Lecture 4 - Astro Pas Rochester

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Opacity and luminosity in stars At the high densities and temperatures found on average in stellar interiors, matter is opaque. The mean free path, or average distance a photon can travel before being absorbed, is about l = 0.5 cm for the Sun’s average density and temperature (given above). Photons produced in the center have to random-walk their way out. How many steps, or mean free paths, does it take for a photon to random-walk from center to surface? (See IAA p.P-25 and PU problem 5.11.) ! Suppose photon starts off at the center of the star, and has an equal chance to go right or left after each absorption and re-emission. Average value of position after N steps is <strong>Astro</strong>nomy 142 18
Diffusion of energy to the surface in the Sun <strong>Astro</strong>nomy 142 19
Page 1 and 2: Today in Astronomy 142: stellar str
Page 3 and 4: Principles of stellar structure (co
Page 5 and 6: These equations can be derived from
Page 7 and 8: Hydrostatic equilibrium Consider a
Page 9 and 10: Another way to derive this, using E
Page 11 and 12: The Sun’s interior, on the averag
Page 13 and 14: Central pressure in a star (continu
Page 15 and 16: For the Sun: Central pressure in a
Page 17: Central density and temperature of
Page 21 and 22: Opacity and luminosity in stars (co
Page 23 and 24: Diffusive approximations @f(x, t) @
Page 25: Summary ! Using hydrostatic equilib

Lecture 4 - Astro Pas Rochester

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