THE Principles of CHAPTER V THE SPECTRA OF THE STARS No unregarded star Into so small a character, Removed far from our humane sight, But if we steadfast looke We shall discerne In it, as in some holy booke, How man may heavenly knowledge learne. HABINGTON. HE principles on which spectrum analysis rests can be stated so concisely that I shall set them forth for the special use of such readers as may not be entirely familiar with the subject. Everyone knows that when the rays of the sun Spectrum pass through a triangular prism of glass or other transparent substance they are unequally refracted, and thus separated into rays of different colours. These colours are not distinct, but each runs into the other by insensible gradations, from deep crimson through red, scarlet, orange, yellow, green, and blue to a faint violet. Analysis. This result is due to the fact that the light of the sun is made up of an indiscriminate mixture of rays of an infinite number of wave-lengths, or, in simpler language, of an infinite number of tints of colour, since to every wave-length corresponds a definite tint. Such a spreading out of elementary colours in the form of a visible sheet is called a spectrum. By the spectrum of an incandescent object is meant the spectrum formed by the light emitted by the object when passed through a refracting prism or otherwise separated into its elementary colours. The interest and value which attach to the study of spectra arise from the fact that different bodies give different kinds of spectra, according to their constitution, their temperature, and the substances of which they are composed. In this manner it is possible, by a study of the spectrum of a body, to reach certain inferences respecting its constitution. trum. In order that such a study should lead to a definite conclusion, we must recall that to each special shade of colour corresponds a definite position in the specThat is to say, there is a special kind of light having a certain wave-length and therefore a certain shade which will be refracted through a certain fixed angle, and will therefore fall into a definite position in the spectrum. This position is, for every possible kind of light, expressed by a number indicating its wave-length. If we form a spectrum with the light emitted by an ordinary incandescent body, a gaslight for example, we shall find the series of colours to be unbroken from one end of the spectrum to the other. That is to say, there will be light in every part of the spectrum. Such a spectrum is said to be continuous. But if we form the spectrum by means of sunlight, we shall find the spectrum to be crossed by a great number of more or less dark lines. This shows that in the spectrum of the sun light of certain definite wavelengths is wholly or partly wanting. This fact has been observed for more than a century, but its true significance was not seen until a comparatively recent time. If, instead of using the light of the sun, we form a spectrum with the light emitted by an incandescent Spectrum gas, say hydrogen made luminous by the Analysis. electric spark, we shall find that the spectrum consists only of a limited number of separate bright lines, of various colours. This shows that such a gas, instead of emitting light of all wavelengths, as an incandescent solid body does, principally emits light of certain definite wave-lengths. It is also found that if we pass the light of an incandescent body through a sufficiently large mass of gas cooler than the body, the spectrum, instead of being entirely continuous, will be crossed with dark lines like that of the sun. This shows that light of certain wave-lengths is absorbed by the gas. A comparison of these dark lines with the bright lines emitted by the same gas when incandescent led Kirchhoff to the discovery of the following fundamental principle: Every gas, when cold, absorbs the same rays of light which it emits when incandescent. An immediate inference from this law is that the dark lines seen in the spectrum of the sun are caused by the passage of the light through gases either around the sun or forming the atmosphere of the earth. A second inference is that we can determine what these gases are by comparing the position of the dark lines with that of the bright lines produced by different gases when they are made incandescent. Hence arose the possibility of spectrum analysis, a method which has been applied with such success to the study of the heavenly bodies. So far as the general constitution of bodies is concerned, the canons of spectrum analysis are these: Firstly, when a spectrum is formed of distinct bright lines, the light which forms it is emitted by a transparent mass of glowing gas. Secondly, when a spectrum is entirely continuous the light emanates either from an incandescent solid, from a body composed of solid particles, which may be ever so small, or from a mass of incandescent gas so large and dense as not to be transparent through and through. Thirdly, when the spectrum is continuous, except that it is crossed by fine dark lines, the body emitting the light is surrounded by an atmosphere formed of gases cooler than itself. The chemical constitution of these gases can be determined by the position of the lines. Fourthly, if, as is frequently the case, a spectrum is composed of an irregular succession of bright and shaded portions, the body is probably a gaseous mass under great pressure. It will be seen from the preceding statement that a mass of gas so large as not to be transparent may not be distinguishable from a solid body. It is therefore not strictly correct to say, as is sometimes done, that an incandescent gas always gives a spectrum of bright lines. It will give such a spectrum only when it is transparent through and through.' A gaseous mass, so large as to be opaque, would, if it were of the same temperature inside and out, give a continuous spectrum, without any dark lines. But the laws of temperature in such a mass show that it will be cooler at the surface than in the interior. This cooler envelope will absorb the rays emanating from the interior, as in the case when the latter is solid. We conclude, therefore, that the fact that the great majority of stars show a spectrum like that of the sun, namely, a continuous one crossed by dark lines, does not throw any light on the question whether the matter composing the body of the star is in a solid, liquid, or gaseous state. The fact is that the most plausible theories of the constitution of the sun lead to the conclusion that its interior mass is really gaseous. Only the photosphere may be to a greater or less extent solid or liquid. The dark lines that we see in the solar spectrum are produced 1 As this principle is not universally understood, it may be well to remark that it results immediately from Kirchhoff's law of the proportionality between the radiating and absorbing powers of all bodies for light of each separate wave-length. When a body, even if gaseous in form, is of such great size and density that light of no colour can pass entirely through it, then the consequent absorption by the body of light of all colours shows that throughout the region where the absorption occurs there must be an emission of light of these same colours. Thus light from all parts of the spectrum will be emitted by the entire mass. |