ON THE FIXED STARS. 393 



about a centre of attraction ; and clusters by a still greater central com- 

 pression. A seventh class includes such nebulae as have not yet been 

 resolved into stars, some of which Dr. Herschel supposes to be so remote, 

 that the light emitted by them must actually have been two millions of 

 years in travelling to our system. The nebulae of another description 

 resemble stars surrounded by a bur, or a faint disc of light ; a diffused 

 milky nebulosity, apparently produced by some cause distinct from the 

 immediate light of any stars, is the next in order : and Dr. Herschel has 

 distinguished other more contracted nebulous appearances, in different 

 states of condensation, into the classes of nebulous stars and planetary 

 neDulae, with and without bright central points. Many of these distinc- 

 tions are perhaps too refined to be verified by common observers ; but the 

 discovery of the existence of double and triple stars, revolving round a 

 common centre, will, if it be confirmed, add one more to the catalogue of 

 Dr. Herschel's important improvements.* 



It is, however, fully ascertained, that some of the stars have periodical 

 changes of brightness, which are supposed to arise either from the tem- 

 porary interposition of opaque bodies revolving round them, or still more 

 probably, from a rotatory motion of their own, which brings at certain 

 periodical times a less luminous part of the surface into our view. Thus, 

 the star Algol, which is usually of the second magnitude, becomes, at 

 intervals of 2 days and 21 hours each, of the fourth only, and occupies 7 

 hours in the gradual diminution and recovery of its light, f A less pro- 

 bable conjecture respecting this change of brightness was advanced by 

 Maupertuis,^ who imagined that the disc of the star might be greatly 

 flattened by a rapid rotation, and its edge occasionally presented to us, in 

 consequence of the disturbances produced by the attraction of planets re- 

 volving round the luminary. Other irregular variations may possibly be 

 occasioned by the appearance and disappearance of spots, occurring, like 

 the spots of the sun, without any determinate order or assignable cause ; 

 and many stars have in the course of ages wholly disappeared, and some- 

 times have been again recovered ; others have made their appearance for a 

 short time, where no star had before been seen. Such a temporary star was 

 observed by Hipparchus, 120 years before our era, and the circumstance 

 suggested to him the propriety of making an accurate catalogue of all the 

 stars, with their respective situations, which is still extant, having been 

 preserved by Ptolemy, who added 4 stars to the 1022 that it contained. In 

 1572, Cornelius Gemma discovered a new star in Cassiopeia, which was 

 so bright as to be seen in the day time, and gradually disappeared in six- 

 teen months. Kepler, in 1604, observed a new star in Serpentarius, more 



* Catalogue of Double Stars, Ph. Tr. 1782, p. 112 ; 1785, p. 40; 1811, 1814, 

 1817. On their changes, ibid. 1803, p. 339 ; 1804, p. 353. Also Mem. of 

 the Astronomical Society, 1822. Bessel, Konigsberg Obs. Pat. 10. Astronomische 

 Nachrichten, No. 88. Struve, Catalogus Novus Stellarum Duplicium, &c. fol. 

 Dorpat, 1827. J. Herschel, Ph. Tr. 1824, 1830. Ast. Soc. 1821, &c. &c. South, 

 Ph. Tr. 1824-6. 



t Goodricke, Ph. Tr. 1783, Ixxiii. 474, and Ixxiv. 287. See also Ixxvi. 48; and 

 ,Lalande, Hist, et Mem. 1788, p. 240. 



J Ph. Tr. 1732, p. 240. 



See Ph. Tr. 1715, xxix. 354. 



