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Is it true that the visible light we see from the Sun comes from the electron-electron shell changes from the element iron as they absorb and emit energy in the form of photons. This energy derived from the fusion reaction?

Is this process occurring deep in the sun so as to delay the final emmission of the photon out of the Sun by several hundred thousand years because of the density?


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2 Answers 2

No. The basic energy comes from the conversion of four protons to a Helium nucleus in the sun's core. It is true that the opacity, which is the per unit mass absorbtivity is heavily influenced by the amount of "metal" present in the star. Astrophysicists refer to everything heavier than Lithium as Z (metal), so the abundance of these heavier elements contributes to the thermal structure. A lower abundance would mean lower opacity -easier for the heat to escape, which counterintuitively means the star would contract and burn even faster/hotter. But there are a lot more intermediate weight atoms (such as neon) in the sun than just iron. see: so I really doubt the iron dominates the opacity.

In any case the photons we see, are emitted from the photosphere (the upper few thousand Km of the sun proper).

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I pulled out my 40some year old stellar structure book. AFAICT the opacity should be dominated by a combination of electron scattering, and free-free transitions (unbound electrons interacting with photons as they pass near atomic nuclei). The later scales as atomic number squared, so atom per atom, the iron is several times as important as say (Oxygen or Neon), but the numbers are also much smaller, so the iron contributes, but only marginally so. – Omega Centauri Jan 22 '11 at 15:16
You might add a few words to the effect that the visible surface is simply glowing because it is hot. Just like the gas in a neon (or sodium or mercury) lamp. – dmckee Jan 22 '11 at 15:20

I'd like to address the question of the delay between energy production by fusion and light emission at the visible surface.

To start with, while the primary power source of every star is fusion, they also heat up by Virial contraction. This is important because it means that proto-stars are very hot before fusion ever sets in (and indeed the heat is needed to trigger the fusion). Accordingly the following description applies to all stars, even at the very beginning of their life.

The interior of a star is composed of very hot, very dense, ionized gas (that is a plasma). This stuff is very opaque, meaning that the mean range of a photon is short (on the order of centimeters). So it is appropriately in some averaged point-of-view, to say that a photon travels a distance proportional to its range, interacts and is re-emitted in a random direction. Wash, rinse, repeat.

You can calculate how long is required for the random walk to move the photon up to the relatively transparent outer layer and allow emission into space. It turns out to be a long time: many thousands of year for a star like the sun.

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I didn't want to add the gravitational energy part, as for a manisequence star like the sun the energy release via contraction is pretty small during the mainsequence phase. Ive never liked the random walk analogy, as the radiation field in the interior is to a very high degree just the Plank black body distribution. What escapes is the heat, the photons have been absorbed and thermally re-emitted a huge number of times. And in convective parts of the star, convection does a lot of the work of heat transport. – Omega Centauri Jan 22 '11 at 16:13
@Omega, I assumed the OP was working from a pop-sci source, and wanted to get t a sense of how the long time constant could be, but I wanted to avoid talking about structure issues. Perhaps I should have left Virial heating out, but I was telling a story: star-stuff is opaque because it is ionized because it is hot... – dmckee Jan 22 '11 at 18:13
I liked the fact that you put in the virial heating, I had considered saying something about it but didn't. – Omega Centauri Jan 23 '11 at 0:49

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