All stars derive their energy through the thermonuclear fusion of
light elements in to heavy elements. High temperatures are required so that the mutual electrostatic repulsion of the protons
in each fusing atomic nucleus is overcome. This requires a high velocity collision that can only be achieved under conditions
of high temperature. The minimum temperature require for the fusion of Hydgrogen is 5 million degrees. Elements with more
protons in their nuclei require higher temperatures, since the higher nuclear masses must be raised to higher temperatures
to attain the particle velocity needed for this fusion to occur. For instance, to fuse carbon requires a temperature of about
1 billion degrees. Most of the heavy elements, from oxygen up through iron, are thought to be produced
in stars that contain at least ten times as much matter as our Sun.
Our Sun is currently burning, or fusing, hydrogen to helium. This is the process that occurs during most of a star's lifetime.
After the hydrogen in the star's core is exhausted, the star can burn helium to form progressively heavier elements, carbon
and oxygen and so on, until iron and nickel are formed. Up to this point the process releases energy. The formation of elements
heavier than iron and nickel requires the input of energy. Supernova explosions result when the cores of
massive stars have exhausted their fuel supplies and burned everything into iron and nickel. The nuclei with mass heavier
than nickel are thought to be formed during these explosions.
Hydrogen To Helium via the Proton-Proton Chain:
Step 1. 2 protons collide at high velocity, overcoming
electrostatic repulsion and fusing into a deuterium nucleus held together by the strong nuclear force while simultaneously
releasing a positron and antineutrino.
Step 2. One deuterium nucleus fuses with one additional
proton to form Helium-3 nucleus. A photon is also generated with this fusion step that forms the main electromagnetic energy
resulting from stellar processes. This is the light you see from the sun.
Step 3. Two Helium-3 nuclei fuse to form one Helium-4
nucleus, the normal isotope of helium. Two remaining protons left over can reinitiate the chain again, starting at step
1. This chain recycles over and over with the net result of converting hydrogen nuclei (protons) into Helium-4
Hydrogen to Helium via the CNO cycle:
Step 1. One proton fuses with a Carbon-12 nucleus
to create a Nitrogen-13 nucleus (seven protons plus six neutrons) plus releasing a single gamma ray photon.
Step 2. Unstable Nitrogen-13 rapidly decays into
Carbon-13 (six protons and seven neutrons). Also created are a positron and a neutrino.
Step 3. One proton fuses with the Carbon-13
nucleus to form a Nitrogen-14 nucleus and a gamma ray photon. Nitrogen-14, the common isotope of nitrogen has 7 protons and
Step 4. Nitrogen-14 fuses with one proton to form Oxygen-15
nucleus plus another gamma ray photon.
Step 5. Unstable Oxygen-15 rapidly decays into a Nitrogen-15
nucleus plus a neutrino and a positron.
Step 6. One proton fuses with Nitrogen-15
nucleus to form Carbon-12 plus Helium-4.
Helium to Carbon:
Three Helium-4 fuses to Carbon-12 in the core of a red giant
type star after most of the hydrogen has been used up during repeated cooling and reheating cycles. Two photons
are given off in this process.
These are the reactions that lead from star formation to the end of
the normal stellar life cycle. When a star finally runs out of fuel, "out of gas" so to speak, it cools down and contracts.