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. [Illustration: 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. [Illustration: Fig. 430.] 372. _Annular Nebulæ._--Occasionally ring-shaped nebulæ have been observed, sometimes with, and sometimes without, nebulous matter within the ring. They are called _annular_ nebulæ. They are both circular and elliptical in form. Several specimens of this class of nebulæ are given in Fig. 431. [Illustration: Fig. 431.] 373. _Nebulous Stars._--Sometimes one or more minute stars are enveloped in a nebulous haze, and are hence called _nebulous stars_. Several of these nebulæ are shown in Fig. 432. [Illustration: Fig. 432.] 374. _Spiral Nebulæ._--Very many nebulæ disclose a more or less spiral structure, and are known as _spiral_ nebulæ. They are illustrated in Fig. 433. There are, however, a great variety of spiral forms. We shall have occasion to speak of these nebulæ again (381-383). [Illustration: Fig. 433.] 375. _Double and Multiple Nebulæ._--Many _double_ and _multiple_ nebulæ have been observed, some of which are represented in Fig. 434. [Illustration: Fig. 434.]