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The Sunlight is an electromagnetic radiation.

Is it known what is the origin of this radiation? Can it be adequately described by classical electrodynamics (Maxwell's equations) as a motion of electric charges in the Sun? Is it necessary to take into account quantum effects described by quantum electrodynamics? Or is it necessary to take into account other processes?

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    $\begingroup$ I really wish someone had just answered "The Sun." $\endgroup$ – tobyink Nov 8 '20 at 0:48
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The light from the Sun comes from the photosphere; a relatively thin layer, a few hundred km thick.

The photosphere of the Sun is in radiative equilibrium, getting neither hotter or colder on average. What this means is that the emission processes that produce the radiation that escapes from the photosphere, are the inverse of the absorption processes that stop radiation from deeper, hotter layers reaching us.

The dominant continuum process is bound-free photoionisation of H$^{-}$ ions that form when hydrogen atoms capture electrons released from the ionisation of potassium and sodium atoms in the atmosphere. There are some other bound-free photoionisation processes of other species that contribute continuum opacity, and bound-bound transitions between energy levels in a variety of atoms and ions that contribute opacities at discrete wavelengths.

The principle of detailed balance means that these absorption processes are balanced by free-bound photorecombination of H$^-$ ions contributing light over a continuum of wavelengths and bound-bound downward transitions in atoms and ions at specific wavelengths.

The understanding of these processes certainly requires quantum physics and cannot be described by classical electromagnetism.

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    $\begingroup$ Still, the effect of all this advanced plasma physics is to reasonable accuracy a black body spectrum. $\endgroup$ – my2cts Nov 5 '20 at 21:33
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    $\begingroup$ @my2cts Sure. But "blackbody emission" is not a process. $\endgroup$ – ProfRob Nov 5 '20 at 21:59
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    $\begingroup$ +1 This is the only answer that addresses what OP seems to be asking about. The question was about sun light, not about fusion. And a "black body spectrum" can arise from different underlying physics. An incandescent light bulb and the photosphere can both be thought of as black bodies, but for different reasons. Similar to how a piece of metal and an electrolyte solution are both good conductors of electricity, but for different reasons. $\endgroup$ – jkej Nov 6 '20 at 15:38
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    $\begingroup$ @jkej You may have overlooked my answer. $\endgroup$ – my2cts Nov 6 '20 at 17:42
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The origin of sunlight is the hot plasma at and near its surface. It can be reasonably well described by Planck's black body radiation.

Check out https://en.m.wikipedia.org/wiki/Sunlight and sources therein.

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  • $\begingroup$ Perhaps a source would be appropriate in this answer $\endgroup$ – electronpusher Nov 5 '20 at 14:56
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    $\begingroup$ This answer is correct if the source of the light from a classic electric lamp is the wolframium filament. And is wrong if the origin is the electric power plant which is several miles away. $\endgroup$ – Anonymous Coward Nov 6 '20 at 11:20
  • $\begingroup$ Several comments removed. Please remember to be kind to each other, everyone. $\endgroup$ – ACuriousMind Nov 6 '20 at 17:07
  • $\begingroup$ @AnonymousCoward : So your position is that is is sensible to claim the electric power plant sends light down the electrical grid? $\endgroup$ – Eric Towers Nov 8 '20 at 18:01
  • $\begingroup$ @EricTowers Not at all. My position is that "power plant sends light down the electrical grid" has the same level of correctness as "Origin of light in the sun is fusion at the core". To be more explicit I think both are incorrect. $\endgroup$ – Anonymous Coward Nov 9 '20 at 12:52
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Stars undergo nuclear fusion of chemical elements in their cores. The outward pressure of the resultant radiation and the thermal pressure from the plasma counter-act the impending collapse from the inward gravitational pressure of the star. Thus, a star is a delicate dance between outward and inward pressures. Eventually, the star runs out of nuclear fuel and the gravitational collapse wins over and the stars ends its life - depending on its mass and metallicity - as either a white dwarf star, a neutron star, or a black hole (or maybe even more exotic, undiscovered possibilities).

In terms of quantum mechanics there's a rich history of scientific progress about stellar models. For a nice historical review, I recommend this article by Arny. Therein, he discusses how purely classical models prior to the 1900's were used to try to estimate the age of the sun using thermodynamics. But this resulted in ~10^6 years, which they knew was incorrect due to the geological record. Some advancements were made in the thermodynamic understanding of plasmas by Jeans, Lambden and others.... But the real breakthrough came from quantum mechanics in two ways:

  1. spectroscopy allowed for the precise characterization of stars according to their composition - certain wavelengths of light are emitted by certain energy differences of the states of certain atoms. This paved the way to a nuclear theory of how stars work.

  2. Eddington and others computed how much energy would be required to keep the sun shining at its luminosity, and this led to numerous avenues of research. I won't go into detail here, the article by Arny is great. But the punchline is that the energy could be acquired through a nuclear process (such as the proton-proton chain reaction which requires 4 hydrogen atoms to produce helium isotope in low-mass stars such as our sun, or the CNO cycle which is thought to dominate in higher mass stars, and this is a field of its own since the late 1930's called stellar nucleosynthesis).

Eddington, in The Internal Constitution of the Stars (1926), calculated without knowing how the energy was released, the energy yield of hydrogen burning. He further noted that hydrogen was the most efficient fuel in terms of energy per gram and that it would be able to power the sun for some 100 billion years.

But there were many details that were unexplained. Most importantly, how do they overcome the Coulomb barrier? The famous physicist George Gamow found the answer in quantum theory:

The key to the solution to this dilemma was found by G. Gamow in 1928 when he showed that quantum mechanical tunneling allowed fusion to occur at temperatures far lower than previously seemed plausible.

EDITED: Although fusion occurs in the core and produces photons as a by-product, these photons are rapidly absorbed by the plasma and re-emitted continually. It takes hundreds of thousands of years for photons to scatter through the opaque interior. Light leaving the surface of the star scatters from the photosphere of the stellar atmosphere, where the density of the plasma becomes sufficiently low for the photons to escape (a "last scattering surface"). Their path can be modeled as a random walk.

@my2cts points out that the spectrum of the photosphere is well-modeled by the black-body radiation spectrum.

EDIT: per the OP's question in the comments, as to whether a certain nuclear reaction occurs depends on what nuclear force mediates it (among other things...). This free article explains quite nicely in detail. For the simple example, consider the proton-proton reaction: 4 protons simultaneously colliding is extremely unlikely, so a series of 2-body interactions (chains) instead,

strong interaction: p + p $\rightarrow$ $^2$He, this does not work since $^2$He is highly unstable, i.e. $^2$He $\rightarrow$ 2p immediately (for the same reason p + $^4$He $\rightarrow$ $^5$Li is impossible).

weak interaction (Bethe 1938): p + p $\rightarrow$ D + e$^{+}$ + $\nu$, and this does work, where D is deuterium and $\nu$ is a neutrino.

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    $\begingroup$ This is like saying that the photons emitted by a light bulb originate at the power plant. Unless the bulb is powered by solar cells and then the photons emitted by the light bulb originate in the core of the sun. $\endgroup$ – nasu Nov 5 '20 at 19:16
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    $\begingroup$ Totally misleading. The light from the Sun comes from the photosphere and has nothing to do with fusion. If you turned fusion off the Sun would get gradually smaller and hotter with roughly constant luminosity. $\endgroup$ – ProfRob Nov 5 '20 at 20:45
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    $\begingroup$ Of course the sun emits light ultimately because it is heated by fusion. However, the photons it emits come from the hot plasma at its surface. I believe this is what the OP asked. $\endgroup$ – my2cts Nov 5 '20 at 21:28
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    $\begingroup$ Agree on this being misleading. Far, far more photons are emitted from the photosphere than are produced in the core. This is obvious when you consider that the total power emitted from the photosphere must be the same as that produced in the core for a star at equilibrium, and the energy per photon in the core is many times higher. $\endgroup$ – Christopher James Huff Nov 6 '20 at 3:20
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    $\begingroup$ The very first sentence of your answer makes this claim. Stars do not shine electromagnetic radiation generated by fusion in their cores. $\endgroup$ – Christopher James Huff Nov 6 '20 at 14:18

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