# Star question

1. Aug 23, 2004

### CBR600RR

I was just thinking, nuclear fusion in the core of a star produces a gamma ray burst that takes about one million years to reach the surface of the star where it becomes visible light. Why does it take the gamma ray one million years to reach the surface if it is traveling at light speed? I know it has something to due with the fact that the gamma ray gets absorbed and then re-emmited through its journey to the surface, but I would still think it would reach its destination faster than one million years. What am I missing?

2. Aug 23, 2004

### kurious

I think it is to do with large number of photon wavefunctions being in close proximity to one another and making the probability of any one photon getting to the surface small.

3. Aug 23, 2004

### chroot

Staff Emeritus
No, kurious, it is due only to photon-electron scattering.

The mean time for a photon to reach the surface of the Sun is probably less than a million years; it's more likely around 100,000 years.

What you're probably missing are three things, CBR:

1) The mean time of a photon's journey to the surface is about 100,000 years. Some lucky photons suffer only one or two collisions and escape in fractions of a second.

2) Collisions don't just gently nudge photons around; collisions can actually completely reverse the direction of a photon and send it back through the star the other way.

3) The Sun's gas is entirely ionized all the way out to its photosphere (the part you see). Ionized gas is extremely good at scattering light, since there are plenty of free electrons around. You can actually calculate the mean free path of photons (the average distance between collisions) quite easily, and from it, the average time it takes for a photon from the core to reach the photosphere. Depending upon the simplifying assumptions you make, the number is at least somewhere in the tens of thousands of years.

- Warren

4. Aug 23, 2004

### Nereid

Staff Emeritus
Absorption and re-emission is the right answer. Think of heat from the Earth's core ... it travels by conduction in the solid inner core, then convection in the outer core, then both in the mantle ... inside stars 'heat' is also carried by both conduction and convection from the core (where it's produced as gammas) to the surface (where it's emitted as light, UV, IR, ...).

Another way of thinking of this - the energy in the gamma makes its way to the surface, and undergoes many changes on the way, sometimes moving as a scattered photon, sometimes as the bulk movement of the gas; you may not consider 'conduction' in a gas to be the same as in a solid (where you have 'phonons'), but the principle is similar.

How long does heat from the core of the Earth take to get to the surface?

5. Aug 23, 2004

### CBR600RR

I kind of get what you are saying about the light getting bounced in all different directions along its journey throughout the sun, but it seems to me that since atoms are mostly just "empty space" between the electrons and nucleus the chances of gamma rays actually hitting an electron are so minimal that it should not be a factor.

Also shouldn't the gamma rays have enough energy to "push" aside all of the particles in the way?

Like I have said before I am not trying to be stubborn, your answers are all excellent, but I'm just missing something really easy. Answering my new questions will really help me understand more. If the questions are stupid, just bare with me. Thanx.

Last edited: Aug 23, 2004
6. Aug 23, 2004

### chroot

Staff Emeritus
1) The chances of a gamma ray interacting with a free electron in the dense plasma of the Sun is in fact very high. The average gamma ray goes no further than about a centimeter in the denser part of the Sun before scattering off an electron.

2) Gamma rays can't just push things out of the way. Are you perhaps thinking of gamma rays as "bullets" and the plasma of the Sun as "paper?" If so, you must realize that particle interactions don't work that way; particles can't break apart like a piece of paper. When a photon interacts with an electron, both the photon and the electron fly off in new directions. Although I really hate to use the "billiard ball" analogy, the situation is really much more like billiard balls in this case. Imagine a pool table with all of the balls careening wildly all over the table at high speeds, and trying to shoot your eight ball through all of them.

- Warren

7. Aug 23, 2004

### Nereid

Staff Emeritus
So let's see which part is causing you trouble.

Start with the Earth - I expect that you have no difficulty with the idea that heat might take a long time - hundreds, thousands, hundreds of thousands of years - to get from the core to the surface. I also expect that you have no difficulty with gammas being stopped by (say) 10 km of air, or 100 m of lead.

Now if you learned that the density in the core of the Sun is greater than that of lead (at the surface of the Earth), would you be surprised that gammas don't get very far? When gammas are absorbed into lead (or air), what happens to their energy? When something gets really hot (say, 100 million degrees), it will be more than 'white hot', right? If it were a 'black body', what would be the frequency (or wavelength) at which most photons were emitted? (compare 'red hot' - this frequency will be somewhere in the red part of the spectrum).

Finally, think about how heat moves from a hot place to a cooler one.

8. Aug 23, 2004

### CBR600RR

I'm still not completely there, but I'm defenately much closer to the answer than when I started . You did a good job in explaining the process to me. Thanx.

P.S. If anyone has anything else to add, please feel free to do so.

9. Aug 24, 2004

### Nereid

Staff Emeritus