The Sun is a gigantic ball of gas and plasma , hot enough inside to allow the fusion of hydrogen nuclei and hot enough on its surface to emit light capable of heating the Earth’s surface and allowing the presence of water. liquid and life on it. However, for the energy produced inside to get out of the star, a long time must pass . The energy of the Sun and of any star is produced in its core, where the temperatures and pressures necessary for nuclear fusion to take place occur. The fact that the Sun’s surface is shiny and so hot (compared to Earth’s, at least) tells us clearly that the energy produced in the core must find a way to reach the surface , to dissipate. . Now let’s see how exactly it happens.
The very high temperatures inside the Sun, which can reach up to 15 million degrees in its core , guarantee abundant and violent collisions between the gas particles that compose it. These particles will move at very high speeds, colliding with each other constantly. Also, these temperatures will mean that the core gas will be ionized . The electrons will have broken away from the atomic nuclei, forming a dense plasma. Under normal conditions, in which the electrons are still attached to the atomic nucleus, the photons of light can be absorbed by the atoms , exciting these electrons or removing them, if the photons have enough energy.
Therefore there are two opposite conditions determining how a photon travels through this region of the Sun. On the one hand, since there are no non-ionized atoms, photons will be able to travel more freely , since it will be much less likely to meet a tiny proton or electron than with a relatively gigantic atom. But on the other hand, since such incredible densities exist in the nucleus, they will not be able to move without quickly finding a particle to interact with. When we move away from the nucleus, the situation changes, although not in the direction that the photons would like. Upon leaving the nucleus, the temperatures will drop rapidly , so that the particles present there will collide with less and less energy and less frequency, causing more and more electrons to remain attached to an atomic nucleus. On the other hand the density will also decrease rapidly , increasing the separation between particles.
This will have the consequence that, although conditions have changed, the middle layers of the Sun remain opaque to light. However, there will be an important change about 200,000 kilometers deep from the photosphere, the outermost layer of the Sun, which we could consider as its surface. Until then, in the deepest layers of the Sun, energy transport was by radiation. The energy was being carried by photons, by light. In these regions the heavy matter (protons, neutrons, electrons) did not have a net movement, but was packed and crushed by the incredible pressures, but the light could dissipate the energy produced in the nucleus towards the outer layers . At these 200,000 kilometers of depth things change, and now there is a net transport of matter .
At this depth, a pattern of convection cells is created, in which the hotter and lower density material rises towards the surface and cooler and denser material falls towards the interior of the Sun. In this way, the energy is transported from the interior to the Sun. outside, through the movement of large amounts of matter . This of course is not as simple as it might seem from the explanation above and these convection cells are distributed in different layers, of different sizes . At a depth of 200,000 kilometers these cells can be several tens of thousands of kilometers in size, larger than Earth. The cells progressively reduce in size until, at a depth of about a thousand kilometers, they are approximately a thousand kilometers in size. The upper part of this last layer of cells is in contact with the photosphere, which we can observe directly and which we can study in detail to try to understand the interior of the Sun.
The beginning of the photosphere would in fact be marked by the point at which the density of the gas drops too low to continue to allow energy transport by convection. In the photosphere the density can be less than one gram per cubic meter , thousands of times less than the density of the Earth’s surface atmosphere. This lower density also makes the gas transparent again , allowing light to escape unimpeded to outer space and, occasionally, to Earth.
In all this energy transport from when it is produced in the core of the Sun until it is emitted abroad in the photosphere, there will be countless processes of absorption and emission of photons by the material that forms the different layers of the Sun. Depending on the specific region , each photon will be able to travel only a few millionths of a centimeter or several centimeters before being absorbed and re – emitted . All this process can be extended to the point that a photon created in the nucleus takes 100,000 years , or more, to completely leave the star. Of course, this amount will be different for each individual proton, but what is certain is that it will take much longer than the two-and-a-half seconds that it would take to leave if it could travel freely or that it takes for the trillions of neutrinos that are produced each time to leave. second in the inner layers of the Sun.