THE Sc N. Coining now to a closer inquiry into the details of our subject, we begin with the sun (q.v.), the central body of our own system. (See SOLAR SYSTEM.) From a terrestrial point of view, being the source of our light and heat, the sun is certainly the most important of all the heavenly bodies. But considered as a cosmic body the sun is simply a blazing star, and not even a very great one as compared with the average of all the stars. Numerous theories as to its constitution have been advanced. The best authorities now agree that whatever may be the unknown structure of the central mass, the sur face layer directly visible to *us (the 'photo sphere') consists of brightly incandescent mat ter, partly liquid or even solid, and giving out the sun's light. Outside of this is a truly•gase ou.4 layer (the `ehromosphere'). very hot, bub bling, boiling, and at times throwing jets of luminous vapor hundreds of thousands of miles into space. Beyond the chromosphere extends another envelope of tenuous matter (the corona, q.v.), so slightly luminous that it can be seen only when a total eclipse shuts out altogether the stronger light of the photosphere. The most con spicuous phenomena upon the photosphere are the sunspots. These are great temporary breaks or openings, sometimes a hundred thousand miles in diameter. They look like the effects of storms or explosions. Their cause is not definitely
known, but it has been found that magnetic storms on the earth usually occur at periods of great spot-frequency on the sun. No connection with the weather or other terrestrial phenomena has, however, been demonstrated.
Solar chemistry has been studied by means of an instrument called the spectroscope (q.v.) (see SPECTROSCOPY) . which enables us to determine the chemical constituents of a luminous body like the sun. The conclusion reached is that the sun con thins no chemical elements other than those found on the earth, a fact of great interest in connection with the question of a possible com mon origin for the whole solar system. One of the most interesting questions in eonneetion with the sun is the probable source of its great and continuing heat. Why does it not become com pletely consumed as a result of continuous com bustion? This question was answered by Helm holtz (q.v.),who showed that the energy required to generate the heat could be derived from a gradual contraction or shrinkage of its bulk. So great is this hulk that a contraction sufficient to produce the heat in question might go on for ten thousand years without producing a dimi nution in the sun's apparent diameter large enough to be perceived even by our greatest telescopes.