448 ASTROPHYSICS 



quantity and quality, are the means to the ends in view. And the 

 great generalizations of scientific truth, the doctrines of evolution 

 and of the conservation of energy, for example, have been no less 

 helpful here than elsewhere. 



The study of our sun forms the principal basis of astrophysical 

 research. The sun is an ordinary star, comparable in size and condi- 

 tion with millions of other stars, but it is the only one near enough to 

 show a disk. The point-image of a distant star must be studied as an 

 integrated whole; whereas the sun may be observed in considerable 

 geometrical detail. We cannot hope to understand the stars in general 

 until we have first made a thorough study of our own star. 



We are unable to study the body of the sun, except by indirect 

 methods. The interior is invisible. The spherical body which we 

 popularly speak of as the sun is hidden from view by the opaque 

 photosphere. This photospheric veil, including the sun-spots; the 

 brilliant faculse and flocculi, projecting upward from the photosphere; 

 the reversing layer, in effect immediately overlying the photosphere; 

 the chromosphere, a stratum associated with and overlying the revers- 

 ing layer; the prominences, apparently ejected from the chromo- 

 sphere ; and the corona, extending outward from the sun in all direc- 

 tions to enormous distances; these superlatively interesting features 

 of the sun constitute the only portions accessible for direct observa- 

 tion; and they are an insignificant part of its mass. They are liter- 

 ally the sun's outcasts. Our knowledge of the sun is based almost 

 exclusively upon a study of these outcasts. Nevertheless, we are able 

 to formulate a fairly simple and satisfactory theory of its constitu- 

 tion. 



The materials composing the sun appear to be the same as those 

 forming the earth's crust. Of the eighty known elements, slightly 

 more than half have been observed in the reversing layer and chromo- 

 sphere, by means of their spectra. The existence of others remains 

 unproved, but there are no reasons to doubt that they too are present. 

 Our most complete study of the sun's composition was made by 

 Rowland, 1 and he has said that, if the earth were heated to the 

 temperature of the sun, the terrestrial and solar spectra would be 

 virtually identical. 



The force of gravity at the sun's surface is well known, but the 

 radial pressures at interior points are somewhat uncertain, as they 

 depend upon the unknown law of increasing density with increasing 

 depth. The minimum value of' the pressure at the sun's centre is 

 thought to be fully ten thousand million times the pressure of our 

 atmosphere at sea-level. 2 The most probable value of the effective 



1 Physical Papers, pp. 521-524. 



2 Arrhenius, Lehrbuch der Kosmischen Physik, p. 123. 



