514 



SCIENCE 



[N. S. A'OL. XTuY. No. 1169 



Fjg. 1. (fl) N.G.C. 1501. (6) N.G.C, 418. (c) N.G.C. 6-543. 



Planetary Nebulaj. Composite drawings by Curtis from his photographs with the Crossley Beflector of 



the Liclc Observatory. 



The classes of objects with which astron- 

 omers have to deal are verj^ few. In our 

 solar system we have the sun, the planets 

 and their moons, the comets, the zodiacal 

 light, and the meteors. To the best of our 

 knowledge that exhausts the list. When we 

 look bej^ond the solar s.ystem and out into 

 the great stellar system we see only two 

 classes of objects : the stars and the nebulae ; 

 but there is an extremelj^ great variety of 

 each class, tens of millions of stars and tens 

 of thousands of nebula, probably no two of 

 either class exactly alike. 



The serious studj^ of the nebulas began 

 with Sir William Herschel in the 1780 's. 

 In less than two decades his famous sweep- 

 ings of the sky had rewai'ded him with the 

 finding of 2,500 nebulfe and star clusters. 

 He did not separate them into a list of neb- 

 ula and a list of clusters, as he was not 

 clear about their relations to each other. 

 When he observed certain of them with 

 small telescopes and low magnif jang power 

 they looked like continuous structures, as 

 if tliej'' were little clouds of luminous gases ; 

 but when some of the same objects were sub- 

 jected to greater magnification they were 

 resolved into star clusters. Here was the 

 serious beginning of the hypothesis that all 

 the nebula would be resolved into stars if 

 only our telescopes were sufficiently jDOwer- 

 ful. Herschel was not satisfied with this 



view, and in 1791 he proposed the hypothe- 

 sis that nebula evolve into stars. He 

 thought that nebula of great size would 

 condense very gradually, or break up, into 

 smaller nebula ; that the smaller ones would 

 condense into nebula ever more and more 

 regular in outline; and that these would 

 eventually pass into the small, nearly sjnii- 

 metrical objects which he called planetary 

 nebula, because in telescopes of low power 

 they presented discs resembling the discs 

 of our planets. He said the planetaries 

 were the immediate forerunners of the 

 stars, and that they would evolve into stars. 

 Herschel actually classified a considerable 

 nvnnber of the known nebula in accordance 

 with this hypothesis. Speaking of a star 

 surrounded by nebulosity, M'hich is the con- 

 dition existing in most planetary nebula 

 (see Fig. 1)-, he said that the nebulous mat- 

 ter seemed "more fit to produce a star by 

 its condensation than to depend upon the 

 star for its existence." 



Herschel 's mind was profoundl.y philo- 

 sophic, and his ideas about nebula attracted 

 wide attention. They may easily have sug- 

 gested Laplace's celebrated nebular hy- 

 pothesis of the origin of our solar system, 

 announced a few years later. Herschel 

 thought of the birth of many stars from the 

 nebula; Laplace's Iwpothesis ventured to 

 describe in detail the process of the develop- 



