44 



ASTRONOMY. SIR WILLIAM HERSCHEL. 



[Diss. VI. 



(190.) 

 Nebulae. 



Greater precision has been in some respects attained, but 

 little, if anything, added to the theory of double stars. 



II. On Nebulce and the (so-called) Nebular Hy- 

 pothesis. The cloudy patches of mild light which 

 are visible in clear but moonless nights in various 

 parts of the sky are called nebulae from their resem- 

 blance to light vapours moderately illuminated. 

 Some regions of the sky abound more with them than 

 others. The Milky Way presents to the naked eye 

 a nebulous appearance extending over large tracts, 

 and nebulae more or less dense are frequent within and 

 near its borders. These facts were well known from 

 an early period. Some conspicuous nebulae were 

 described in the sixteenth century ; that in Andro- 

 meda by Marius and the conspicuous nebula in 

 Orion by Huygens. Halley added several, but the 

 first descriptive catalogue of these objects was pub- 

 lished by Messier in 1783 (in the Connaissance des 

 Terns) , and it forms the basis of the modern obser- 

 vations. 



Sir W. Herschel took up the subject with his cus- 

 tomary zeal. In 1786 he published a catalogue of 

 1000 nebulae, which he had increased by 1802 to 

 2500. The first step to some appreciation of the 

 nature of these extraordinary appearances was of 

 course to classify them, and this Herschel did with 

 such success that it may be affirmed that his classes 

 remain the best at the present time, as generally re- 

 presenting the phenomena. Without entering into 

 all his details, the following division embraces the 

 main facts : 



A. Clusters of stars. 



B. Nebulae proper. 



a. Having a certain regularity, as 1. Circular or 

 planetary nebulae ; 2. elliptic ; 3. annular. 



/3. Wholly irregular, as the nebula of Orion, 

 the Magellanic clouds, &c. 



C. Nebulous stars. 



Remarks. A number of faint stars produces a 

 nebulous haze. This is a physiological fact. Parts 

 of the Milky Way which present this appearance to 

 the naked eye are at once shown by a telescope of 

 small power to be composed of stars. L Patches which 

 appear nebulous in such a telescope are shown by a 

 better one to be composed of small stars. This is 

 called the Resolution of a nebula. Such resolutions 

 become more and more numerous as the apertures of 

 telescopes are enlarged. Lord Rosse's telescope has 

 resolved several which withstood the highest power 

 of Herschel's. One inference might therefore be that 

 all nebulas are clustered stars, at a greater or less 

 distance. Such an opinion is very general, and very 

 plausible. It may on the other hand be observed 

 that Herschel abandoned this his earlier inference, 



whilst, at the same time, he proceeded diligently with 

 the resolution of successive nebulae, by means of his 

 powerful telescopes. He maintained on the contrary to 

 the last that nebulas belong to two classes, resolvable 

 and irresolvable. The latter he believed to be composed 

 of diffuse self-luminous matter, more or less con- 

 densed, whilst the milky light of the former is occa- 

 sioned by the blended gleam of numerous close stars. 

 With this difference, we find his views regarding the 

 changes in the sidereal system and the progress of 

 the condensation and breaking up of nebulse, nearly 

 the same in his earlier and later papers, particularly 

 those of 1785 and 1811. 



The opinions of Herschel on these changes were 

 the following : Having observed that the more 

 regular nebulae (whether with a central star or 

 not) presented almost invariably an appearance of 

 gradually increasing brilliancy towards its centre, 

 he was led to conclude that this is due to the aggre- 

 gation of the self-luminous parts in virtue of their 

 mutual gravitation ; and the conclusion would so 

 far be the same, whether the parts tluis coalescing 

 were in fact numerous distinct masses, or were like 

 a vapour diffused through space. This was the pro- 

 cess of condensation, which he inferred on the one 

 hand from the law of gravity, on the other from the 

 generally symmetrical arrangement of the luminous 

 matter in nebulae of the first class. In order to de- 

 tect, by actual observation, the progress of condensa- 

 tion, he invented a simple and highly expressive 

 terminology, indicating the general brightness, the 

 amount of its gradation, and the law of its gradation 

 from the circumference towards the centre. In 

 whatever degree Lord Rosse's late observations may 

 be considered to deprive nebulas of the character of 

 symmetry which Herschel had conferred on them, 

 the general fact of an apparent condensation in a 

 great majority of instances cannot be denied. 



The breaking up of nebulae (also described in his 

 paper of 1785, as well as in his later ones) is likewise 

 an inferred change anticipated in large irregular 

 nebulse, which usually exhibit bright spots or centres, 

 \vith irregular vacant spaces, which sometimes seem 

 like portions of a black and distant profundity seen 

 through the rents of a nebulous veil. Herschel sup- 

 poses that the luminous centres are also places of 

 greatest attraction, and must gradually draw away 

 the star-like substance from the rarer spaces. 



It is curious that these speculations of Herschel 

 (which are substantially common to his papers of 

 1785 and 1811) had been at least imagined at a very 

 early period. Arago has pointed out that Tycho 

 and Kepler conceived the New Stars of 1572 and 

 1604 to have resulted from a sudden condensation 



(193.) 



Herschel 

 inferred 

 their pro- 

 gressive 

 condensa- 

 tion, 



(194.) 

 and break- 

 ing up in 

 some cases. 



(195.) 

 Early anti- 

 cipation of 

 this hypo- 

 thesis. 



1 The group of the Pleiades presents a peculiar instance. When the eye is turned full upon it, no more than six stars (as is 

 well known) can be counted, but when the eye is turned aside through 10 or 15, by a peculiarity of indirect vision by means of 

 the lateral parts of the retina, the group appears a mass of nebulous light. This arises from an immense crowd of stars just 

 below visibility, which, probably from the greater sensitiveness of this part of the eye, make a sensible though somewhat indis- 

 tinct impression. This observation as to the Pleiades, and the inference which explains by it the seeming countlessness of the 

 stars when viewed cursorily, were published by the present writer in Brewster's Journal for 1826. 



