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The Sun as a Star

The Sun as a Star . The “Surface” of the Sun and Its Structure. Outer Layers – 3 distinct region Photosphere Chromosphere Corona . Photosphere -- light sphere. The surface in “visible” light T ~ 6500 - 4000 K Depth 100’s kms. Granulation – cellular pattern due to convection .

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The Sun as a Star

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  1. The Sun as a Star The “Surface” of the Sun and Its Structure Outer Layers – 3 distinct region Photosphere Chromosphere Corona

  2. Photosphere -- light sphere The surface in “visible” light T ~ 6500 - 4000 K Depth 100’s kms Granulation – cellular pattern due to convection

  3. Chromosphere – color sphere, seen at solar eclipse T ~ 6000 - 100,000 K, 2000 km thick Hot, low density gas, also granular appearance – supergranulation

  4. The Corona or Halo T ~ 1-2 x 106 K , extends millions kms Very hot, low density gas Source of energy to heat chromosphere and corona ?? Coronal holes -- in X ray images Flux of charged particles – solar wind - from the holes, governed by magnetic fields Heating via magnetic waves and mechanical flux from convective layers deep in interior

  5. Sunspots and the Solar Activity Cycle Appear dark – lower temp. ~ 4500 K vs 6000 K Strong magnetic fields ~ several thousand Gauss – normal Sun – few Gauss Opposite polarity between sides of a large spot group Magnetic disturbance or storm

  6. The 11 yr sunspot cycle -- magnetic cycle every 22 yrs.

  7. Maunder Minimum and the “ little ice age”

  8. Solar Activity – Prominences associated with large spot groups

  9. Solar Flares -- most violent form of solar activity A sudden brightening, above large spot group, between regions of opposite polarity Outburst of charged particles (cosmic rays), increase in high energy radiation

  10. Sunspots, prominences, flares all associated with magnetic fields All increase and reach maximum with 11 yr solar activity cycle

  11. The Solar Interior and the energy source of the Sun and Stars Gravitational (contraction/collapse) Nuclear Fission --- radioactive elements Nuclear Fusion --- ??? p+ + p+ -> ? How? the Coulomb barrier

  12. Nuclear Fusion and Nucleosynthesis in the Stars The proton-proton chain or hydrogen fusion, requires 107o K 1. p+ + p+ np+ + e+ + neutrino np+ = deuteron (deuterium) 2. d+ + p+  n2p+ + gamma ray n2p+ = 3He 3. 3He+ + 3He+ 4He+ + 2 p+ Net Result -- 4H  1He

  13. Alternative -- CNO cycle in more massive stars > 2 Msun The CNO cycle converts hydrogen to helium The mass-12 isotope of C captures a proton and emits a gamma-ray producing the mass-13 isotope of N. N-13 is unstable and beta decays to the mass-13 isotope of C with a half-life of approximately 10 minutes. The mass-13 isotope of C captures a proton and emits a gamma-ray to become the mass-14 isotope of N. The mass-14 isotope of N captures another proton and emits a gamma-ray to become the mass-15 isotope of O. The mass-15 isotope of O undergoes a beta decay to become the mass-15 isotope of N. The mass-15 isotope of N captures a proton and emits an alpha-particle (that is, a nucleus of helium) to close the cycle and return to C-12.

  14. Beyond Hydrogen Fusion He Fusion to Carbon, Oxygen3He4 C12 C12 + He4  O16 requires 108o K C, O fusion to heavier elements up to Fe (iron) requires 5 x 108o K C12 + C12  Mg24 ,O16 + He4  Ca20

  15. The Solar Interior Hydrostatic Equilibrium (Pressure) gas pressure out = gravity in Thermal Equilibrium (Temperature) Energy (heat) in = Energy out => Energy production rate = luminosity of the star

  16. Transfer of energy Radiative (inner) and convection (outer) Random walk of photons

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