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,
Dominic - The surface of the Sun is very hot of course. It’s so hot that hydrogen becomes ionised into plasma so that you have photons and electrons as separate bodies, rather than bound together into atoms. And as those different charges interact, they exchange energy at the surface of the Sun and in the process of saving energy, they can lose energy and that 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 and so, you have another process which is convection which is carrying that heat which is generated at the centre of the sun out of 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 these protons and electrons can interact with that light. And that means the photons produced deep down can only actually travel a few centimetres before they're reabsorbed.
Chris - And Bryan Fulton who was on this programme - he’s professor of Astrophysics at the University of York - 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 - we’d 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 it’s only hopping a few centimetres at a time and we don't know what direction it’s 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. It’s quite a random walk for that energy to get to the surface.
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.
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 :)
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