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Subject to, or produced by → photoionization.
A process in which → absorption of photons at → ultraviolet (UV) / → optical wavelengths is followed by → electronic transitions associated with the emission of longer wavelength optical and → near-IR photons. Photoluminescence has two types: → phosphorescence and → luminescence. The excitation of the photoluminescence process under astrophysical conditions results from the absorption of a single UV/optical photon, leading to an electronic transition from a → ground state (1) to a higher state (2). State (2) typically is a bound, high-lying vibrational-rotational level of the first or second electronically excited state of a molecule or molecular ion, or a high state in the → conduction band of a semiconductor particle. The excited system relaxes through a series of → vibrational-rotational transitions until the electron finds itself in an intermediate state (3), from where an optical electronic transition back to the ground state (1) is possible. In a → polycyclic aromatic hydrocarbon (PAH) molecule, for example, state (3) can either be the lowest state in the → singlet or → triplet vibrational-rotational manifold of the first excited electronic level (Witt, A. N., Vijh, U. P., 2003, astro-ph/0309674).
An instrument for measuring the amount of light.
šidsanji, šidsanjik, nursanji, nursanjik
Pertaining to or related to → photometry.
bând-e šid-sanjik, ~ nur-sanjik
Fr.: bande photométrique
dorin-e šidsanjik, ~ nursanjik
Fr.: binaire photométrique
A binary star whose binarity is detectable from its variability and light-curve that has certain specific characteristics.
kabizeš-e šidsanjik, ~ nursanjik
Fr.: calibration photométrique
A calibration which converts the measured relative magnitudes into an absolute photometry.
Fr.: parallaxe photométrique
râžmân-e šidsanjik, ~ nursanjik
Fr.: système photométrique
A system of → magnitudes, each of them characterized by a set of
well-defined → passbands
(or → filters) with known
→ response curves. The system is defined by the values given for
the → standard stars.
In astronomy, the measurement of the light of astronomical objects, generally in the visible or infrared bands, in which a wavelength band is normally specified.
Electronic tube which converts photons into electrons, multiplies the electrons via a series of electrodes, and produces a measurable current from a very small input signal.
The → quantum of the → electromagnetic field, which mediates the interaction between charged particles. It is the mass-less → boson with zero → electric charge, which propagates with the → speed of light in vacuum. The energy of a photon is connected to its → frequency ν, through the formula E = hν, where h is → Planck's constant.
From phot-, variant of → photo- before a vowel + → -on a suffix used in the names of subatomic particles (gluon; meson; neutron), quanta (photon, graviton), and other minimal entities or components. The term photon was coined by Gilbert N. Lewis in 1926 in a letter to the editor of Nature magazine (Vol. 118, Part 2, December 18, page 874).
photon escape time
zamân-e goriz-e foton
Fr.: temps d'échappement des photons
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.
Fr.: durcissement des photons
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.
Fr.: bruit de photons
An intrinsic noise caused by the quantum nature of light. Same as → quantum noise.
Fr.: sphère de photons
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 RS/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and RS 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 tiring limit
hadd-e xastegi-ye foton
Fr.: limite par fatigue du photon
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: Lwind = Mdot (v∞2/2 + GM/R) = Mdot (v∞2/2 + vesc2/2). For Lwind = L*, the mass loss rate is Mdotmax = 2L*/(v∞2 + vesc2). Following Owoki & Gayly (1997), Mdottir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v∞ tending toward zero. Mdottir 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.
Fr.: plasma photon-baryon
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.
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.