The atomic structure[2]: the birth of quantum mechanics

By the end of the 19th century, Maxwell's theory of electromagnetism wasn't able to explain several phenomena, such as:

1. Black body radiation;
2. Photoelettric effect;
3. Compton scattering;
4. Emission and adsorption spectrum of atoms.

Those phenomena were interpreted using quantum mechanics, that was based on the idea that light could behave in a particle way, and not only as a wave. Max Plank was the first one to introduce the idea of quantization, that allowed to explain black  body radiation.

If we take in account any metal that melts at high temperatures, we'll see that he emits radiation in a continuous spectrum, that varies with temperature. Black body is a theoretical object that behaves in a similar way: it adsorbe all the incoming radiation, and its emission depends only on temperature. Its spectrum will present a maximal emission at a certain wavelength, that will vary according to Wien's Law.

In 1899 Max Plank succeeded in interpretating the experimental results about black body radiation. He assumed that energy could not vary in a continuous way, but that it was quantized.

In 1905 use this new idea to solve the paradox of Photoelectric effect. This effect was based on the fact that electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. On experiments it was observed that electron emission took place only if the incident radiation had a bigger frequency than a certain minimum one (typical of the irradiated body). The emitted electrons had a certain kinetic energy, that varied from zero, to a maximum value, dependent from the frequency of the radiation. The intensity of emitted electrons was proportional to incident radiation's intensity, meanwhile their speed (and so their kinetic energy) was independent from it.

According to classical physics, electrons of superficial layers could be stimulated  by incident radiation, but the speed of emitted electrons should vary proportionally with the intensity of incident radiation. This was in contrast with experimental results, so a new conception of electromagnetic waves was needed.

Einstein proposed that electromagnetic waves, while interacting with matter, had a corpuscular behavior,  as if it was composed by quanta of light, photons. While intensity of radiation increases, photon's energy remains the same, and gains the number of photons per unity of surface. A photon can indeed give his energy to surface electron: if it is bigger than the minimum required, the electron will be emitted, and will assume a kinetic energy equal to the difference of photon's kinetic energy and the minimum energy required.

This explains why to an increasing intensity of incident radiation, corresponds a gaining of emitted electrons, while their kinetic energy is not  changed. This interpretation led to the introduction of wave particle duality of light.

Thermonuclear fusion: deep inside the heart of stars [2]

The process of Thermonuclear fusion requires high temperatures and high pressure. Those conditions until now  have been found only inside the hearth of stars. Here the elements are under the physical form of a ionized gas. It means that the  different particle have a certain electric charge, and so before nuclear interaction could begin to work, coulombian repulsion has to be defeated.

Considering the simplest situation, with two hydrogen atoms, the energy required to exceed the coulombian barrier is 1000 KeV. Inside a star the temperature is usually around 10^7 K. Heat energy of mono-atomic gas can be calculated as follow:

According to this data, nuclear reaction should be impossible at those condition. However three other factors can combine to bring a certain probability of success for those reactions:

• Particles are characterized by a Maxwell speed distribution. It means that a certain amount of particles have a energy greater than the medium one, and a certain amount can reach the level required;
• According to quantum mechanics there's a little probability that a particle with low energy could  exceed the coulombian barrier, by Quantum Tunnelling;
• Stars are made by a large amount of particles. Even though the medium level isn't enough to exceed the barrier, a great amount of particles could have enough energy.

Nevertheless most of the energy produced by a star is related to the Proton-Proton chain, that occurs at 3x10^7 K. The reaction is basically the transformation of 4 proton in a nucleus of Helium, according to the following process:

The total energy produced by this process is 26 MeV per cicle, even though 0,26 MeV are emitted under  the form of particles known as Neutrino.

This cycle is prevalent in star only through the first period of their life. When temperature gets closer to 10 million degrees another chain takes place, and it is known as CNO chain (Carbon Nitrogen Oxigen).

This chain consists, like the previous one, in the transformation of 4 protons in an atom of Helium. In total the energy production is similar to the previous one, 25 MeV. So what are the factors that mark the difference between one chain and the other? In the CNO chain heavier atoms are used as catalyst. Because of that higher energies are require to exceed the coulombian barrier, and so also higher temperatures.

Anyhow the process that is fundamental for  the production of energy by thermonuclear fusion on earth is the p-p chain. The CNO chain in fact requires higher temperatures, too high for our actual technologies. Consequently the following article will always refer to this cycle.