The strong interaction is responsible for 99 percent of the visible mass in the universe. Most of the mass of a proton or neutron does not come from the quarks themselves, which contribute only about 1 percent of the total weight. Instead, the vast majority of that mass arises from the energy of the strong interaction binding those quarks together.
Murray Gell-Mann and George Zweig independently proposed that protons and neutrons were composed of smaller entities called quarks in 1964. Gell-Mann chose the name quark from a line in James Joyce's Finnegans Wake, while Zweig referred to them as aces. These quarks carry a property called color charge, which has no relation to visible light but comes in three types: red, green, and blue.
Quarks are never found in isolation due to a property known as color confinement. If one attempts to pull two quarks apart, the energy required to separate them increases until it becomes sufficient to create a new quark-antiquark pair. This process results in the formation of new hadrons rather than the release of free quarks.
The residual strong force, also known as the nuclear force, is mediated by mesons, specifically virtual pions and rho mesons. This force is much weaker than the fundamental force because it is mostly neutralized within the nucleons. It diminishes rapidly with distance, following a negative exponential power known as the Yukawa potential.
The strong interaction drives the sun and other stars through nuclear fusion. In the core of the sun, protons are forced together under immense pressure and temperature, allowing the residual strong force to overcome electromagnetic repulsion. When the nuclei fuse, the resulting mass defect is converted into energy, releasing gamma rays and sustaining the star's luminosity.
Asymptotic freedom is a property wherein the strong interaction diminishes at higher energies or shorter distances. This means that quarks behave almost as free particles when they are very close to each other, but become tightly bound as they move apart. The strength of the interaction is parameterized by the strong coupling constant, which changes depending on the energy scale.