There is an occasional star, like chi Carinae, whose spectrum consists almost wholly of bright lines, in general bearing no apparent relationship to the bright lines in the spectra of the gaseous nebulae except that the hydrogen lines are there, as they are almost everywhere. There is reason to believe that such a spectrum indicates the existence of a very extensive and very hot atmosphere surrounding the main body, or core, of the star in question. This particular star is remarkable in that it has undergone great changes in brilliancy and is located upon a background of nebulosity. The chances are strong that the star has rushed through the nebulosity with high rate of speed and that the resulting bombardment of the star has expanded and intensely heated its atmosphere.
There are the Wolf-Rayet stars, named from the French astronomers who discovered the first three of this class, whose spectra show a great variety of combinations of continuous spectrum and bright bands. We believe that the continuous spectrum in such a star comes from the more condensed central part, or core, and that the bright-line light proceeds from a hot atmosphere extending far out from the core.
The great majority of the stars have spectra which are continuous, except for the presence of dark or absorption lines: a few lines in the very blue stars, and an increasing number of lines as we pass from the blue through the yellow and red stars to those which are extremely red.
Secchi in the late 60's classified the spectra of the brighter stars, according to the absorption lines in their spectra, into Types I, II III and IV, which correspond: Type I, to the very blue stars, such as Spica and Sirius; Type II, to the yellow stars similar to our Sun; Type III, to the red stars such as Aldebaran; and Type IV, to the extremely red stars, of which the brightest representatives are near the limit of naked-eye vision. Secchi knew little or nothing concerning stars whose spectra contain bright lines, except as to the isolated bright-line spectra of a few nebulae, and as to the bright hydrogen lines in gamma Cassiopeia, and his system did not include these.
One of the most comprehensive investigations ever undertaken by a single institution was that of classifying the stars as to their spectra, over the entire sky, substantially down to and including the stars of eighth magnitude, by the Harvard College Observatory, as a memorial to the lamented Henry Draper. Professor Pickering and his associates have formulated a classification system which is now in universal use. It starts with the bright-line nebulae, passes to the bright-line stars, and then to the stars in which the helium absorption lines are prominent. The latter are called the helium stars, or technically the Class B stars. The next main division includes the stars in which hydrogen absorption is prominent, called Class A. Classes B and A are blue stars. Then follows in succession Class F, composed of bluish-yellow stars, which is in a sense a transition class between the hydrogen stars and those resembling our Sun, the latter called Class G. The Class G stars are yellow. Class K stars are the yellowish-red; Class M, the red; and Class N, the extremely red. Each of these classes has several subdivisions which make the transition from one main class to the next main class fairly gradual, and not per saltum; though it should be said that the relationship of Class N to Class M spectra is not clear. The illustration, Fig. 17, brings out the principal features of the spectra of Classes B to M. The spectrum becomes more complicated as we pass from Class B to the Class M, and the color changes from blue to extreme red, because the violet and blue radiations become rapidly weaker as we pass through the various classes.
GENERAL COURSE OF EVOLUTIONARY PROCESS
The general course of the evolutionary processes as applied to the principal classes of celestial bodies is thought to be fairly well known. With very few exceptions astronomers are agreed as to the main trend of this order, but this must not be interpreted to mean that there are no outstanding differences of opinion. There are, in fact, some items of knowledge which seem to run counter to every order of evolution that has been proposed.
The large irregular nebulae, such as the great nebula in Orion, the Trifid nebula, and the background of nebulosity which embraces a large part of the constellation of Orion, are thought to represent the earliest form of inorganic life known to us. The material appears to be in a chaotic state. There is no suggestion of order or system. The spectroscope shows that in many cases the substance consists of glowing gases or vapors; but whether they are glowing from the incandescence resulting from high temperature, or electrical condition, or otherwise, is unknown, though heat origin of their light is the simplest hypothesis now available. Whether such nebulae are originally hot or cold, we must believe that they are endowed with gravitational power, and that their molecules or particles are, or will ultimately be, in motion. It will happen that there are regions of greater density, or nuclei, here and there throughout the structure which will act as centers of condensation, drawing surrounding materials into combination with them. The processes of growth from nuclei originally small to volumes and masses ultimately stupendous must be slow at first, relatively more rapid after the masses have grown to moderate dimensions and the supplies of outlying materials are still plentiful, and again slow after the supplies shall have been largely exhausted. By virtue of motions prevailing within the original nebular structure, or because of inrushing materials which strike the central masses, not centrally but obliquely, low rotations of the condensed nebulous masses will occur. Stupendous quantities of heat will be generated in the building-up process. This heat will radiate rapidly into space because the gaseous masses are highly rarefied and their radiating surfaces are large in proportion to the masses. With loss of heat the nebulous masses will contract in volume and gradually assume forms more and more spherical. When the forms become approximately spherical, the first stage of stellar life may be said to have been reached.
It was Herschel's belief that by processes of condensation, following the loss of heat by radiation into surrounding space, formless nebulae gravitated into nebula of smaller and smaller volumes until finally the planetary form was reached, and that planetaries were the ancestors of stars in general. That the planetaries do develop into stars, we have every reason to believe; but that all nebulae, or relatively many nebulae, pass through the planetary stage, or that many of our stars have developed from planetaries, we shall later find good reason for doubting. The probabilities are immensely stronger that the stars in general have been formed directly from the irregular nebulae, without the intervention of the planetaries. The planetary nebula seem to be exceptional cases, but to this point we shall return later.
It is quite possible, and even probable, that gaseous masses have not in all cases passed directly to the stellar state. The materials in a gaseous nebula may be so highly attenuated, or be distributed so irregularly throughout a vast volume of space, that they will condense into solids, small meteoric particles for example, before they combine to form stars. Such masses or clouds of non-shining or invisible matter are thought to exist in considerable profusion within the stellar system. The nebulosity connected more or less closely with the brighter Pleiades stars may be a case in illustration. Slipher has recently found that the spectra of two small regions observed in this nebula are continuous, with absorption lines of hydrogen and helium. This spectrum is apparently the same as that of the bright Pleiades stars. Slipher's interpretation is that the nebula is not shining by its own light, but is reflecting to us the light of the Pleiades stars. That this material will eventually be drawn into the stars already existing in the neighborhood, or be condensed into new centers and form other stars, we can scarcely doubt. The condensation of such materials to form stars large enough to be seen from the great distance of the Pleiades cluster must generate heat in the process, and cause these stars in their earliest youth to be substantially as hot as other stars formed directly from gaseous materials. It is possible, also, that the spiral nebulae will develop into stars, perhaps each such object into many, or some of the larger ones into multitudes, of stars.