IAL 9: The Life of the Sun

Don't Panic


  1. Introduction
  2. The Main-Sequence Life of the Sun
  3. The Fate of the Sun
  4. The Fate of the Earth

  1. Introduction

  2. Just as an overview here is an illustration of Sun's life.

    The history of formation of the
    Sun is part of the general history of star formation.

    Therefore at this point we will just click over to IAL 21: Star Formation for that history and return to this lecture to pick up the particular story of the Sun on the main sequence and afterward.

  3. The Main-Sequence Life of the Sun

  4. The main-sequence life (i.e., core hydrogen-burning life) of stars is their longest nuclear burning phase.

    It seems about 90 % of their nuclear-burning life is on the main sequence (Se-230).

    The more massive the star, the faster it lives.

    Stars more massive than about 8 solar masses end as core-collapse supernovae after only about 30??? or less million years.

    A 0.1 solar mass star is theoretically predicted to live of order 6000 Gyr (Gyr = gigayear = 10**9 years) on the main sequence (Se-265).

    It's odd to think that there are stars whose future is only known theoretically since NOT enough time has passed for them to have left the main sequence in the lifetime of the observable universe.

    1. Representative Nuclear Burning Lifetimes:

      Some representative nuclear burning lifetimes of stars are given in the figure below (local link / general link: star_lifetimes.html).

      Sun is a middling mass star and will live about 10 Gyr on the main sequence and will live as a nuclear burning star for about 1.5 Gyr thereafter (FK-493). These numbers are, of course, subject to revision.

    2. The Evolving Sun on the Main Sequence:

      On the main sequence, the Sun evolves very slowly.

      The main thing that is happening is that hydrogen in the core (i.e., the region out to about 0.2 solar radii) is slowly being transformed to helium by hydrogen burning (i.e., H-to-He-4 burning). See the figure below (local link / general link: nuclear_burning_ppi_chain.html).

      The decrease in fuel in the core has the seemingly paradoxical result that the
      Sun will get brighter (i.e., increases in luminosity) over its main-sequence lifetime. To explicate:

      1. Pressure support depends on the number of particles.
      2. When hydrogen is fused to helium, 4 particles are converted to 1.
      3. This causes a tendency to loss in pressure which in turn causes a tendency to contraction due the gravitational force on the Sun's own mass.
      4. But contraction increases the temperature essentially due to gravitational potential energy being converted into heat energy.
      5. The higher the temperature and the higher the density due to contraction, the higher the pressure and thus collapse is resisted.
      6. But higher temperature and density increases the collision rate of hydrogens and thus the hydrogen burning rate despite the decreasing abundance of hydrogen. Of course, there is a point where the rate must decrease when the hydrogen is nearly exhausted.
      7. So a competition of effects causes there to be a higher rate of energy production and the Sun gets brighter to compensate and stay in a quasi-steady-state.
      8. In general, main-sequence stars burn brighter as they exhaust their fuel.
      9. The argument for the positive feedback effect is only plausible. Proof requires a numerical calculation, and those have been done.
      10. Reference Se-246; FK-467.

      The Sun is now about 30 % brighter than when initially on the main sequence about 4.6 Gyr ago. Then it was a zero-age main sequence (ZAMS) star.

      It will be about 30 % brighter than now in about 3.5 Gyr (WB-106; FK-493).

      The luminosity evolution of the Sun is illustrated in the figure below (local link / general link: sun_evolution.html).

  5. The Fate of the Sun

  6. When the hydrogen in the core of the Sun becomes exhausted in about 6 Gyr (FK-493), the main-sequence life of the Sun ends. Then there is a rather complex evolution---to say the least.

    Now we will go step by step through post-main-sequence evolution of the Sun and stars of of stellar-mass in the insert below (local link / general link: post_main_sequence_sun.html).

    The SOLAR
    white dwarf will be mostly carbon and oxygen from helium burning. The Sun CANNOT burn carbon and oxygen: it's core never gets hot and dense enough.

    Stars that start out more massive than about 4 solar masses can burn carbon and oxygen (FK-497), but they too end as white dwarfs if they arn't massive enough to become core-collapse supernovae: i.e., have masses greater than about 8 solar masses.

    But although the SOLAR white dwarf will be mostly carbon and oxygen, the lighter helium and hydrogen atoms float on the top and form a skin, and so white dwarfs appear as hydrogen or helium objects (see Wikipedia: White dwarf: Atmosphere and spectra).

    The interior composition of white dwarfs is a modeling inference from the whole theory of stellar evolution.

    White dwarfs are NOT held up by ordinary gas and EMR pressure, but by a quantum mechanical pressure of degenerate electrons that is NOT very temperature dependent. This gives white dwarfs an unusual mass-radius relation. This mass-radius relation is shown in the figure below (local link / general link: white_dwarf_mass_radius_relation.html).

    White dwarfs are initially very hot with a surface temperature of order 30,000 to 100,000 K (Ni-178).

    The location of white dwarfs on the Hertzsprung-Russell (HR) diagram is shown in the figure below (local link / general link: star_hr_lum.html).

    Initially, a
    white dwarf's EMR peaks in the UV and in the visible they appear white: hence the "white" in white dwarf.

    But they arn't very bright because of their small size.

    White dwarfs have no nuclear energy sources. They can generate only a little heat by contraction.

    Essentially, they lose heat energy and cool off forever.

    Eventually they will drop to such low temperatures that they only radiate in the microwave or radio. They will then be called black dwarfs. For the cooling of a white dwarf to a black dwarf, see the figure below (local link / general link: white_dwarf_aging_video.html).

  7. The Fate of the Earth

  8. What happens to the Earth when the Sun dies?

    Well its a bit uncertain. It depends on how large Sun gets in its red giant phase and its asymptotic giant branch phase. See the figure below (local link / general link: sun_red_giant.html).

    If the
    Sun's radius is always less than 1 AU, the Earth might survive as a cold rock for many billions of years after the Sun becomes a white dwarf. It's orbit will have to change somehow as the Sun loses mass. The orbital radius will probably increase since the gravitational force will be lower.

    If the Sun exceeds 1 AU as some sources claim (Ze2002-344), then the Earth will be enveloped by the Sun.

    Once inside the Sun's atmosphere, there will be drag force that will cause the Earth to spiral into deeper layers and there it will be evaporated. The time scale after envelopment is about 200 years (Ze2002-344)---an sad case of orbital decay.

    "So the glory of this world passes away: sic transit gloria mundi."