Astrophysics (Index)About


(nuclear fusion)
(formation of an atomic nucleus nuclei from multiple smaller nuclei)

Fusion (specifically, nuclear fusion) is a type of nuclear reaction, specifically, the formation of an atomic nucleus from smaller nuclei. Some fusion reactions release energy in the form of electromagnetic radiation and/or particles such as neutrinos, while others require energy to take place. Fusion (the former kind, releasing energy) is the primary source of energy in the long life of stars and is a factor in explosions such as supernovae. In both cases, it constitutes nucleosynthesis of the elements, e.g., that planets are ultimately made of.

The opposite nuclear reaction, division of a nucleus into multiple nuclei, is nuclear fission (or just fission). In both cases, certain conditions make the event more likely: for fusion, in addition to the particular particles that are input to the particular reaction, the particles must have enough kinetic energy in the form of heat. The requirements for fission are similar, but the kinetic energy is needed in an extra incoming particle (e.g., neutron) to trigger the fission event.

In both cases, a new nucleus (or nuclei) results, of a different element (or elements), i.e., with a different atomic number (number of protons), and a different mass number (number of nucleons, i.e., protons and neutrons). The true atomic mass is approximately the mass number, but is actually a touch larger or smaller because of the binding energy, i.e., the energy that holds this particular nucleus configuration together, which varies with the combination of atomic number and mass number. It is the difference in required binding energy for the initial nuclei and the final nuclei, when it results in an excess of energy, that produces fusion or fission power. A new configuration that requires more binding energy than the initial nuclei draws kinetic energy from the collision (and requires that much kinetic energy and thus a sufficient temperature).

In astrophysics, fusion is often informally referred to as burning, i.e., a star "burning hydrogen" or hydrogen burning to mean hydrogen nuclei fusing to produce something else. The word burning is being used metaphorically: hydrogen fusion is something like the process of chemically combining with oxygen to produce a different compound, but this burning is at the nuclear level: a nuclear process that produces a different nucleus and element.

Further reading:

Referenced by pages:
asymptotic giant branch (AGB)
alpha capture
binding energy
black hole (BH)
brown dwarf (BD)
carbon star (C)
CNO cycle
compact object (CO)
Compton scattering
cross section
cataclysmic variable star (CV)
deuterium burning
electron screening
endothermic reaction
Gamow peak
giant star
globular cluster (GC)
gravitational collapse
horizontal branch (HB)
hydrogen burning
helium (He)
helium flash
helium runaway
inflated radii
iron (Fe)
isothermal core
kappa mechanism (κ-mechanism)
Kelvin-Helmholtz mechanism
lithium (Li)
main sequence star (MS)
mean free path
Milky Way
neutrons from carbon-13
nitrogen (N)
nuclear energy generation rate (ε)
oxygen (O)
PG 1159 star
planetary nebula (PN)
post-main-sequence star
proton-proton chain
quantum number
quantum tunneling
random walk
red clump (RC)
red-giant branch (RGB)
Schönberg-Chandrasekhar limit
silicon (Si)
supernova (SN)
sodium (Na)
stellar core
stellar evolution
stellar structure
strong force
sulfur (S)
thermal pulse
thermal runaway
timescale (t)
triple alpha process
T-Tauri star (TTS)
Type Ia supernova
white dwarf (WD)
X-ray burster (XRB)
zero-age main sequence (ZAMS)