The nucleus is a fundamental source of energy. Nuclear reactions are typically a million times more energetic than chemical reactions. Light produced by the Sun's nuclear reactions drives the Earth's weather and is converted by Earth's organisms to food and fuel.

The nucleus of an atom consists of protons and neutrons. A proton or neutron is made of quarks, held together by the strong interaction mediated by gluons. A proton consists of two up quarks and one down quark, whereas a neutron consists of a single up quark and two down quarks. In a nucleus that has protons and neutrons, the residual interactions between the quarks mediated by gluons is responsible for holding the protons and neutrons together.

Phases of Nuclear Matter
Like matter in its more familiar forms, the behavior of protons and neutrons varies depending upon their state. Water exists in three states or phases: solid, liquid and gas—known to us simply as ice, water, and steam. Temperature and pressure determine which of the phases water exhibits. Similarly, nuclear matter has different behavior depending on the temperature and density. Different regimes in which nuclear matter can find itself include neutron stars, the early universe, a nucleon gas, a quark-gluon plasma, and normal nuclear matter. Scientists study these phases by colliding accelerated particles to produce extreme conditions.

The Big Bang
Nuclear processes that occurred in the Big Bang and in stars produced all the elements on Earth. One microsecond after the Big Bang the universe was populated predominantly by quarks and gluons. As the universe expanded, the temperature dropped, eventually cooling enough to allow quarks and gluons to condense into nucleons, which subsequently formed hydrogen and helium. The universe continues to expand and cool; its present average temperature is 2.7 K.

Radioactivity
An atom is radioactive if its protons and neutrons can be converted or expelled to produce a system of lower energy. For nuclei with less than 20 protons, the number of neutrons required to maintain a stable balance is roughly equal to the number of protons. For large numbers of protons in the nucleus, the repulsive electric force between protons lead to nuclear energy states that favor neutrons over protons. A radioactive atom seeks a more stable arrangement of protons and neutrons through radioactive decay. Radioactive decay occurs randomly, but large collections of radioactive materials have predictable average lifetimes. The common radiation modes are named after the first three letters of the Greek alphabet—alpha, beta, and gamma. In an alpha decay, a helium nucleus (2 protons + 2 neutrons) escapes from a nucleus, with alpha emission reducing by two both the number of protons and the number of neutrons in the nucleus. Beta decay can proceed either by emission of an electron and an antineutrino or by emission of their antiparticles, a positron and a neutrino. Beta decay changes the number of protons and the number of neutrons in the nucleus by converting one into the other. Inverse beta decay involves the capture of an electron by a nucleus and the emission of a neutrino. In a gamma decay, a high energy photon is emitted from the nucleus to attain a lower energy configuration.

Fission
Fission occurs when a nucleus splits in two. This can take place spontaneously or can be induced when a nucleus captures a neutron. For example, an excited state of uranium (created by neutron capture) can split into two smaller nuclei. With a large number of uranium atoms in close proximity, it is possible for the neutrons resulting from the first fission event to be captured and to cause other uranium atoms to fission. During this process, the number of uranium atoms that fission increases exponentially. Each fissioning uranium releases energy, so it is possible to extract considerable energy. In a nuclear bomb, the fission process is extremely rapid. In a nuclear reactor, fission is done in a controlled manner where control rods absorb excess neutrons, preventing these neutrons from causing another uranium fission. Nuclear explosions cannot occur in conventional reactors because the density of fissionable material is too low. In addition, heating the fuel results in an increased separation of fissionable materials as temperature increases. This process automatically lowers the reaction rate.

Fusion
Fusion occurs when two nuclei combine to form a new nucleus. Fusion of low mass nuclei can release a considerable amount of energy. In the Sun, four hydrogen nuclei (protons) combine through several multistep processes to form a helium nucleus. Since the energy required to overcome the repulsion of the two nuclei is enormous, fusion occurs only under extreme conditions, such as those found in the cores of stars. To fuse higher mass nuclei requires even more extreme conditions, such as those generated in supernovae. Stars are the source of all the elements more massive than lithium.