BY
C. V. L. CHARLIER
SCIENTIA PUBLISHER
LUND 1921
HAMBURG 1921
PRINTED BY LÜTCKE & WULFF[Pg 3]
Our knowledge of the stars is based on their apparent attributes,obtained from the astronomical observations. The object of astronomyis to deduce herefrom the real or absolute attributes of the stars, whichare their position in space, their movement, and their physical nature.
The apparent attributes of the stars are studied by the aid of theirradiation. The characteristics of this radiation may be described indifferent ways, according as the nature of the light is defined. (Undulatorytheory, Emission theory.)
From the statistical point of view it will be convenient to considerthe radiation as consisting of an emanation of small particles from theradiating body (the star). These particles are characterized by certainattributes, which may differ in degree from one particle to another. Theseattributes may be, for instance, the diameter and form of the particles,their mode of rotation, &c. By these attributes the optical and electricalproperties of the radiation are to be explained. I shall not here attemptany such explanation, but shall confine myself to the property whichthe particles have of possessing a different mode of deviating from therectilinear path as they pass from one medium to another. This deviationdepends in some way on one or more attributes of the particles. Letus suppose that it depends on a single attribute, which, with a terminologyderived from the undulatory theory of Huyghens, may be called thewave-length (λ) of the particle.
The statistical characteristics of the radiation are then in the firstplace:—
(1) the total number of particles or the intensity of the radiation;
(2) the mean wave-length (λ0) of the radiation, also called (or nearlyidentical with) the effective wave-length or the colour;[Pg 4]
(3) the dispersion of the wave-length. This characteristic of the radiationmay be determined from the spectrum, which also gives the variationof the radiation with λ, and hence may also determine the meanwave-length of the radiation.
Moreover we may find from the radiation of a star its apparentplace on the sky.
The intensity, the mean wave-length, and the dispersion of thewave-length are in a simple manner connected with the temperature (T)of the star. According to the radiation laws of Stephan and Wien wefind, indeed (compare L. M. 41[1]) that the intensity is proportional to thefourth power of T, whereas the mean wave-length and the dispersionof the wave-length are both inversely proportional to T. It follows thatwith increasing temperature the mean wave-length diminishes—the colourchanging into violet—and simultaneously the dispersion of the wave-lengthand also even the total length of the spectrum are reduced(decrease).
The apparent position of a star is generally denoted by itsright ascension (α) and its declination (δ). Taking into account theapparent distribution of the stars in space, it is, however, more practicalto characterize the position of a star by its galactic longitude (l) andits galactic latitu