Nucleosynthesis(p-p cycle)
The temperature inside the Sun is about 16 million degrees, and is enough for protons to become so close as to overcome their mutual electric repulsion and fuse. We recall that the simplest and most abundant atom in the Universe, hydrogen, has an electron (negatively charged particle) moving around a nucleus composed of only a positively charged particle, the proton. Heavier atoms, such as Helium (He), which has 2 electrons and 2 protons, have also neutrons in the nucleus.( Neutrons are chargeless particles with mass approximately the mass of the proton).Four hydrogen atoms (each has a nucleus composed of only a proton with mass 938.26 MeV) give rise to a He atom (which has a nucleus with two protons and two neutrons with a mass of 3,726 MeV) with a gain of energy E=mc2, where m is the mass difference between the 4 H atoms and the atom of He.
Note: mass here is given in terms of energy! The energy is calculated using E = m c2(Einstein's relation), with m (mass of proton) = 1.67 10-27 Kg, c (speed of light) = 3 108 m/sec; the result, in Joules, is then converted in Mev - millions of electronvolts, using 1 Mev = 1.6 10-17 Joules. Electrons have about 1/2000 the mass of a proton.
This energy then moves outside the core through a "radiation zone" and then through a "convection zone", where transport of heat is like the transport of air by currents. These reactions take place inside the Sun where the temperature is about 16,000,000 K. Going from the center of the Sun to the its surface , the temperature decreases to about 5,700 K. Thus, the energy that makes stars such as the Sun shine comes from the net energy gain in a fusion process
The figure shows the typical process of helium formation. Photons emitted in the process are not shown.
The reaction follows these steps:
1) Two protons combine together to form a deuterium nucleus (D in the figure). This nucleus is composed by a proton and a neutron and in the reaction a positive electron (e+) and an antineutrino (v) are released.
2) One proton is added to the nucleus and 3He get formed. 3He is composed by two protons and one neutron.
3) Two 3He atoms formed in different regions get together to form 4He. In this process two protons are ejected out.
Neutrinos are particles that interact very weakly with matter; they are chargeless and have little or no mass. A huge number of neutrinos is released by the nuclear reactions inside the Sun and they can travel through its material without being absorbed. The number of neutrinos ejected by the Sun can give us an estimate of which kind of reactions that are taking place in its center.
For more details about neutrinos click here.
Note: there is a minimum and maximum size of a star. A mass below which no ignition occurs is of the order of 0.08 times the mass of the Sun. In this case the gravitational force cannot compress and heat particles at the ignition temperature, about 10 million degrees. On the other side, a star 80-100 times the mass of the Sun becomes unstable and eventually falls apart quickly. Stars more massive than the Sun fuse also other elements besides hydrogen.