Science Questions

How does the sun produce photons?

Sat, 4th Jun 2011

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Shee Hong asked:

I was wondering, how are photons created in the sun, and how do they get propelled at the speed of light towards the Earth?

With great thanks,

Hong Shee



Dominic -   The surface of the Sun is very hot of course.  Its so hot that hydrogen becomes ionised into plasma so that you have protons and electrons as separate bodies, rather than bound together the suninto atoms.  As those different charges interact, they can lose energy which is radiated as the photons that we see.  Now that's not actually the powerhouse that drives luminosity of the Sun.  That is the fusion of hydrogen atoms into helium which occurs at the core of the Sun - in fact, only in the central 20% or so of the Sun; you have another process, which is convection, which is carrying that heat generated at the centre of the sun out to the surface to keep the surface hot so that it continues to shine.

Chris -   Are there no photons being produced deep inside the Sun?  Presumably there are, but they just can't get out.

Dominic -   Yes.  Photons are being produced all throughout the Sun; but the Sun is made of a cloudy material because the protons and electrons inside the Sun can interact with those photons.  And that means the photons produced deep down can only actually travel a few centimetres before they're reabsorbed.

Chris -   Brian Fulton, professor of Astrophysics at the University of York, when he was on this programme he made the point that the photons that get made in the Sun are actually a million years old plus by the time they emerge because they have spent their entire life being bombarded around and absorbed and reabsorbed, ad infinitum almost before they finally escape.  So, if the Sun went out tomorrow - as in all reactions stopped - wed still have a million years of the light locked inside.

Dominic -   That's absolutely right.  The light is travelling at the speed of light, but its only hopping a few centimetres at a time and we don't know what direction its going to come back out again.  It may end up going back towards the centre of the Sun again and it takes a million years to get out.  Its quite a random walk for that energy to get to the surface.


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It's basically the same principle as light emitted from any hot black body. It was explained mathematically by Max Planck in 1901 and by Einstein in 1909. Each approached the problem from very different angles. For a less mathematical approach, watch this video.

The spectrum of the sun is not a perfect match to the black body curve. The gasses near the surface absorb specific wavelengths, causing them to become ionized; as they cool, they emit specific wavelenghts. The black absorption lines in the spectrum are easily seen, but the emission lines are lost in the brighter black body spectrum. Phractality, Sun, 1st May 2011

Wow Phractality, nice explanation. Made me look up how the 'ionization' would work. Strange how one thinks one 'knows', only to find that one needs to recheck it again. Like I thought I 'knew' something about this, but then someone writes something that makes me see it in a different way, and I have to back to basics to see if I really understood it. Could be galloping senility too of course :)

"The corona is known to be very hot because it has ions with many electrons removed from the atoms. At high enough temperatures the atoms collide with each other with such energy to eject electrons. This process is called ionization.

At very high temperatures, atoms like iron can have 9 to 13 electrons ejected. Nine-times ionized iron is only produced at temperatures of 1.3 million K and 13-times ionized iron means the temperature gets up to 2.3 million K! During strong solar activity, the temperature can reach 3.6 million K and lines from 14-times ionized calcium are seen.

Most of the corona is trapped close to Sun by loops of magnetic field lines. In X-rays, those regions appear bright. Some magnetic field lines do not loop back to the Sun and will appear dark in X-rays. These places are called ``coronal holes''.

And the Corona is the utmost layer of the sun, its atmosphere as it is called, and as those ions (atoms) leave that atmosphere they create the 'solar wind'. And the suns ions are made of positively charged ions (atoms) of hydrogen, H+, formed by removal of the electron from atomic hydrogen. the definition of ions come from Michael Faraday describing those substances that will 'allow a current to pass ("go") between electrodes in a solution, when an electric field is applied. It is from Greek ιον, meaning "going." '

And in general an ion is 'an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.'

So now I have it pin pointed again, well, probably the outset of early senility :) This one is a nice description of the sun The Sun--The Closest Star and if you want the math hiding behind our descriptions then this one will fill you up :) yor_on, Wed, 4th May 2011

Atoms and ions in solids liquids and gases are dynamic and always in thermal agitation bumping into each other.  When things with electrical charges accelerate (as they do when the bump into each other) they emit photons.  In the centre of the matyerial these photons interact with the material and are reabsorbed.  At the surface thy can radiate into space.  The frequency of these photons depends on how hot things are. You are very familiar with central heating radiating warmth you can feel  also seeing pokers get red and light bulb filaments white (really quite bright red!) hot.  This is just the same with the sun where the surface is around 6000 K.  There the peak radiation is yellow. Soul Surfer, Wed, 4th May 2011

Sir i got a lot help from your answers. You said hydrogen is present in ionized form i.e. in the form of electron and proton separately but what about the neutron, what happens to it?
Another thing that i want to know is as we are having particles such as electrons and proton in the atom, from where do the photon come in picture? How is it formed, what is actually producing the particle named photon? trupal.patil, Tue, 5th Apr 2016

The Sun is mostly Hydrogen & Helium, with some Oxygen, Carbon, Neon and Iron.

Almost all Hydrogen has just a proton and electron, with no neutrons.
About 1 in 6,000 Hydrogen atoms does have a neutron; this special kind of Hydrogen is called Deuterium, or "Heavy Hydrogen".

All the stable atoms (apart from "light" Hydrogen) have neutrons, locked up with the proton.

The temperature of the Sun's surface is hot enough (about 5,500 C) to rip the electrons off an atom. However, it requires far higher temperatures (millions of degrees C) to disturb the nucleus of the atom. So the neutrons just stick to the protons, and it doesn't really affect how the Sun produces light from its surface.

The charged atomic nuclei (+ charge) and charged electrons (- charge) are zipping around in the Sun in a plasma "soup".
When two charged particles zip by each other, their paths are bent. This change in momentum causes emission or absorption of a photon, which is required by conservation of momentum, and conservation of energy.
This process is called Bremsstrahlung radiation. evan_au, Tue, 5th Apr 2016

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