The picture of the sun which we have so far drawn will fit almost any star, and indeed will be found with greater elaboration of detail in the article STAR. We proceed to particularise for our own luminary. The temperature at the centre of the sun has been estimated at 30 or 6o million degrees, according to the view taken of the conditions of stability and generation of energy. The density 'at the centre is at least 20 times the mean density. The rate of radiation corresponds to a loss of mass of more than 4.000,000 tons per second—an enormous loss, but one which the sun will be able to sustain, if no sudden change occurs, for millions of years to come.
Application to Detailed Features.—Such, in outline, is the idea which is now held of the sun's constitution. The concepts out of which it is formed are atomic nuclei, electrons, and radia tion. If it were complete, the existence of sunspots, prominences and the corona, the sun's rotation, equatorial acceleration and magnetic field, and the phenomena of the atmosphere would follow as inevitable consequences. They do not do so because we have either not yet specified sufficient fundamental concepts or failed to realise the full potentialities of those we have specified. Yet some progress has been made, especially in the consideration of the atmosphere, where, thanks largely to Megh Nad Saha. Ralph Howard Fowler, Edward Arthur Milne, and Henry Norris Russell, ideas consistent with those applied to the interior have shown how the more prominent of the observed phenomena arise. The atom of each element requires a certain amount of energy to detach each of its electrons successively, and the conditions favouring detach ment, or ionisation, are high temperature and low pressure. Thus the spectrum lines of sodium are produced only up to a certain height because, in the lower pressures above that height, all the atoms of sodium are ionised, in which state they produce different spectrum lines lying outside the range of wave-length which the transparency of our atmosphere allows us to observe. The precise connection between the degree of ionisation and the physical conditions has been formulated, and this enables the temperature and pressure in the sun's atmosphere to be estimated from the observed heights reached by the various elements. It appears that the pressure in the sun's atmosphere is very low—less than one thousandth of that of the earth's atmosphere—while the tem perature is not far below that of the photosphere. When the selective pressure of radiation also is taken into account the rela tively great heights reached by the chromospheric gases become intelligible, and in the hands of Milne a consistent theory of the chromosphere is in process of construction (see CHROMOSPHERE).
Although there is at present no sign of an explanation of sun spots along the lines of the general solar theory, the work of Hale has greatly clarified our ideas of the nature of those objects. Hale pictures a vortex motion just below the photosphere, in which the revolution of electrified particles produces a magnetic field.
Photospheric matter is projected upwards along the axis of the vortex, becoming cool by sudden expansion and so appearing darker than the rest of the photosphere. This forms the umbra of a spot, and the outspreading and circulation of the material in the atmosphere constitutes the penumbra. Vilhelm Firman Koren Bjerknes has supplemented this view by assuming that during each I I-year cycle there exist, immediately beneath the photosphere, two vortex rings, one in each hemisphere, each lying along a circle of latitude. When, at a certain point, a ring rises up to the photosphere and is intersected by it the two sections of the ring form spots, which thus occur in pairs and appear to have opposite rotations and therefore opposite magnetic polarities. Periodic movements of the rings in latitude, occurring as part of a definite system of circulating currents of solar matter slightly below the photospheric level, account for the observed latitudinal migration of the spots. This conception is very attractive, but can be regarded at present only as a very plausible hypothesis. It remains to be shown how such rings have developed in a sun having the constitution described above.
latest authoritative book devoted exclusively to the sun is Eclipses of the Sun by S. A. Mitchell (1925) ; this deals comprehensively with solar observation and theory in a non-technical way. The chapter on "The Constitution of the Sun," by C. E. St. John, in the biography of Sir Norman Lockyer (1928) contains a valuable resume of modern ideas. For general accounts see chapters in Astronomy, by H. N. Russell, R. S. Dugan, and J. Q. Stewart (1927) ; F. J. M. Stratton, Astronomical Physics (1925) and H. Dingle, Modern Astrophysics (1927). (H. D.) the name of a group of small birds forming the passerine family Nectariniidae. They inhabit the Ethioviarb In dian and Australian regions. One species occurs in Baluchistan, which is perhaps outside of the Indian region, but the fact of its being found there may be a reason for including that country within the region. They are considered to have their nearest allies in the Meliphagidae (see HONEY-EATER) . Some of them are popularly called "humming-birds" by Anglo-Indians and colonists, but with that group the sun-birds have nothing to do. The most wonderful combination of the brightest colours— scarlet, purple, blue, green and yellow—is often seen in one and the same bird. One group, however, is dull in hue. Graceful in form and active in motion, sun-birds flit from flower to flower, feeding on small insects and nectar; but this is usually done while perched and rarely on the wing as is the habit of humming-birds. The extensible tongue is converted into an almost tubular organ. The nests of the sun-birds, domed with a penthouse porch, and suspended from the end of a bough or leaf, are neatly built.
The Nectariniidae form the subject of a Monograph by G. E. Shelley.