THUS far our description of the stellar universe has been confined to its geometrical properties. A serious study of the evolution of the stars must seek to determine, first of all, what the stars really are, what their chemical constitutions and physical conditions are; and how they are related to each other as to their physical properties. The application of the spectroscope has advanced our knowledge of the subject by leaps and bounds. This wonderful instrument, assisted by the photographic plate, enables every visible celestial body to write its own record of the conditions existing in itself, within limits set principally by the brightness of the body. Such records physicists have succeeded to some extent in duplicating in their laboratories; and the known conditions under which the laboratory experiments have been conducted are the Rosetta Stones which are enabling us to interpret, with more or less success, the records written by the stars.
It is well known that the ordinary image of a star, whether formed by the eye alone, or by the achromatic telescope and the eye combined, contains light of an infinite variety of colors corresponding, speaking according to the mechanical theory of light, to waves of energy of an infinite variety of lengths which have traveled to us from the star. In the point image of a star, these radiations fall in a confused heap. and the observer is unable to say that radiations corresponding to any given wave-lengths are present or absent. When the star's light has been passed through the prism, or diffracted from the grating of a spectroscope, these rays are separated one from another and arranged side by side in perfect order, ready for the observer to survey them and to determine which ones are present in superabundance and which other ones are lacking wholly or in part. The following comparison is a fair one: the ordinary point image of a star is as if all the books in the university library were thrown together in a disorderly but compact pile in the center of the reading room: we could say little concerning the contents and characteristics of that library; whether it is strong in certain fields of human endeavor, or weak in other fields. The spectrum of a star is as the same library when the books are arranged on the shelves in complete perfection and simplicity, so that he who looks may appraise its contents at any or all points. Let us consider the fundamental principles of spectroscopy.
1. When a solid body, a liquid, or a highly-condensed gas is heated to incandescence, its light when passed through a spectroscope forms a continuous spectrum: that is, a band of light, red at one end and violet at the other, uninterrupted by either dark or bright lines.
2. The light from the incandescent gas or vapor of a chemical element, passed through a spectroscope, forms a bright-line spectrum; that is, one consisting entirely of isolated bright lines, distributed differently throughout the spectrum for the different elements, or of bright lines superimposed upon a relatively faint continuous spectrum.
3. If radiations from a continuous-spectrum source pass through cooler gases or vapors before entering the spectroscope, a dark-line spectrum results: that is, the positions which the bright lines in the spectra of the vapors and gases would have are occupied by dark or absorption lines. These are frequently spoken of as Fraunhofer lines.
To illustrate: the gases and vapors forming the outer strata of the Sun's atmosphere would in themselves produce bright-line spectra of the elements involved. If these gases and vapors could in effect be removed, without changing underlying conditions, the remaining condensed body of the Sun should have a continuous spectrum. The cooler overlying gases and vapors absorb those radiations from the deeper and hotter sources which the gases and vapors would themselves emit, and thus form the dark-line spectrum of the Sun. The stretches of spectrum between the dark lines are of course continuous-spectrum radiations.
These principles are illustrated in Fig. 12. The essential parts of a spectroscope are the slit—an opening perhaps 1/100th of an inch wide and 1/10th of an inch long—to admit the light properly; a lens to render the light rays parallel before they fall upon the prism or grating; a prism or grating; a lens to receive the rays after they have been dispersed by the prism or grating and to form an image of the spectrum a short distance in front of the eye, where the eye will see the spectrum or a sensitive dry-plate will photograph it. If we place an alcohol lamp immediately in front of the slit and sprinkle some common salt in the flame the two orange bright lines of sodium will be seen in the eyepiece, close together, as in the upper of the two spectra in the illustration. If we sprinkle thallium salt in the flame the green line of that element will be visible in the spectrum. If we take the lamp away and place a lime light or a piece of white-hot iron in front of the slit we shall get a brilliant continuous spectrum not crossed by any lines, either bright or dark. Insert now the alcohol-sodium-thallium lamp between the lime light and the slit, and the observer will see the two sodium lines and one thallium line in the same places as before, but as dark lines on a background of bright continuous spectrum, as: illustrated in the lower of the two spectra. Let us insert a screen between the lamp and the lime light so as to cut out the latter, and we shall see the bright lines of sodium and thallium reappear as in the upper of the two spectra. These simple facts illustrate Kirchhoff's immortal discovery of certain fundamental principles of spectroscopy, in 1859. The gases and vapors in the lamp flame are at a lower temperature than the lime source. The cooler vapors of sodium and thallium have the power of absorbing exactly those rays from the hotter lime or other similar source which the vapors by themselves would emit to form bright lines.
When we apply the spectroscope to celestial objects we find apparently an endless variety of spectra. We shall illustrate some of the leading characteristics of these spectra as in Figs. 13 to 18, inclusive, and Figs. 21, 22, 23 and 24. The spectra of some nebulae consist almost exclusively of isolated bright lines, indicating that these bodies consist of luminous gases, as Huggins determined in 1864; but a very faint continuous band of light frequently forms a background for the brilliant bright lines. Many of the nebular lines are due to hydrogen, others are due to helium; but the majority, including the two on the extreme right in Fig. 13, which we attribute to the hypothetical element nebulium, and the close pair on the extreme left, have not been matched in our laboratories and, therefore, are of unknown origin. Most of the irregular nebulae whose spectra have been observed, the ring nebulae, the planetary and stellar nebulae, have very similar spectra, though with many differences in the details.[1]
[1] My colleague, Wright, who has been making a study of the nebular spectra, has determined the accurate positions of about 67 bright nebular lines.
The great spiral nebula in Andromeda has a continuous spectrum crossed by a multitude of absorption lines. The spectrum is a very close approach to the spectrum of our Sun. It is clear that this spiral nebula is widely different from the bright-line or gaseous nebulae in physical condition. The spiral may be a great cluster of stars which are approximate duplicates of our Sun, or there is a chance that it consists, as Slipher has suggested, of a great central sun, or group of suns, and of a multitude of small bodies or particles, such as meteoric matter, revolving around the nucleus; this finely divided matter being visible by reflected light which originates in the center of the system.