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This is an extract from the astrophysics chapter in my book:

Hydrogen atoms fuse to form helium. At the same time, lots of of gamma photons and neutrinos are produced. The photons take thousands of years to "fight" their way to the surface of the Sun, but then escape into space as visible or near visible photons at the speed of light.

I am really confused about how the sun produces light. I understand that through nuclear fusion, energy is formed, but isn't this energy in the form of thermal and gamma photons? I do not understand how visible light is emitted which is the reason to why we see the sun. Could someone please elaborate the paragraph from my book.

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Blackbody radiation. The sun is hot and so radiates with the spectrum that it does. en.wikipedia.org/wiki/Black-body_radiation –  Brandon Enright Mar 18 at 4:33
Be aware that the language used in that passage is evocative, but technically sloppy. The energy of the photons will eventually emerge from the sun as light (i.e. photons) but they won't be "the" photons in any useful sense. –  dmckee Mar 18 at 13:15

2 Answers 2

Hydrogen atoms fuse to form helium through the proton-proton chain which fuses four protons into one alpha particle (nucleus of ${}^{4}He$) and releases two neutrinos, two positrons and energy in the form of gamma photons. Although photons travel at the speed of light, the random motions they experienced inside the sun takes them thousand of years to leave the Sun' center. This random motion is due to the dense plasma in the Sun's interior since each photon permanently collides with an electron and gets deviated from its original path. The Energy released by fusion moves outward up to the top of the radiation zone, where the temperature drops to about 2 million K, then the photons get absorbed by the plasma more easily and this creates the necessary conditions for convection. This creates the convection zone of the zone. Then the plasma rises and the photons are carried to the photosphere where the density of the gas is low enough that they can escape. They mostly escape as visible photons, as their initial energy is lost through the random motion in the radiactive zone, and the absortion in the convective zone.

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so, gamma photons due to the loss of energy are emitted as visible photons... Just one more thing, are these the photons that are absorbed by the surface elements of the sun and then re-emitted... i.e. the black body radiation. I am having difficulties linking the concept of the source of the light of the sun and the black body radiation –  Eliza Mar 18 at 5:36

The photons we see are the result of black body radiation. The light from the Sun is emitted by basically the same process that emits light from an incandescent light bulb.

The energy released by fusion at the core is rapidly randomised as photons interact with the charged particles in the plasma, and you end up with just a hot plasma. The heat is gradually transferred outwards and ends up as a surface temperature of about 5800K. I won't go into the mechanism of black body radiation because this is addressed in the answers to the question What are the various physical mechanisms for energy transfer to the photon during blackbody emission?. Suffice to say that the thermal motion of charged particles in the plasma cause random oscillating electric dipoles, and these then emit electromagnetic radiation corresponding to the energy of these oscillations. Because the oscillations are random the result is emission of a broad spread of wavelengths with a peak at about 500nm.

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