The general problem
.—“Stellar atmosphere" is the name given loosely to the outer portions of a star. The stellar atmosphere is divided observationally into three superincumbent layers, named the photospheric layers, the reversing layer and the chromosphere, in order of increasing level. The boundaries between these are only roughly defined, but broadly speaking the photospheric layers give rise to the continuous spectrum of the star, the reversing layer to the absorption-line spectrum and the chromosphere (when seen edgeways) to the flash spectrum. Mathematical analysis of the way in which gaseous material comprising the outer portions of a star may be expected to thin out into space confirms this threefold division. It also brings to light certain dynamical and thermal characteristics of the three layers. For example a definite temperature gradient in the photospheric layers shades off into an approximately isothermal state in the chromosphere; “local thermodynamic equilibrium” in the photospheric layers shades off into “monochromatic radiative equilibrium” in the upper chromosphere; and a somewhat unimportant general radiation pressure in the photospheric layers augments to a strong selective radiation-pressure in the reversing layer and chromosphere. The reversing layer is in most cases the transition layer. Assumptions valid for either photospheric layers or chromosphere separately cease to be so near their upper and lower boundaries respectively and so far it has not been possible to give a treatment which accurately deals with the regions of transition. In the present lecture it is proposed to consider chiefly the photospheric layers and the reversing layer. For these regions the dominant need is the determination of the general opacity—the fogginess—for this determines the depth we see into the star and so the pressures, densities, etc., at which the observed spectral phenomena originate. The abstract problem of the stellar atmosphere may be stated as follows. For many purposes the curvature of the outer regions of a star may be neglected and we consider only material stratified in parallel planes. The material is subject to (
a
) a gravitational field of acceleration
g
, (
b
) a net flux of energy of amount πF per unit area, incident on it from below and emergent into space above. This is determined by the evolution of energy in the interior of the star. The amount of energy actually incident on the atmospheric layers from below exceeds
π
F, but a portion is re-radiated downwards by the atmospheric layers,
π
F being the net amount passing through. If the atmospheric layers are in a steady state there is no accumulation of energy, and the net amount of energy crossing any surface of stratification is equal to that crossing any parallel surface, namely
π
F. The quantity F itself is the mean value of the emergent
intensity
of radiation at any point, or, what is the same thing, the mean intensity of radiation over the stellar disc. The abstract problem is:— Given the two parameters
g
and F, and given also the ultimate chemical composition of the material, to determine the distribution of temperature, pressure, density, ionization and chemical composition in the layers, and to determine also the complete intensity-distribution both in angle and in frequency, of the emergent radiation. The practical problem is to some extent the converse one of inferring the temperature and other physical quantities from the observed emergent radiation,
i.e
., from the observed spectra, measured if possible spectro-photometrically. In many cases we do not know either
g
or F, and these also may have to be determined from the observed spectra.
The Sweeping-out of Dust by Radiation Pressure of Stars and Chemical Composition Peculiarities of Disc Galaxies
