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

The time required for a photon created in the Sun's core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks before arriving at the surface by chance. The typical time is approximately 5 x 10⁴ years for a constant density Sun.

→ photon; → escape; → time.

→ Electromagnetic radiation in equilibrium in a → black body cavity. Photons can be treated as the simplest → ideal gas because all the particles move at the same velocity, the → speed of light. There are, nevertheless, two main differences. 1) Photons are → bosons and → Bose-Einstein statistics must be used. However, photons do not interact with each others so that no approximation is made by neglecting inter-particle forces. 2) Some photons scatter off the walls, with some being absorbed and new ones being emitted continually; so that no constraint can be placed on their number.

→ photon; → gas.

An effect occurring in the outer zones of → H II regions where the number of high-energy ultraviolet photons with energies well above the → ionization potential of hydrogen increases with respect to the number of → Lyman continuum photons. The effect is due to stronger absorption of weaker photons.

→ hard; → photon.

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.

1) The visible surface of the Sun (temperature 5700 K), just below the → chromosphere and just above the → convective zone. The solar photosphere is a thin layer of roughly 300 km wide. Its temperature decreases uniformly with height, from about 6,600 K (pressure 0.868 → millibars) at its bottom, to about 4,400 K (pressure 125 mb), where it merges with the chromosphere. The photosphere has a "rice-grain" appearance, called → granulation, caused by rising (hot) and falling (cool) material in the → convective cells just below the photosphere. Other main features of the photosphere are → sunspots, → faculae, and → supergranulation.
2) The region of a star which gives rise to the continuum radiation emitted by the star.

→ photo- + → sphere.

Of or pertaining to a → photosphere.

→ photosphere; → -ic.

The process in green plants, algae, diatoms, and certain forms of bacteria by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct.

→ photo- + → synthesis.

Magnitude defined for the combination of a photographic plate and a yellow filter, approximating the spectral sensitivity of the eye.

→ photo- + → visual; → magnitude.

A detector usually constituted by a p-n junction. Upon irradiation, the electron-hole pairs which are created, are immediately separated by the strong electric field across the junction, and a current is generated, which is proportional to the number of incident photons per second.

→ photo- + → voltaic; → detector.

A sequence of two or more words arranged in a grammatical construction and acting as a unit in a → sentence.

From L.L. phrasis "diction," from Gk. phrasis "speech, way of speaking, enunciation," from phrazein "to express, tell," from phrazesthai "to consider."

Vatpâr, literally "part of speech," from vat-, "to speak, say;" cf. (Kurd.) wittin "to speak, say," → letter, + pâr "piece, part, portion," → partial.

Pertaining to the physical sciences, especially physics.

→ physics + → -al.

Same as → physisorption.

→ physical; → adsorption.

The branch of chemistry dealing with the relations between the physical properties of substances and their chemical composition and transformations.

→ physical; → chemistry.

The state of a → physical system regarding its temperature, density, pressure, etc. at a given time.

→ physical; → condition.

A fundamental → physical quantity that is generally believed to be both universal in nature and constant in time.

→ physical; → constant.

Any of basic physical quantities, such as mass, length, time, electric charge, and temperature in terms of which all other kinds of quantity can be expressed.

→ physical; → dimension.