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The Question

(Submitted April 14, 1998)

Why does it take so long for a photon to escape from the sun? How would the amount of interactions affect this time?

The Answer

The core of the Sun is extremely hot, many millions of degrees, so electrons are stripped from their atoms. This means that there are a lot of free electrons whizzing around in the Sun. With the densities that are typical for the core of the Sun (a few gm/cm3) there will be many interactions with photons. It is the density and the effective cross section of an electron , and the average velocity of an electron (usually determined from the temperature) which determine the amount of time for an interaction (or, actually, between interactions). The size of the Sun, and the average distance a photon travels between interactions ("mean free path") determines the number of interactions (its actually proportional to the square of (radius of sun)/ (mean free path). Once we plug in the right numbers for the Sun, I get a number of about 100,000 years. I think the more "official" number is about 1 million years.

Steve Bloom
for Ask an Astrophysicist

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