An Etymological Dictionary of Astronomy and Astrophysics
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The antiparticle of a proton, identical in mass and spin but of opposite (negative) charge.

From → anti- + → proton.

An → isotope of → helium that consists of two → protons, without any → neutrons. It is extremely → unstable.

→ di-; → proton.

A particle of the hadron family which is one of the two particles that make up atomic nuclei. It has an electric charge of one positive fundamental unit, a diameter of about 1.65 x 10^-13 cm, and a mass of about 1.67 x 10^-24 g (about 938 MeV c^-2).

From Gk. proton, neuter of protos "first." Coined by Eng. physicist Ernest Rutherford (1871-1937).

The temperature in the → solar wind, as derived from the mean kinetic energy of protons: mv²/2 = (3/2)kT_p, where k is → Boltzmann's constant. There are two types of proton temperature: parallel temperature, measured from protons moving parallel to the magnetic field, and perpendicular temperature relating to protons at right angles to the magnetic field. The proton temperature is usually derived using particle detectors on board space probes that determine the velocity → distribution function of the particles from their energies (N. Meyer-Vernet, 2007, Basics of the Solar Wind, Cambridge Univ. Press). See also → electron temperature.

→ proton; → temperature.

A series of → thermonuclear reactions, taking place mainly in → low-mass stars, such as the Sun, which transforms four hydrogen nuclei (protons) into one helium (⁴He) nucleus and thereby generates energy in the stellar core. First, two protons (¹H) combine to form a → deuterium nucleus (²H) with the emission of a → positron (e⁺) and a → neutrino (ν): ¹H + ¹H → ²H + e⁺ + ν. The deuterium nucleus then rapidly captures another proton to form a helium-3 nucleus (³He), while emitting a → gamma ray (γ): ²H + ¹H → ³He + γ. There are three alternatives for the next step. In the PP I chain, occurring in 86% of the cases, two ³He nuclei fuse to a final ⁴He nucleus while two protons are released: ³He + ³He → ⁴He + ¹H + ¹H. The mass of the resulting ⁴He nucleus is less than the total mass of the four original protons used to produce ⁴He (→ mass defect). The difference, ~ 0.7% of the total mass of the protons, is converted into energy and radiated by the Sun. In this process, the Sun loses some 4 million tons of its mass each second. See also → CNO cycle.

→ proton; → chain.

A → thermonuclear reaction in which two protons collide at very high velocities and combine to form a → deuterium. See also → proton-proton chain.

→ proton; → reaction.

A compact, hot, and → neutrino-rich object that results from a → supernova explosion and is a transition between an → iron core and a → neutron star or → black hole. The life span of a protoneutron star is less than one minute.

→ proto-; → neutron; → star.