Image Above: Barnard 68, a molecular (black) cloud of gas and dust, which will become a star, perhaps in the next 100,000 years. At the moment, it is still very cold (around 4 Kelvin), and so the particles are moving around very slowly, and gravity has not yet began to dominate. Once gravity dominates, the star will begin to collapse (known at this stage as a protostar), and eventually the core will be hot enough, and the pressure will be high enough for the star to ignite, as hydrogen fuses into helium.
Strong Nuclear Forces
Nuclear energy, is stored within the energy binding the nucleus together. It is the strong nuclear force (also known as strong interaction or strong force) that both slightly repels the nucleon particles (the protons and neutrons in the nucleus) from each other, but predominately holds them together (mediated by photons) - holding them, and in the case of more than one proton, overcoming the repelling electrostatic force between the protons. The nuclear force also, although in different ways (mediated by gluons), also holds the quarks together - the fundamental subatomic elements that make up nucleon particles.
The strong nuclear force is one of the four fundamental forces in the universe, and is estimated to be 1038 times stronger than gravity (another of the four fundamental forces); but, unlike gravity and electromagnetic forces, it only acts over very small distances (10-15m or 1 femtometer) - roughly the width of a proton.
The extreme pressure and temperature (above 10,000 Kelvin) at the centre of the Sun is often too hot for the protons and the electrons surrounding the nucleus to stay properly connected, and the electrons are consequently being ripped off the nuclei. This can be imagined like a soup of loosely connected and disconnected electrons and protons - known as plasma (the fourth phase of matter - after gas, liquid, and solid).
All particles that have the same charge (positive or negative) are naturally repelled by one another (and opposite charges attract). The force that drives them apart is electromagnetism. In these extreme conditions in the Sun's core, relatively small numbers of protons (the hydrogen nuclei disconnected from their partner electrons) are able to collide - they are able to overcome this repulsive force between protons with the same charge, and a initial fusion reaction can occur (less than 2% of the fusion process is through another reaction process known as the CNO cycle). In this process, the electromagnetism force has been overcome, as the collision speed has made the distances between the atoms small enough, so that the nuclear strong force can take over, and pull the two protons together to form a new larger nucleus.
This chain of events (explained in more detail below) lead to the conversion of Hydrogen into Helium. Helium is left as a byproduct (a form of "ash"), as the fusion of Helium (with another Helium) forms Beryllium-8, which is very unstable, and extremely quickly breaks part - back into two Helium atoms again, making further fusion into heavier elements impossible until later on in the life of star (also explained further below - if the star has sufficient mass). This build-up of helium means that the composition of the core is different from that of the rest of the Sun - an estimated 35% Hydrogen and 63% Helium (by mass).
Although Helium is heavier than Hydrogen, it is about four times lighter than the 4 protons required to create it. Some of the mass deficit is, impart, due to the loss of positrons and neutrinos, which are produced and released in the process; however, even accounting for this, there is still a small amount of mass missing. This remanding mass has been converted into energy, which can be shown in the famous equation: E=mc2.
Star Energy Production: Proton-Proton Chain
Phase 1 of the proton-proton chain reaction is the fusion of two repelling (same charge) protons (1H nuclei). For this to occur, the two individual protons have been able to get very close together - almost wrapping around each other - and have bonded together into an unstable Helium-2 - diproton (2He), which has two protons and no neutrons.
Now a very rare event can occur: one of the protons turns into a neutron. The change from a proton to a neutron is made possible by the third of the four forces of nature: the weak nuclear force - it basically changes one of the three quarks (the fundamental subatomic particles that make up a nucleon) within a proton from a 'down quark' into an 'up quark,' transforming the proton into a neutron, which releases an excess positive electric charge as a positron (e+ - these are identical to electrons except they hold a positive charge), a neutrino is also released (which has virtually no mass).
The resulting unstable diproton (2He) decays very quickly either back into a hydrogen-1 through proton emission, as is the most frequent scenario, or far more rarely into deuterium (2H) through beta-plus decay, the next element required for the proton-proton chain to continue.
These three rare events that need to occur: proton-proton bonding, weak nuclear force proton to neutron switching, and diproton to deuterium decay; make this first phase very rare, and therefore very slow.
The emitted positrons (e+) usually annihilate immediately, through impacts with free electrons (e-), which releases two gamma rays, with a combined mass energy of 1.02 MeV.
Phase 2 the resulting deuterium (2H) is able to fuse with another proton (1H nuclei), producing a lighter isotope of Helium (3He), and releasing the excess energy as a Gamma Ray Photon (γ) (5.49 MeV). This reaction can happen very fast - perhaps every second, on average.
Phase 3 there are four possible paths to generate the Helium isotope (4He), however, the following, so called ‘P-P I branch’ is the most frequent (83.3%). Here, the Helium isotope (3He) can react with another Helium isotope (3He), which produces two protons (1H nuclei) and the heavier Helium isotope (4He), and releases 12.86 MeV of energy.
The resulting Helium 4 atom (4He) has 0.7% less mass than the four protons from which it was created, as the lost mass was expelled from the core during the fusion stages as energy (26.73 MeV net), in the form of Neutrinos, and high energy Gamma Photons (γ).
This released energy will interact with electrons and protons in the core, with some being transmitted out, however, that which is retained will increase kinetic energy of fusion products and other elements, and increase the temperature of the plasma, helping to prevent it from collapsing under its own weight.
Hydrostatic Equilibrium: A Stable Star
This energy expelled from the Core produces hydrostatic equilibrium, a corresponding outward force, that equally counters the Gravitational Inward Pressure, and suspends Gravitational Compression and collapse – making the Sun a stable star.
This hydrostatic equilibrium is self-regulating (see image to the right): when the rate of fusion is high, the core expands as the increased inner forces pushing outwards become greater than the gravitational pressure pushing inwards. This then reduces the relative density (mass to volume density) in the core, and therefore, decreases collision probability and therefore frequency, reducing the rate of fusion; and when the rate of fusion is low there is less heat produced, and a lower corresponding outward force, causing the core to cool and shrink, which increases density, and therefore begins to increase fusion again.
The released Neutrino particles continue almost directly, without resistance through the Suns' outer layers, as they have no electrical charge (although a very small mass), practically at the speed of light. The Positrons and Photons, on the other hand, do not make it very far as they enter into the Sun’s next zone: the Radiation Zone. The all important vibrational energy and/or gamma rays pass to the next stage.