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Optical depth and actual depth, and respectively, can vary widely depending on the absorptivity of the astrophysical environment. Indeed, τ {\displaystyle \tau } is able to show the relationship between these two quantities and can lead to a greater understanding of the structure inside a star .
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 ...
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a ...
The subdisciplines of theoretical astrophysics are: Compact objects – this subdiscipline studies very dense matter in white dwarfs and neutron stars and their effects on environments including accretion. Physical cosmology – origin and evolution of the universe as a whole. The study of cosmology is theoretical astrophysics at its largest scale.
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions.
Illustration of the use of interferometry in the optical wavelength range to determine precise positions of stars. Courtesy NASA/JPL-Caltech. Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies.
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 ...
The explanation is that the interstellar medium is optically thin. Starlight traveling through a kiloparsec column undergoes about a magnitude of extinction, so that the optical depth ~ 1. An optical depth of 1 corresponds to a mean free path, which is the distance, on average that a photon travels before scattering from a dust grain.