An Overview of Post-Main Sequence Evolution

Sections:

1. Introduction
2. Evolution to the Post-Main Sequence on the Hertzsprung-Russell (HR) Diagram
3. Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the AGB Phase
4. Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the Planetary Nebula Phase
5. Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the White Dwarf Phase
6. Post-Main-Sequence Evolution of the Sun
7. Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to the Supernova Phase
8. Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to Leaving a Neutron Star or Black Hole Compact Remnant
9. Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to Leaving a Supernova Remnant
10. The Main-Sequence Rule
11. Further Understanding of Post-Main-Sequence Evolution

1. Introduction:

   In this insert, we give an overview of the post-main-sequence evolution of stars.

2. Evolution to the Post-Main Sequence on the Hertzsprung-Russell (HR) Diagram:

   Image 1 Caption: A Hertzsprung-Russell (HR) diagram showing a representative sample post-main-sequence evolutionary tracks for single stars of various initial stellar masses, all with initial fiducial solar metallicity Z=0.02 and zero angular momentum.

   Note the following HR diagram bands:

   1. AGS = asymptotic giant branch.
   2. BSG = blue supergiants.
   3. LBV = luminous blue variable.
   4. MS = main sequence.
   5. RC = red clump: The stars in this HR diagram band are cool horizontal branch stars.
   6. RG = red giant.
   7. RSG = red supergiants.
   8. SubG = subgiants. The subgiants that evolve from main-sequence stars ⪅ 0.9 M_☉ are hypothetical in that none exist at cosmic present t_0 (equal to the age of the observable universe = 13.797(23) Gyr (Planck 2018)), but they will exist in cosmic future (Wikipedia: Future of an expanding universe).
   9. WR = Wolf-Rayet star.
   10. YSG = yellow supergiant.

3. Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the AGB Phase:
   1. The evolutionary tracks of lower-mass stars (⪅ 8 M_☉??? on the main sequence) leave the main sequence mostly moving toward the upper right half of the HR diagram. So the lower-mass post-main-sequence stars are typically lower in photospheric temperature and brighter than than main-sequence stars.

      Note, all stars born with ⪅ 0.9 M_☉ since the Big Bang era at lookback time the age of the observable universe = 13.797(23) Gyr (Planck 2018) have main-sequence lifetimes longer than the age of the observable universe = 13.797(23) Gyr (Planck 2018), and so NONE have ever left the main sequence. So their fates are entirely model-dependent. It is thought that the smaller ones the red dwarf stars (M ⪅ 0.6 M_☉) will evolve to being blue dwarfs (composed of helium (He) and ∼ 2 % metals). NO blue dwarfs exist yet.

   2. Actually, lower-mass stars have 3 phases red giant phases:
      1. The red giant phase proper where they are hydrogen burning in a shell surrounding an unburning helium (He, Z=2) core. We understand in a computer modeling why these stars expand to have stellar radii of ∼ 1 astronomical unit (AU) = 1.49597870700*10**11 m when they have this nuclear burning behavior. You put the right ingredients into the computer model and this is what you get. There is NO simple word explanation.

      2. The horizontal branch star phase (labeled RC in the image). This occurs when the helium core becomes hot and dense enough to have helium burning to carbon (C, Z=6) and oxygen (O, Z=8). computer modeling tells this makes the star contract, decrease in luminosity, and increase in photospheric temperature. For a cartoon of a red giant and the horizontal branch star, see the figure below (local link / general link: redgiant_hb.html).

         ---

      3. When the helium in the core is exhausted, the asymptotic giant branch (AGB) star phase. In this phase, computer modeling tells us the star expands again, and becomes more luminous than in the red giant phase proper. The AGB phase is relatively short: e.g., only ∼ 10**6 years for the Sun. Because the AGB star phase is so short, AGB stars are rare. However, because AGB stars are very luminous, they are conspicuous and some well-known naked-eye stars are AGB stars: e.g., Antares (Alpha Scorpii), Arcturus (Alpha Booetis), Rigel (Beta Orionis), and Mira (Omicron Ceti). The lower-mass lower-mass stars like the Sun are NEVER hot and dense enough in the core to nuclearly burn carbon (C, Z=6) and oxygen (O, Z=8). However, higher-mass lower-mass stars can nuclearly burn higher atomic number elements, but NOT iron (Fe,Z=26).

         For a cartoon of an AGB star, a planetary nebula, and white dwarf, see the figure below (local link / general link: agb_white_dwarf.html).

      ---

4. Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the Planetary Nebula Phase:
   1. During the AGB phase the lower-mass stars typically lose much of their stellar mass in multiple thermal pulses (AKA helium shell flashes) that eject their outer layers???. The thermal pulses (which are explosive events) are the result of unstable runaway helium burning that periodically afflicts these lower-mass stars. Helium burning is less stable than hydrogen burning. The ejected mass becomes a complex expanding shell which dissipates into the interstellar medium (ISM) on the time scale of tens of thousands of years.

   2. The shells are called planetary nebulae since they looked a bit like planets to observers in the 18th century and 19th century, but otherwise they have nothing to do with planets.

   3. In planetary nebulae the gas (which has, of course, largely cosmic composition (meaning inside modern galaxies: fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals)) is excited by electromagnetic radiation (EMR) from the remnant AGB stars and emits an emission line spectra. They are resolved astronomical objects in small telescopes and are quite pretty. Every planetary nebula looks different because of the chaotic nature of the thermal pulses, because of the different time periods since ejection, and because shells of ejecta from different thermal pulses can overlap.

   4. For example planetary nebulae, see the figures below (local link / general link: planetary_nebula_ring.html; local link / general link: planetary_nebula_cats_eye.html).

      ---

      ---

5. Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the White Dwarf Phase:
   1. The remnant AGB stars stars on the HR diagram move to higher photospheric temperatures, then to lower luminosity and become white dwarfs.

   2. White dwarfs on average have about 0.6 M_☉ and are about Earth-size. Thus, they have very high density are thermodynamically cold (even though their photospheric temperatures and internal temperatures are quite comparable to main sequence stars). Being thermodynamically cold means they are pressure supported by a Fermi gas (AKA degenerate gas) of electrons.

   3. White dwarfs have NO nuclear burning and just cool off forever. But they cool very slowly and the coldest ones have photospheric temperatures of ∼ 6000 K????.

      Far in cosmic future (Wikipedia: Future of an expanding universe), all white dwarfs will cool off and become black dwarfs (see Wikipedia: White dwarf: Radiation and cooling).

   4. The mass-radius relationship of white dwarfs is explicated in the figure below (local link / general link: white_dwarf_mass_radius_relation.html).

      ---

6. Post-Main-Sequence Evolution of the Sun:

   As a example of the post-main-sequence evolution of stars ⪅ 8 M_☉ on the main sequence, we consider the computer model prediction of the post-main-sequence evolution of the Sun (symbol ☉) (really Sun-like star) in the figure below local link / general link: sun_evolution_hr.html).

   ---

7. Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to the Supernova Phase:
   1. As seen in Image 1, the evolutionary tracks of stars ⪆ 8 M_☉ (on the main sequence) tend to be horizontal with the post-main-sequence stars moving back and forth in photospheric temperature in a rather complicated way in general.

      Typically, in ⪅ 10**6 years, they become core-collapse supernovae leaving compact remnants that are neutron stars (if on the main sequence, the progenitor star has ⪅ 20 M_☉???) or black holes (if on the main sequence, the progenitor star has ⪆ 20 M_☉???). Actually, the deciding factor between the neutron star and black hole fates is NOT certain???.

   2. For an explication of core-collapse supernovae, see the figure below (local link / general link: sne_core_collapse_core.html).

      ---

   3. For an actual image of of core-collapse supernovae, see the image of SN 1987A (name meaning first supernova observed in 1987) see the figure below (local link / general link: sn_1987a.html).

      ---

8. Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to Leaving a Neutron Star or Black Hole Compact Remnant:

   Compact remnants (AKA compact stars) are white dwarfs, neutron stars, and black holes (here meaning stellar mass black holes).

   As discussed in the above section Post-Main-Sequence Evolution of Stars Less Than 8 Solar Masses to the White Dwarf Phase, white dwarfs are the compact remnants of stars ⪅ 8 M_☉??? on the main sequence.

   On the other hand, to recapitulate from the above section Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to the Supernova Phase, core-collapse supernovae leave compact remnants that are neutron stars (if on the main sequence, the progenitor star has ⪅ 20 M_☉???) or black holes (if on the main sequence, the progenitor star has ⪆ 20 M_☉???). Actually, the deciding factor between the neutron star and black hole fates is NOT certain???.

   For a brief introduction to neutron stars, see figure below (local link / general link: neutron_star_cutaway.html).

   ---

   For a brief introduction to stellar mass black holes, see the figure below (local link / general link: black_hole_accretion_disk.html).

   ---

9. Post-Main-Sequence Evolution of Stars More Than 8 Solar Masses to Leaving a Supernova Remnant:

   The expanding ejecta of supernova explosion becomes a supernova remnant. There seems to be NO specified condition for the expanding ejecta to be a considered supernova remnant, but probably most people would of order 100 to 300 days after the explosion.

   For a famous example of a supernova remnant the Crab nebula, see the figure below (local link / general link: crab_nebula.html).

   ---

10. The Main-Sequence Rule:

    Note, in understanding stellar evolution including post-main-sequence evolution, you should always remember the main-sequence rule (local link / general link: star_main_sequence_rule.html).

    EOF

11. Further Understanding of Post-Main-Sequence Evolution:

    For further understanding of post-main-sequence evolution, see Post-main-sequence evolution keywords below (local link / general link: post_main_sequence_keywords.html):

    EOF