369. Four Types of Stellar Spectra.—The spectra of the stars have been carefully observed by Secchi and Huggins. They have found that stellar spectra may be reduced to four types, which are shown in Fig. 428. In the spectrum of Sirius, a representative of Type I., very few lines are represented; but the lines are very thick.
Next we have the solar spectrum, which is a representative of Type II., one in which more lines are represented. In Type III. fluted spaces begin to appear, and in Type IV., which is that of the red stars, nothing but fluted spaces is visible; and this spectrum shows that something is at work in the atmosphere of those red stars different from what there is in the simpler atmosphere of Type I.
Lockyer holds that these differences of spectra are due simply to differences of temperature. According to him, the red stars, which give the fluted spectra, are of the lowest temperature; and the temperature of the stars of the different types gradually rises till we reach the first type, in which the temperature is so high that the dissociation (161) of the elements is nearly if not quite complete.
III. NEBULÆ.
Classification of Nebulæ.
370. Planetary Nebulæ.—Many nebulæ (328) present a well-defined circular disk, like that of a planet, and are therefore called planetary nebulæ. Specimens of planetary nebulæ are shown in Fig. 429.
Fig. 429.
371. Circular and Elliptical Nebulæ.—While many nebulæ are circular in form, others are elliptical. The former are called circular nebulæ, and the latter elliptical nebulæ. Elliptical nebulæ have been discovered of every degree of eccentricity. Examples of various circular and elliptical nebulæ are given in Fig. 430.