STARS. 



counted above forty ftais ; one unformed near the fting of 

 the Scorpion ; the eye of Sagittarius, in which two ftars may 

 be feen in a clear fky with the naked eye, and feveral more 

 with the telefcope ; and one in the head of Orion, in which 

 Gahleo counted twenty-one ftars. 



Flamitead obferved a cloudy itar before the bow of Sagit- 

 tarius, which confifts of a great number of fmall ftars, and 

 the ftar d above Sagittarius's right ftioulder is encompalled 

 with feveral more. 



Caflini and Fiamftead difcovered one between the Great 

 and Little Dog, which is very full of ftars, vifible only by 

 the telefcope. 



But the moll i-emarkable of all the cloudy ftars is that in 

 the middle of Orion's fword, in wliich Huygens, and fince 

 Dr. Long, obferved twelve ftars, feven of which (three of 

 them, now known to be four, being very clofe together) 

 feem to fhine through a cloud, very lucid near the middle, 

 but faint and ill-defined about the edges. (See Galaxy, 

 Magellanic Clouds, and Lucid Spots.) Caflini is of 

 opinion, that the brightnefs of thefe proceeds from ftars fo 

 minute, as not to be diftinguiftiedby the beft glafles : others 

 more probably think, they are large fpaces in the ether 

 through which a lucid medium is difiufed. Although moft 

 of thefe fpaces are but a few minutes of a degree in breadth, 

 yet, fince they are among the fixed ftars, they are probably 

 fpaces not lefs than our whole folar fyftem ; \n which there 

 feems to be a perpetual uninterrupted day, which may fur- 

 nifti matter of fpeculation, as well to the curious naturalift, 

 as to the aftronomer. 



There are alfo feveral ftars which appear fingle to the 

 naked eye, but are difcovered by the telefcope to be double, 

 &c. Of thefe feveral have been obferved by Caflini, Hooke, 

 Long, Ma&elyne, Hornfby, Pigott, Mayer, &c. ; but Dr. 

 Herfchel has been much the moft fuccefsful in obfervations 

 cf this kind. Befides the (kill and induftry by which he is 

 no lefs diltinguifhed than other aftronomers who have been 

 employed in celeftial obfervations, his fuccefs has been 

 chiefly owing to the very extraordinary magnifying powers 

 of the Newtonian feven-feet refleftor which he has ufed, and 

 the advantage of an excellent micrometer of his own con- 

 llruflion. The powers which he has ufed have been 146, 

 227, 278, 460, 754, 932, 1 159, 1536, 2010, 3168, and even 

 6450. His obfervations appear to commence with the year 

 1776, but almoft all of them were made in the years 1779, 

 1780, 1781 ; and they have been fince continued with the 

 fame fkill and general afliduity. For the refult, we refer to 

 the articles Double Stars, Galaxy, Heavens, Nebula, 

 Nebulosity, and the fequcl of this article. 



Star, Double, or a Binary Sidereal Sjjlem, is formed by 

 two ftars fituated fo near to each other, as to be kept toge- 

 ther by their mutual gravitation. The two bodies may 

 revolve round their common centre of gravity in circles, 

 or in fimilar eUipfes, the dimenfions of their orbits being pro- 

 portional to their relative quantities of matter. From a fe- 

 ries of obfervations on double ftars. Dr. Herfchel has found 

 that they have changed their fituations with regai'd lo each 

 other, that the one performs a revolution round the other, 

 aud that the motion of fome of them is direft, while that 

 of others is retrograde. In the courie of obfervations pur- 

 fued for twenty-five years, he difcovered that in more than 

 fifty of the double ftars, there is a change cither in the dif- 

 tance of the two ftars, or in the angle made by a line join- 

 ing them with the direftion of their daily motion, which he 

 calis " the angle of pofition." The interefting obfervations 

 that have been publiftied (Phil. Tranf. vol. Ixxiii.) relate to 

 fix double ftars, a, Geminorum, y Leonis, e Bootes, ^ Her- 

 cuUs, i Serpentis, and y Virginis. In refpeft to the firft 



of thefe, Caftor or a Geminorum, the angle of pofition was 

 found to have diminiftied in the fpace of 43 years 142 days, 

 45° 39' '■> ^""^ irom the regularity of its decreafe, he thinks 

 it highly probable that the orbits in which the two ftars 

 move round their common centre of gravity are nearly cir- 

 cular, and at right angles to the line in which we fee them ; 

 and that the time of a whole apparent revolution of the fmall 

 ftar round Caftor will be nearly 342 years and 2 months, in 

 a retrograde direftion. From the year 1778 to 1 803, he 

 never could perceive any variation in the diftance of the two 

 ftars, which was uniformly i^ of the diameter of the large 

 one. The diftance of the two ftars which compofe y Leonis 

 has undergone a decided change from February i6th, 1782, 

 to March 26th, 1803. The diameters of the two ftars were 

 as 5 to 4. The angle of pofition had diminilhed in this time 

 from 7" 37' N. following, to 6° 2 1' S. following. From the 

 interval between the two ftars, the ratio of their diameters, 

 and the variation in the angle of pofition, Dr. Herfchel has 

 found that the apparent orbit of the fmall ftar is elliptical, 

 and that it performs a whole revolution in about 1 200 years, 

 in a retrograde direftion. The beautiful double ftar of 

 £ Bootes is compofed of two ftars, one of which is of a light 

 red, and the other of a fine blue, having the appearance of 

 a planet and its fatellite. The diftance between the two ftars, 

 obferved with a power of 460, and an aperture of 6.3 inches, 

 in 1781, was I5 the diameter of the large ftar, and in 1803, 

 the interval had increafed to i|- of that diameter. The ratio 

 of the fize of the ftars is as 3 to 2. The angle of po- 

 fition appeared to have fuffered a change of 12° 33', in the 

 fpace of 22 years and 207 days. From thefe fafts Dr. 

 Herfchel concludes that the orbit of the fmall ftar is ellip- 

 tical, and performs its revolution, according to the order 

 of the figns, in 1681 years. The double ftar 1^ Hercules is 

 compofed of a greater and a lefler ftar ; the former being of 

 a beautiful blueilh-white, and the latter of a fine afti-colour. 

 On the 1 8th July, 1782, the interval between the two ftars 

 was one half the diameter of the fmaller one, with a power of 

 460. On the 2ift July of the fame year, their diftance re- 

 mained the fame, but with a power of '987 ; they were one 

 full diameter of the fmall ftar afunder. In 1795, Dr. Her- 

 fchel found it difficult to perceive the fmall ftar. In the 

 month of Oftober 1795', however, he faw it diftinftly with 

 a power of 460. In 1802, he could no longer perceive the 

 fmall ftar, but in a clear night in September of that year, 

 with a power of 460, the apparent difc of ^ Hercules feemed 

 to be a lictle lengthened in one direition. With the ten-feet 

 telefcope, and a power of 600, it had the appearance of a 

 lengthened, or rather wedge-formed, ftar. With a power of 

 2140, he again examined it on the 1 ith of April, 1803, and 

 found the difc a little diftorted ; but he was convinced that 

 not more than three-eighths of the apparent diameter of the 

 fmall ftar was wanting to a complete occultation. The 

 angle of pofition, on the 21ft July, 1782, was 20° 42' north 

 following. 



The double ftar of ^Serpentis has, like i Bootes, undergone 

 a confiderable change in the angle of pofition, without any 

 variation in the diftance between the two ftars. On the 5th 

 September, 1782, the angle of pofition was 42° 48' fouth 

 preceding ; and on the 7th February 1802, it was 61° 27' 

 fouth preceding, having fuftained a diminution of 18° 39' in 

 the fpace of 19 years and 155 days. Hence the period of a 

 complete revolution of the fmaller ftar will be about 375 

 years. 



The double ftar of y Virginis, which has long been known 

 to aftronomers, io compofed of two ftars, which Dr. Herfchel 

 at firft confidered as nearly equal, though he has fince afcer- 

 taiaed that the one is a little larger than the other. Their 



diftance, 



