post-main-sequence file: post_main_sequence_sun.html.