An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

An intrinsic noise caused by the quantum nature of light. Same as → quantum noise.

→ photon; → noise.

A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently → compact objects where the → curvature of → space-time is very important. In other words, a body can take a stable orbit around a → black hole provided that it moves with the → speed of light. However, only photons can have such a velocity; hence the term "photon sphere." For a non-rotating → Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c² = 3 R_S/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and R_S is the → Schwarzschild radius. For a rotating, → Kerr black hole, the situation is much more complex due to the → Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction.

→ photon; → sphere.

The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: L_wind = Mdot (v_∞²/2 + GM/R) = Mdot (v_∞²/2 + v_esc²/2). For L_wind = L_*, the mass loss rate is Mdot_max = 2L_*/(v_∞² + v_esc²). Following Owoki & Gayly (1997), Mdot_tir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v_∞ tending toward zero. Mdot_tir is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10^-4 Msun yr^-1 in even the most luminous stars.

→ photon; tiring, from tire "to weary; become weary," → tired; → limit.

The plasma filling space before the → recombination epoch that mainly consisted of → cosmic microwave background radiation photons, electrons, protons, and → light elements.

→ photon; → baryon; → plasma.

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems.

→ photon + → -ics.

A disk or cylindrical part tightly fitting and moving within a cylinder, either to compress or move a fluid collected in the cylinder, as air or water, or to transform energy imparted by a fluid entering or expanding inside the cylinder, as compressed air, explosive gases, or steam, into a rectilinear motion usually transformed into rotary motion by means of a connecting rod (Dictionary.com).

From Fr. piston, from M.Fr. piston "large pestle," from O.It. pistone "a piston," from pestare "to pound," from L.L. pistare, from pistare "to pound."

The theory supported by a wide range of evidence in which the Earth's crust is composed of several large, thin, relatively rigid plates that move relative to one another. The interaction of the plates at their boundaries causes seismic and tectonic activity along these boundaries. See also → continental drift.

→ plate; → tectonics.

The time required for a complete revolution of the Earth's pole on the celestial sphere as the result of → precession. A Platonic year is equal to 25 800 years.

Of or pertaining to Gk. philosopher Plato, from Gk. Platon "broad-shouldered," from platys "broad." → year.

A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction: ₉₂U²³⁸ + ₀n¹→ ₉₃Np²³⁹ + β^- (23.5 minutes) → ₉₄Pu²³⁹ + β^- (2.36 days). The → half-life of ₉₄Pu²³⁹ is 2.44 × 10⁴ yr. Plutonium-239 is a → fissile isotope.

The name derives from the planet → Pluto. It was selected because it is the next planet in the solar system beyond the planet → Neptune and the element plutonium is the next element in the → periodic table beyond → neptunium.

→ post-Newtonian formalism.

→ post-; → Newtonian; → expansion.

An approximate version of → general relativity that applies when the → gravitational field is → weak, and the matter → velocity is → small. Post-Newtonian formalism successfully describes the gravitational field of the solar system. It can also be applied to situations involving compact bodies with strong internal gravity, provided that the mutual gravity between bodies is weak. It also provides a foundation to calculate the → gravitational waves emitted by → compact binary star systems, as well as their orbital evolution under radiative losses. The formalism proceeds from the Newtonian description and then, step by step, adds correction terms that take into account the effects of general relativity. The correction terms are ordered in a systematic way (from the largest effects to the smallest ones), and the progression of ever smaller corrections is called the → post-Newtonian expansion.

→ post-; → Newtonian; → formalism.

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.

The → Eddington limit of luminosity for a → rotating star in which both the effects of → radiative acceleration and rotation are important. Such objects mainly include → OB stars, → LBV, → supergiants, and → Wolf-Rayet stars. It turns out that the maximum permitted luminosity of a star is reduced by rotation, with respect to the usual Eddington limit (Maeder & Meynet, 2000, A&A, 361, 159).

→ rotational; → Eddington limit.

Variously colored → sedimentary rock composed mainly of sand-like quartz grains cemented by calcite, clay, or iron oxide. The sand accumulated originally underwater in shallow seas or lakes, or on the ground along shorelines or in desert regions.

→ sand; → stone.

In → supersymmetry theories, a hypothetical → boson super-partner of a → lepton. See also → squark

s from → supersymmetry; → lepton.

Math., Physics: A solution of a certain type of partial differential equation that represents a solitary wave. A soliton is a self-reinforcing wave that maintains its shape while it travels at constant speed. Solitons are caused by a cancellation of nonlinear and dispersive effects in the medium.

From solit(ary) + → -on.