Either of these methods indicates that the temperature of the sun’s surface is about 6000 degrees absolute, which is nearly twice the temperature of the hottest part of the electric arc. The total amount of light and heat received on earth from the sun shews that the sun’s radiation must be very nearly, although not quite, the “full temperature radiation” (p. 123) of a body at this temperature. This is also shewn by the sun’s radiation being distributed among the various colours in a way which conforms very closely to the theoretical curve for a full radiator at 6000 degrees shewn in fig. 15. The surface-temperature of a star can also be estimated from its spectral type. Many of the lines in stellar spectra are emitted by atoms from which one or more electrons have been torn off by the heat of the star’s atmosphere. We know the temperatures at which the electrons in question are first stripped off their atoms, and so can deduce the star’s temperature. The temperatures which correspond to the different types of stellar spectra as shewn in Plate VIII (p. 51), are approximately as follows: +-------------+-----------+ |Spectral type|Temperature| +-------------+-----------+ | _B_ | 23,000 | | _A_ | 11,000 | | _F_ | 7,400 | | _G_ | 6,000 | | _K_ | 5,100 | | _M_ | 3,400 | +-------------+-----------+ The last three entries in the table refer only to normal stars having diameters comparable with that of the sun. We shall find (p. 276) that a second class of stars (giants) exist, whose diameters are enormously greater than the sun’s. These have the substantially lower temperatures shewn below: +-------------+-----------+ |Spectral type|Temperature| +-------------+-----------+ | _G_ | 5600 | | _K_ | 4200 | | _M_ | 3200 | +-------------+-----------+ In studying stellar structure and mechanism, we are less concerned with the heat of the star’s surface as measured by its temperature, than with the amount of radiation it pours out per square inch.