Nuclear Fusion in Stars
We have found equations for how gravitational energy change much be matched by a radiative energy exchange in a star. But how the radiation is internally created within the star is a subject yet to be covered. This energy is produced by what is known as nuclear fusion. In fusion, two atomic nuclei combine to form new elements and in the process create subatomic particles (neutrons or protons) and in the process release energy through the change in mass of the resulting atoms.
Depending on the mass and age of a star, there are different types of atomic fusions that are common. Most common is the proton-proton, or hydrogen-hydrogen fusion, in which 4 hydrogen particles come together to form 2 helium particles with the help of free electrons. Given that hydrogen is the most basic atom unit, all other elements were originally formed through some form of hydrogen fusion. Today we know that most stars, including our sun, have intense luminosities fueled by hydrogen fusion.
Other common types of fusion include using helium or even carbon isotopes to change mass of interior atomic units. Elements up until iron can be used for fusion as iron sits at the peak of the energy curve that determines whether it takes more energy to produce fusion than is produced from the reaction. The curve below shows the energy required for fusion. It is noted that some heavier elements can still produce energy through fission, a completely different energy process.
The constant fusion of atoms inside of stars requires energy to constantly escape the system and while there are different convective and radiative processes to get rid of the massive quantities of energy, each process is ultimately responsible for the light we see coming off of the star, also known as its luminosity.
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