extinction

Extinction is the phenomena of no light from a star reaching Earth because of absorption or scattering, such as by dust (dust extinction). Extinction due to dust and/or gas clouds in the interstellar medium (ISM) is called interstellar extinction. Atmospheric extinction refers to absorption and scattering affecting observation that's specifically due to the Earth's atmosphere. Wavelength bands less affected by atmospheric extinction are known as atmospheric windows.

The term extinction is also used for a quantity indicating the degree to which light is absorbed or scattered even if some light gets through. Objects bright enough to show through the dust are likely to experience reddening. An extinction curve is the function of degree of extinction against wavelength. A particular galaxy often has a characteristic curve related to characteristics of its ISM and star formation. An extinction efficiency factor (Q_λ, aka extinction coefficient) is a measure of the extinction by grains of dust per given wavelength, which depends upon the characteristics of the grain. It is commonly inversely proportional to the wavelength (reddening):

     σλ
Qλ = ——
     σg

• Q_λ - extinction efficiency factor of one of these grains for wavelength λ.
• σ_λ - effective cross section of a grain for wavelength λ.
• σ_g - actual cross section of a grain, i.e., π r² for grain radius r.

Various spectral features (e.g., bumps, dips, spectral lines) give hints to dust constituents and size, and polarization suggests non-spherical grains that are aligned.