The thermodynamics conditions in the inner parts of the stars (i.e central temperature, density, pressure) are so high, that hydrogen H (the main component of the universe (90%) and so of the original gas from which the star was created, c.f Mendeleev's table) starts thermonuclear burning and creates helium He4 in order to compensate the gravitationnal collapse and maintain the star in equilibrium.
For the Sun, this part extents over 20% of its radius, and over 40% of its total mass.
The main fusion reactions are : The PP chain, and the CNO cycle. We can resume these nuclear reactions by: 4 H -> He4 + 2 e- + 2 nue + Qeff , where e- represent electrons (elementary negative charged particle), nue electronic neutrinos (see section below), and Qeff is the energy release per produced He4 (or alpha particle) and is equal to : 26.732 Mev (minus the energy lost with neutrinos).
Actually, these fusion reactions maintains the stars in a stable equilibrium during a thermonuclear time scale (i.e 10 Giga-years) for a typical star like our Sun, until all the hydrogen is consumed, and exists only in the star envellope, then the stars have to burn all the Helium with a new cycle called 3 alpha (i.e you need 3 He4 to create one carbon atom C12), this stage is shorter and happens during the last 100 millions years of a solar-like star. Then you obtain a dense core with a lot of carbon, nitrogen and oxygen, but to maintain the equilibrium against the gravity, the star needs a temperature higher than 1e9 K, and only massive stars have enough gravitationnal power at this stage of their life to burn carbon, stars like our Sun stops at this stage and becomes white dwarfs (planetar nebulae).
About heavier stars (typically 6 solar masses and above), their life are shorter, and their mass allow them to initiate after the 3 alpha cycle, other nuclear reactions with heavier atoms until iron Fe56, to become neutron stars or black holes.
To conclude, download this nice short movie about fission , but that inside the stars it's not the process occuring (i.e light atoms (H, He...) are transformed to heavier ones with atomic number smaller than 26 (Fe), only explosive phenomena (for example: supernovae) could transformed light elements above Fe, this movie starts with uranium (atomic number 92)!).
In this model, 3 of the 4 fundamental interactions, namely, electromagnetic, weak and strong nuclear forces (the latest being the gravitation) describe via field theory, the behaviour of the matter in a microscopic description. The neutrinos are lepton and there is one neutrino for each familly (i.e electrons, muons and taus), but inside the stars only electronic neutrinos are produced. Neutrinos are weak interacting particles because only the weak and the gravitation forces act on them! There are zero-mass particles (but this is not sure anymore), and have no electric charge and magnetic moment (the latter being also under review).
Between solar modeling and neutrinos experiments there is a discrepancy of 30% (i.e models predict more than we observe), and this could have two explanations. First, we (astrophysists) can make errors in evaluating the central temperature, and then the neutrinos fluxes. The other solutions could come from particles physicists and a modification of the Standard Model, with massive neutrinos and them the possibility that some electronic neutrinos become muon or tau neutrinos, which solar neutrino experiments such as Gallex or SuperKamiokande don t observe (exception of the canadian Sudbury experiment which is now operating). See for example this web site.
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