Solar Model

The Sun, a typical main-sequence star of spectral class G2, with a surface of composition X = 0.73 (mass fraction of hydrogen) and Z = 0.02 (mass fraction of metals). Current estimated age: 4.52e9 years.

The standard solar model applies physical stellar modelling to describe the Sun (see Stellar Modelling).

  • The center of the Sun is rich in He-4 (product of pp-chain), and poor in hydrogen (low X, high Y).
  • The surface of the Sun is rich in hydrogen and poor in He-4. This is due to diffusive settling of heavier elements toward the center.
  • Since the birth of the Sun, its radius has increased by 10%, and its luminosity has increased by 40%.
  • The Sun’s primary energy source is the pp-chain.
  • 90% of the Sun’s mass is located within one half of its radius.
  • The luminosity suddenly increases at around 10% of the solar radius, which means that at this point the energy production is maximum. Two factors to take into account for this:
    (1) The energy production is higher as the mass within one shell increases, this is naturally happening as we get further away from the center.
    (2) The concentration of fuel available to generate energy (hydrogen) decreases as we move further away from the center.
    These two factors cause a maximum in the derivative of the Sun’s interior luminosity (dL/dr) at 10% of the solar radius (i.e. maximum of energy production).
  • At 71% of the solar radius, the energy transport regime changes from radiative transport (r < 0.71R) to convective transport (r > 0.71R).
Dependency of composition and luminosity on solar radius.

Dependency of composition and luminosity on solar radius.

Helioseismology = study of the oscillations of the Sun.

The Sun oscillates with roughly ten million vibration modes, typically with a very low amplitude (surface velocity of < 10cm/s, and luminosity variation  dL/L ~ 1e-6). Two kinds of modes identified.

  • p-modes, or five-minute oscillations, with periods between 3 and 8 minutes.
  • g-modes, with longer periods of about 160 minutes.

More about helioseismology.

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