AN ADDRESS ON ASTROPHYSICS. 299 



to show a disk. The point image of a distant star must be studied 

 a6 an integrated whole; whereas the sun may be observed in consider- 

 able geometrical detail. We can not 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 faculae 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 reversing 

 layer; the prominences, apparently ejected from the chromosphere; and 

 the corona, extending outward from the sun in all directions to 

 enormous distances; these superlatively interesting features of the 

 sun constitute the only portions accessible for direct observation; and 

 they are an insignificant part of its mass. They are literally 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 constitution. 



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 Kow- 

 land, 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 center is 

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

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

 temperature of the sun's radiating surface is 6000° Centigrade, and 

 the minimum value for the center is perhaps five million degrees. In 

 view of these high temperatures, and the low average density of the 

 sun, the interior must be largely gaseous, and perhaps entirely so; 

 although, under the stupendous pressures, a great central core is prob- 

 ably of a viscous consistency, but ready to assume the usual properties 

 of a gas when the convection currents carry the viscous masses up into 

 regions of lower pressure. 



The surface strata are radiating heat into surrounding space. To 

 maintain the supply, it is imperative that convection currents should 

 carry the cooled masses down into the interior, and bring correspond- 

 ing hot masses up to the surface. These currents make the sun a very 



