The fusion of hydrogen to helium by the proton-proton chain or CNO cycle requires temperatures in the order of 10 million degrees Celsius or Kelvin. Only at those temperatures will there be enough hydrogen ions in the plasma with high enough velocities to tunnel through the Coulomb barrier at sufficient rates. There are no stable isotopes of any element with atomic masses 5 or 8; beryllium-8 (4 protons and 4 neutrons) is highly unstable and short-lived. Only at extremely high temperatures of around 100 million K can the sequence called the triple-alpha process take place. It is so called because the net effect is to combine 3 alpha particles, which means standard helium-4 nuclei of two protons and two neutrons, to form a carbon-12 nucleus (6 protons and 6 neutrons). In main sequence stars, the central temperatures are too low for this process to take place, but not in stars in the red giant phase.- - - - - - - - -
The process of converting lower-mass chemical elements into higher-mass ones is called nucleosynthesis. One or more stars can be formed from a large cloud of gas and dust. As it slowly contracts due to gravity, the condensation releases energy which in turn heats up the central region of the cloud. The protostar continues to contract until the core temperature reaches about 10 million K, which constitutes the minimum temperature required for normal hydrogen-to-helium fusion to begin. A main sequence star is then born. When a star exhausts its hydrogen supply the pressure in its core falls and it begins to shrink, releasing energy and heating up further. The next step is core helium-to-carbon fusion, the triple-alpha process, which requires a central temperature of about 100 million K. Helium fusion also produces nuclei of oxygen 16 (8 protons and 8 neutrons) and neon 20 (10 protons and 10 neutrons).
At core temperatures of 600 million K, carbon 12 can fuse to form sodium 23 (11 protons, 12 neutrons) and magnesium 24 (12 protons, 12 neutrons), but not all stars can reach such temperatures. Stars with higher masses fuse more elements than stars with lower masses. High-mass stars have more than 8-9 solar masses; intermediate-mass ones 0.5 to 8 solar masses and low-mass stars 0.1 to 0.5 solar mass. After exhausting its central supply of hydrogen and helium, the core of a high-mass star undergoes a sequence of other thermonuclear reactions at increasingly faster pace, reaching higher and higher temperatures.
When helium fusion ends in the core of a star with more than 8 solar masses, gravitational compression collapses the carbon-oxygen core and drives up the temperature to above 600 million K. Helium fusion continues in a shell outside of the core, and this shell is itself surrounded by a hydrogen-fusing shell. At 1 billion K oxygen nuclei can fuse, producing silicon 28 (14 protons, 14 neutrons), phosphorus 31 (15 protons, 16 neutrons) and sulfur 32 (16 protons, 16 neutrons). Each stage goes faster and faster. At 2.7 billion K, silicon fusion begins. Every stage of fusion adds a new shell of matter outside the core, creating something resembling the layers of a massive onion. The outer layers are pushed further and further out.
Energy production in big stars can continue until the various fusion processes have reached nuclei of iron 56 (26 protons, 30 neutrons), which has one of the lowest existing masses per nucleon (nuclear particle, proton or neutron). The mass of an atomic nucleus is less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. Iron has the most tightly bound nuclei next to 62 Ni, an isotope of nickel with 28 protons and 34 neutrons, and consequently has no excess binding energy available to release through fusion processes.
No star, regardless of how hot it is, can generate energy by fusing elements heavier than iron; iron nuclei represent a very stable form of matter. Fusion of elements lighter than this or splitting of heavier ones leads to a slight loss in mass and a net release of nuclear binding energy. The latter principle, nuclear fission, is employed in nuclear fission weapons (“atom bombs”) by splitting large, massive atomic nuclei such as those of uranium or plutonium, while nuclear fusion of lighter nuclei takes place in hydrogen bombs and in the stars.
When a star much more massive than our Sun has exhausted its fuel supplies it collapses and releases enormous amounts of gravitational energy converted into heat. It then becomes a (Type II) supernova. When the outer layers are thrown back into interstellar space, the material can be incorporated into clouds of gas and dust (nebulae) that form new stars and planets. The remaining core of the exploded star will become a neutron star or a black hole, depending upon how massive it is. It is believed that the heavy elements we find on Earth, for instance gold with atomic number 79, are the result of ancient supernova explosions and were once a part of the Solar Nebula that formed our Solar System almost 4.6 billion years ago.
Read the rest at Vlad Tepes.
See: Part 1, Part 2.