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Optical depth in astrophysics refers to a specific level of transparency. Optical depth and actual depth, τ {\displaystyle \tau } and z {\displaystyle z} respectively, can vary widely depending on the absorptivity of the astrophysical environment.
Spectral optical depth or spectral optical thickness is the natural logarithm of the ratio of incident to transmitted spectral radiant power through a material. [1] Optical depth is dimensionless , and in particular is not a length, though it is a monotonically increasing function of optical path length , and approaches zero as the path length ...
A-type star In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
ACYG – (celestial object) Alpha CYGni, a class of rotating variable stars named after Alpha Cygni (Deneb), the archetype for the class; ADAF – (astrophysics terminology) Advection Dominated Accretion Flow, a mechanism by which matter is slowly accreted onto a black hole; ADC – (organization) Astronomical Data Center
A branch of physics that studies atoms as isolated systems of electrons and an atomic nucleus. Compare nuclear physics. atomic structure atomic weight (A) The sum total of protons (or electrons) and neutrons within an atom. audio frequency A periodic vibration whose frequency is in the band audible to the average human, the human hearing range.
The "Big Bang" scenario, with cosmic inflation and standard particle physics, is the only cosmological model consistent with the observed continuing expansion of space, the observed distribution of lighter elements in the universe (hydrogen, helium, and lithium), and the spatial texture of minute irregularities (anisotropies) in the CMB radiation.
For low optical depth corresponding to low , increasing the thickness of the medium leads to a linear increase of absorption and the equivalent line width grows linearly . Once the central Gaussian part of the profile saturates, τ ≈ 1 {\displaystyle \tau \approx 1} and the Gaussian tails will lead to a less effective growth of W ∝ ln N ...
In astrophysics, L is used for luminosity (energy per unit time, equivalent to power) and F is used for energy flux (energy per unit time per unit area, equivalent to intensity in terms of area, not solid angle). They are not new quantities, simply different names.