red toward the violet end. It would seem, therefore, that the hotter stars should be the white ones and the cooler the yellow or red ones.
There are, however, two circumstances to be considered in connection with the contracting star. In the first place, the light which we receive from a star does not emanate from its hottest interior, but from a region either upon or, in most cases, near its surface. It is, therefore, the temperature of this region which determines the color of the light. In the next place, part of the light is absorbed by passing through the cooler atmosphere surrounding the star. It is only the light which escapes through this atmosphere that we actually see.
In the case of the Sun all the light which it sends forth comes from an extreme outer surface, the photosphere. The most careful telescopic examination shows no depth to this surface. It sends light to us, as if it were an opaque body like a globe of iron. This surface would rapidly cool off were it not for convection currents bringing up heated matter from the interior. It might be supposed that such a current would result in the surface being kept at nearly as high a temperature as the interior; but, as a matter of fact, the opposite is the case. As the volume of gas rises, it expands from the diminished pressure and it is thus cooled in the very act of coming to the surface.
In the case of younger stars, there is probably no photosphere properly so called. The light which they emit comes from a considerable distance in the interior. Here the effect of gravity comes into play. The more the star condenses, the greater is gravity at its surface; hence the more rapidly does the density of the gas increase from the surface toward the interior. In the case of the Sun, the density of any gas which may immediately surround the photosphere must be doubled every mile or two of its depth until we reach the photosphere. But if the Sun were many times its present diameter, this increase would be less in a still larger proportion. Hence, when the volume is very great the increase of density is comparatively slow; there being no well-defined photosphere, the light reaches us from a much greater depth from the interior than it does at a later stage.
The gradual passing of a white star into one of the solar type is marked by alterations in its spectrum. These alterations are especially seen in the behavior of the lines of hydrogen, calcium, magnesium and iron. The lines of hydrogen change from broad to thin; those of calcium constantly become stronger.
Of the greatest interest is the question—at what stage does the temperature of the star reach its maximum and the body begin to cool? Has our Sun reached this stage? This is a question to which, owing to the complexity of the conditions, it is impossible to give a precise answer. It seems probable, however, that the highest temperature is reached in about the stage of our Sun.
