Properties of the Core

The center of the sun is very hot, about 15 million degrees. The density in the center of the sun is about 150 times that of water. These conditions mean that matter is totally ionized (atoms are stripped of all their electrons.) This state of matter is called a plasma. At such high temperatures, the particles are zipping around at high speeds. At such high densities, it also means that they are sometimes crashing into one another.

This heat and density is the result of the crushing force of gravity from the upper layers of the sun above the core. The sun is in a balancing act between the crush of gravity and the outward pushing of the pressure. This balancing act is called hydrostatic equilibrium.

The Coloumb Barrier

Actual collisions between particles are suppressed because all of the particles have electrical charges. Particles with the same charges repel each other. Thus, it is quite difficult for collisions to occur. The repelling force is electrical, and it cause a sort of "barrier" to the collisions. We call it the Coloumb Barrier.

The Strong Nuclear Force: Nuclear Fusion

Once a collision does occur, and two particles actually touch, then the strong nuclear force takes over and the particles stick together. The strong nuclear force is so powerful that it overcomes the electrical pushing apart (repulsion).

When the particles stick together, this is called nuclear fusion. During fusion, energy is released. This energy is what powers the sun and keeps it shining and keeps it from collapsing under its own gravitational weight.

The Weak Nuclear Force: Role in Nuclear Fusion

Some nuclear confiurations (combinations of protons and neutrons bonded together by the strong nuclear force) are unstable. For example, if a nucleus has more protons than neutrons, then the electromagnetic repulsion of the protons is not well mitigated by the presnec of the neutral neutrons. So, to ease the tension of the nucleus wanting to eject a proton (for example), one of the protons will turn into a neutron! This process can happen due to the weak nuclear force.

During the process of a proton turning into a neutron, charge and energy must be conserved. The proton must get rid of its positive charge, and it does so by ejecting that charge as a particle that carries off the charge. This particle is a positron, which is the anti-matter particle of the electron. A positron is basically an electron with positive charge. Also, to conserve energy, the proton ejects a neutrino (neutral weakly interacting particle). Once these particles are ejected from the proton, it becomes a neutron. Since the nuclues has changed its number of protons, the element that it was is now changed to a new element.

The weak nuclear force is the force responsible for radioactivity and radioactive decay of elements. It plays an important role in the first step of the chain of nuclear reactions in the core of the sun.

Einstein's Energy-Mass Equivalence, E=mc2

The energy released comes from some of the mass in the particles when they fuse together. Some mass from the particles is "borrowed". The amount of mass taken from the original particles is converted to energy according to Einstein's famous equation "E=mc2", which reads "energy equals mass times the speed of light squared".

The net result of the hydrogen fusion in the sun is that four hydrogen nuclei (protons) are fused into a single helium nuclei (two protons + two neutrons) and in the process some mass is converted to energy (see pages 157-158 and Figure 8-9 in the 10th Edition of the text book).

Here is the application of E=mc2. The mass of four hydrogens (added together) is more than the mass of the single helium! One would think that they would be equal. Four masses added together equals four times the mass, right? Not in fusion! So, to apply Einstein's equation, find the mass loss in the fusion reaction and multiply it by the speed of light squared, which is a huge number = 9,000,000,000,000! The mass loss is just the difference between the "before fusion" mass and the "after fusion" mass, or

Multiple this mass loss by the speed of light squared to get the energy released in each fusion reaction.

So, each time four hydrogens fuse into a helium, a bit of energy is created. But the numbers are huge. About 1038 of these reactions occur every second. That is 100,000,000,000,000,000,000,000,000,000,000,000,000 fusion reactions per second. Every second of your life, for your whole life... (which is a blink of an eye in time). The sun sustains these reactions for about 10 billion years!

The Solar Neutrino Problem

Neutrinos are produced in the nuclear fusion reaction in the core of the sun. Neutrinos pass through matter without interacting (except in special reactions). Thus, they escape the sun's core and allow us to peak directly into the sun's core. We tested out knowledge of the sun's core by counting these neutrinos. For years, we counted only 1/3rd of those we predicted! This was called the Solar Neutrino Problem. It is now resolved. We now know that neutrinos come in three types, and that they can change from type to another as they travel from the core of the sun to Earth!; at first, we were counting only one of the types and only 1/3rd of them were counted. Actually there is more to it than that, but this is what essentially what led to the problem.