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.
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.
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).
