AND ITS INHABITANTS 9 



relationships to planetary systems. They seem to be related 

 in their origin to New Stars and these in turn are thought to 

 be produced by stars sweeping through clouds of meteoric or 

 gaseous matter and attaining temporarily, from the swift 

 impacts, an enormous brilliancy. The impact is so super- 

 ficial, however, that the extreme brilliance is usually lost 

 in a few days or weeks, and the star subsides through a 

 stage like a planetary nebula into a peculiar type of star 

 known as the Wolf-Rayet stars. The origin of the true plane- 

 tary nebulae has not, however, been observed, as they appear 

 to possess a longer life than those which have originated in 

 the past few centuries from new stars. 



The ring nebulae are few and special, having the form of a 

 vortex ring. 



The stellar nebulae form another small group which look 

 in the telescope like hazy stars. 



By far the greatest number of the nebulae are classified as 

 spiral nebulae, more than 120,000 of which have been made 

 known by photography in connection with the greater tele- 

 scopes. Their actual number must of course be far greater. 

 These objects, unlike the other forms of nebulae, avoid the 

 Milky Way, and are scattered over regions where the stars 

 are more widely spaced. They are very remote and may be 

 entirely beyond the stellar system. This implies enormous 

 magnitude. It seems probable that in general they possess 

 high internal velocities, which implies in turn enormous 

 masses to generate such velocities. These nebulae possess 

 spectra similar to those of stars rather than, like the other 

 types of nebulas, spectra of diffuse clouds of gas. Some 

 astronomers look upon them, therefore, as possibly systems 

 of stars rather than true nebulae ; systems so remote as to give 

 the appearance of faint cloud-like spirals, even when viewed 

 under the highest powers of the telescope. A typical spiral 

 nebula is shown in Plate I, B. 



