Why Does Gamma Ray Take 1M Years to Reach Star Surface?

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Discussion Overview

The discussion revolves around the time it takes for gamma rays produced in the core of a star to reach its surface, specifically addressing the mechanisms involved in this process and the implications of photon interactions within the star's dense plasma. Participants explore theoretical and conceptual aspects of this phenomenon.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that gamma rays take about one million years to reach the surface due to absorption and re-emission processes, questioning why this duration is so long despite the speed of light.
  • Another participant suggests that the close proximity of photon wavefunctions reduces the probability of any single photon reaching the surface quickly.
  • A different viewpoint asserts that photon-electron scattering is the primary reason for the long travel time, estimating the mean time to be around 100,000 years, with some photons escaping much faster after fewer collisions.
  • One participant elaborates on the analogy of heat transfer in the Earth, comparing it to the processes in stars, indicating that energy undergoes various transformations before reaching the surface.
  • Concerns are raised about the likelihood of gamma rays interacting with electrons in the dense plasma, with one participant questioning the effectiveness of gamma rays in pushing particles aside due to atomic structure being mostly empty space.
  • Another participant counters that the interaction probability is actually high due to the density of the plasma, explaining that gamma rays cannot simply push particles aside but instead scatter upon interaction.
  • Further analogies are made to heat transfer in different materials, emphasizing the high density of the Sun's core and its implications for photon behavior.
  • One participant expresses that they are gaining a better understanding of the topic but still have lingering questions, inviting additional contributions from others.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms and timeframes involved in the journey of gamma rays to the star's surface. While some agree on the role of scattering and absorption, there is no consensus on the exact time it takes or the primary factors influencing this process.

Contextual Notes

The discussion includes various assumptions about photon interactions, the density of stellar materials, and the nature of heat transfer, which remain unresolved and may affect the interpretations of the claims made.

Who May Find This Useful

This discussion may be of interest to those studying astrophysics, stellar physics, or anyone curious about the processes involved in energy transfer within stars.

CBR600RR
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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? :confused:
 
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I think it is to do with large number of photon wavefunctions being in close proximity to one another and making the probability of anyone photon getting to the surface small.
 
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
 
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?
 
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.
 
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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
 
CBR600RR said:
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.
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.
 
I'm still not completely there, but I'm defenately much closer to the answer than when I started :approve: . 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. :wink:
 
CBR600RR said:
I'm still not completely there, but I'm defenately much closer to the answer than when I started :approve: . 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. :wink:
Some suggestions which might help you (use google to find good sites with answers):
- what is the temperature and density in the core of the Sun?
- what is the 'black body radiation curve'?
- in what part of the electromagnetic spectrum does a black body the temperature of the core of the Sun mostly radiate?
- how does 'radiative transport' (of heat) work? how does it differ from conduction and convection?
 

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