An Etymological Dictionary of Astronomy and Astrophysics
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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.

A stage in the formation of a → planet, which comes about from the aggregation of → planetesimals. The protoplanet eventually becomes a planet by → accretion of material from a → protoplanetary disk.

→ proto- + → planet.

Of or relating to a → protoplanet or protoplanets.

→ protoplanet; → -ary.

A → circumstellar disk of gas and dust surrounding a → pre-main sequence star from which planetary systems form. Protoplanetary disks are remnants of → accretion disks which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10^-2 solar masses, lifetimes ~10⁶-10⁷ years, and accretion rates ~10^-7-10^-8 solar masses per year. According to the standard theory of planet formation, called core accretion, planets come into being by the growth of → dust grains which stick together and produce ever larger bodies, known as → planetesimals. The agglomeration of these planetesimals of 100 to 1000 km in size into rocky Earth-mass planets is the main outcome of this theory. Beyond the → snow line in the disk, if the masses of these cores of rock and ice grow higher than 10 times that of Earth in less than a few million years, gas can rapidly accrete and give rise to giant gaseous planets similar to → Jupiter. If core building goes on too slowly, the disk gas dissipates before the formation of → giant planets can start. Finally the left-over planetesimals that could not agglomerate into rocky planets or core of giant planets remain as a → debris disk around the central object that has become a → main sequence star. An alternative to core accretion theory is formation of planets in a massive protoplanetary disk by → gravitational instabilities. The validity of these two theories is presently debated. See also → protoplanet.

→ protoplanet; → disk.

→ preplanetary nebula.

→ proto-; → planetary; → nebula.

The fluid substance within the living cell that consists of two major divisions, the cytoplasm and the nucleoplasm (cell nucleus). It is composed mainly of nucleic acids, proteins, lipids, carbohydrates, and inorganic salts.

→ proto- + -plasm, → plasma.

Describing the conditions prior to the formation of the → solar system, or pertaining to the → protostar that became the → Sun.

→ proto- + → solar.

The abundance of a chemical element pertaining to the proto-→ solar nebula from which the → solar system was formed. → CI chondrite; → CAI meteorite.

→ protosolar; → abundance.

A stage in the process of → star formation, after the → gravitational collapse of the dense → pre-stellar core and before the initiation of → nuclear fusion in the central object which will eventually become a star. Protostars are classified into four groups: → Class 0, → Class I, → Class II, and → Class III.

→ proto- + → star.

Of or pertaining to → protostars.

→ proto-; → stellar.

A → gravitational collapse leading to the formation of a → protostar.

→ protostellar; → collapse.

A disk of gas and dust surrounding a → protostar. These structures are rotating → accretion disks through which matter is transferred to protostars.

→ protostellar; → disk.

A high-velocity and highly → collimated jet associated with the earliest phase of → star formation that propagating along the polar axis of the → protostar-→ accretion disk system. Protostellar jets are usually detected in the [S II], [O I], and Hα lines and are therefore referred to as optical jets. They may have more than a parsec in length. Their formation is related to the → magnetohydrodynamics (MHD) of accretion disks. These jets are detected in protostellar sources over a wide range of masses, from the very early stages of formation (sources associated with infalling envelopes whose mass exceeds that of the growing star) all the way to the → classical T Tauri stars, whose envelopes have already dispersed. This phenomenon is thought to play a key role in regulating the star formation process by removing the excess → angular momentum of disk material and enabling matter to flow toward the center. Protostellar winds also provide an important → feedback mechanism between the forming star and the surrounding medium, to which they return mass and energy. Protostellar jets are at the origin of → bipolar outflows. The et sweeps up ambient → molecular cloud material into two thin shells, which manifest themselves as the observed bipolar lobes of → carbon monoxide (CO) emission. Once the molecular cloud material has been swept away (on a timescale of 10⁵ years), the bipolar outflow disappears, leaving the protostellar jet to erratically fire away for a further 10⁶-10⁷ years.

→ protostellar; → jet.

The Sun at its protostellar formation stage, before becoming a main sequence star, nearly 5 billion years ago. The protosun was more luminous than today and larger, with a radius comparable to that of the orbits of the inner planets

→ proto- + → sun.

The original or model on which something is based or formed.
Something that serves to illustrate the typical qualities of a class.

→ proto-; → type.

Capable of being demonstrated or proved.

→ prove; → -able.

To supply proof of, to establish or demonstrate the truth or validity of.

→ proof.

A short popular saying, usually of unknown and ancient origin, that expresses effectively some commonplace truth or useful thought (Dictionary.com).

→ pro-; → verb.