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  1. I understand that star is in Plasma state (all nucleus and electrons are not bound to each other and moving around freely)

  2. Photon is emitted when an excited electron moves back to lower orbit.

  3. So in a star if electrons are not in any orbit then how can photons be produced?

I am sure some of my understanding above is incorrect :) please help me understand.

Thanks

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Your question is very good, even after getting the "correct" answer don't stop thinking about it. What about emission of light in lightning isn't it also plasma? Do you know that until very recently scientists thought that energy of the sun comes from gravity? Research this topic. –  Asphir Dom Apr 25 at 13:53

4 Answers 4

up vote 12 down vote accepted

1.I understand that star is in Plasma state (all nucleus and electrons are not bound to each other and moving around freely)

While hydrogen only has one electron, all other neutral atoms have more than one electron. When one electron is removed, this is referred to as the "first ionization". Removing one of several electrons from an atom still makes it plasma. Also, the term "plasma" is used when a substantial fraction of the atoms are ionized, not necessarily all. So in the sun or other stars, there are still electrons bound to nuclei, as well as free electrons.

For these reason, in the spectrum below, one still sees lines from transtions between electron energy levels of atoms.

enter image description here

2.Photon is emitted when an excited electron moves back to lower orbit.

Yes, and absorbed when going to a higher level, that is why we see the lines in the above spectrum.

3.So in a star if electrons are not in any orbit then how can photons be produced?

The main reason is that gamma ray photons are produced in the core of the sun by hydrogen fusion to helium, and create a cascade of lower energy photons as they travel to the surface. Also, all materials emit black body radiation. The overall shape of the above spectrum fits well to a black body model.

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thx davephd ! 1. in a star some electrons are still in orbit. 2. as gamma rays move outwards, the electrons (in orbit) at the surface for e.g will absorb gamma rays and remit photons (visible light frequency) ? –  user45246 Apr 25 at 12:16
    
yes, that's basically right, but the bound electrons exist mostly in the outer part of the star which is "only" in the ~5000K range, compare to ~10 million degress in the core. –  DavePhD Apr 25 at 12:23
    
@DavePhD Which other neutral atoms you mean? Isn't sun mostly hydrogen and helium? –  Asphir Dom Apr 25 at 13:55
    
@AsphirDom the ones seen in the spectrum above for example, Ca, Mg, Fe (I don't mean they are necessarily neutral in the sun, just that they have more than one electron when neutral). About 67 different elements have been observed spectroscopically in the sun. But my point in the first part of the answer was to explain the mean of the term "plasma". –  DavePhD Apr 25 at 14:10
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@AsphirDom yes, in fact the lines are dips, absorbance lines as I say in response to subquestion 2, so they are making a net negative contribution. –  DavePhD Apr 25 at 14:28

Your point 2 is not the whole truth: A photon can be emitted if a system falls from some excited state to a state with less energy, but the system doesn't have to consist of an electron and a nucleus.

Also nuclei can be described by a mean field model and there also exist different energy states, so photons can be emitted. This is for example what happens if we see gamma radiation.

Additionally, and I think this is the main answer to your question, during the fusion process in a star, a lot of energy is produced, also in the form of photons and neutrinos. Look for instance at this wiki article of the p-p chain: http://en.wikipedia.org/wiki/Proton-proton_chain_reaction

ppchain

All the gammas in this picture which shows one of the fusion processes in a star denote photon emission.

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thanks noldig ! in the wiki image, i see gamma-ray, but where is the photon denoted? –  user45246 Apr 25 at 12:02
    
You're welcome. A gamma ray is electromagnetic radiation at high energies, so it is simply a photon with a short wave length. We use this term to denote high energy photons, so for your question it is equivalent. –  Noldig Apr 25 at 12:05
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great. but i understand that 'visible light' is just a part of the em radition spectrum. we cannot see the gamma rays.....so how is the 'visible light' produced? because from earth we can see the stars. i guess we cannot see the gamma rays. –  user45246 Apr 25 at 12:08
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As DavePhD explained in his answer: The photons are not able to leave the star immediately ,the undergo several events before where they can loose energy and produce the black body spectrum we can see –  Noldig Apr 25 at 12:12
    
Should all the events (denoted by "bang" yellow starburst) show a gamma ray being emitted, or does it only happen with a D-H collision? Would these be different energy gamma rays (from different kinds of events)? A side note: p-p fusion like this powers the Sun, but more massive stars principally use the C-N-O cycle. –  Phil Perry Apr 25 at 17:20

Emission of photons can be discrete, as in transitions from one quantized state to another, or continuous.

Continuous radiation for example is the synchrotron radiation of electrons moving in magnetic fields in accelerators.

Bremsstrahlung is when :

is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is also used to refer to the process of producing the radiation.

Both these effects exist in a plasma, atoms and electrons scattering off each other, thus charged particles braking, and photons come out.

By the way, similar continuous spectra happen in a gas too, which has a black body radiation of infrared, not visible to our eyes, atoms and molecules brake in the spill over electric and magnetic fields of each other when scattering and infrared photons are emitted.

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The main source of radiation of stars like the sun are nuclear reactions (pp chain etc) in their core, which heat up the upper layers, which in turn radiate as a black body. Synchrotron is observed only in strong magnetic fields with relativistic charged particles, which applies for supernovae, AGNs, pulsars etc. –  auxsvr Apr 25 at 13:36
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@auxsvr It takes years for the light from the center of the sun to come out to the surface. Plasma carries magnetic fields with it.en.wikipedia.org/wiki/Sun has a good enough description –  anna v Apr 25 at 13:52
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Most of the Sun's interior transfers energy by radiation; that it takes several million years to reach the surface is irrelevant here, since the Sun's age is a thousand times that. Also, a quick glance at the (lack of) literature demonstrates that synchrotron radiation from the Sun is negligible, if observable at all. –  auxsvr Apr 25 at 14:39
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@auxsvr I cannot see how one could separate radiation form accleration/deceleration in magnetic fields from acceleration/deceleration from electric fields at these distances. –  anna v Apr 25 at 15:37

Thanks for a great question, and thanks to everyone for great answers! :-)

A very simple answer is, as someone above has already mentioned, there are lots of collisions of nuclei happening in a star. When these nuclei fuse, the binding energy is given off (this is the energy we get from fusion); and photons are also given off as the (now bound-state) compound nucleus de-excites to its ground state. These fusion-followed-by-deexcitation reactions are responsible for most of the energy and light from stars.

As we know, Electromagnetic radiation doesn't just have to come from electrons de-exciting, but comes from oscillation of any charged particles (e.g. protons inside the nucleus).

Keep thinking! It's great to see someone question things.

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