Solar Core - Energy Transfer

Energy Transfer

The high-energy photons (gamma rays and x-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years. However, the concept of photon travel is not a well-defined one, since photons are not conserved, and one photon at a high temperature normally turns into many photons at a lower temperature, during passage of heat out of the solar core to the Sun's photosphere. The long periods of time (tens of millions of years) refer to the characteristic time for the entire solar temperature distribution to change, as a result of changing heat generation rate in the core. This is far longer than the average time for transport of heat through the Sun because most of the Sun's heat capacity is in the kinetic energy of the particles in its plasma, not in the electromagnetic radiation present within it. The shorter estimates of photon travel time (tens of thousands of years) refer to the relatively rapid mean time needed for radiation to travel from the center of the Sun to the photosphere, even though the Sun's heat cannot pass from core to surface at this rate, due to the large heat capacity needed to be heated or cooled in the process, as mentioned above.

After a final trip through the convective outer layer to the transparent surface of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.