An Etymological Dictionary of Astronomy and Astrophysics
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A nuclear reaction in which two or more atomic nuclei fuse into a single heavier nucleus by a collision of the interacting particles at extremely high temperatures. Chains of thermonuclear reactions, such as the → proton-proton chain and the → CNO cycle, account for the energy radiated from the Sun and more massive stars.

→ thermo- + → nuclear; → reaction.

The minimum energy, for an incident particle or photon, below which a particular reaction does not occur.

→ threshold; → reaction.

Same as → common fraction.

M.E., from L. vulgaris, from vulgus "the common people," + -aris, → -ar.

→ common; → fraction.

One of the fundamental forces of nature that accounts for some particle interaction, such as → beta decay (→ radioactivity), the decay of free → neutrons, → neutrino interactions, and so forth. It is short-ranged, dominating at distances of 10^-16 cm and occurs at a rate slower than that of the → strong interaction by a factor of about 10^-13, hence its name. Although the weak interaction also includes interactions in which no neutrinos are emitted, neutrino emission accompanies all weak interactions of interest to astrophysics. Weak interaction plays an important role in the evolution of the stars from birth to death. For example, the → proton-proton reaction is a weak interaction. Also called → weak force or → weak nuclear force.

→ weak; → interaction.

Same as → weight concentration.

→ weight; → fraction.

Same as → weight concentration.

→ weight; → fraction; → concentration.

The diffraction of X-rays by the atoms or ions of a crystal. The wavelength of X-rays are comparable to the size of interatomic spacings in solids. Since the atoms in a crystal are arranged in a set of regular planes, crystals serve as three-dimensional diffraction gratings for X-rays. Planes of repetition within the atomic structure of the mineral diffract the X-rays. The pattern of diffraction thus obtained is therefore used to identify minerals by bombarding them with X-rays.

→ X-ray; → diffraction.