Schuster-Schwarzchild Model Summary

Definitions:

W_λ = Equivalent width in Angstroms,  λ = feature wavelength in Angstroms,
X₀ = α₀N (a dimensionless number proportional to Nf), where
N = column density of absorbers in cm^-2,  f = line oscillator strength,
α₀ (“fictitious absorption coefficient”) = (πe²/m_ec)(f /(Δν_D))  [cm^-2],
Δν_D (Doppler width in frequency units) = (1/λ)(2kT/M)^½ = α/λ,
α (most probably thermal speed) = (2kT/M)^½ [cm/s].

Assumptions:

        1.  The continuum is formed in the "photosphere" and has no absorption features as it is emitted from the photosphere
             surface.
        2.  The lines are formed in a "reversing layer" which lies entirely above the "photosphere."  No emission occurs in the
             "reversing layer."
        3.  The absorption coefficient per atom, α_ν, is constant throughout the "reversing layer."

We get three separate approximations which apply in three different cases:

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