V All! ABLE STARS.] 



ASTRONOMY. 



955 



latter in 12 '8 days. The period of the latter star was 

 first stated to be 6d. 9h. by Goodricke, who detected its 

 variation, which he at first supposed to be from the third 

 to the fifth magnitude. In this determiuation of the 

 length of the period he was partly correct ; for it is one 

 of those stars in which there is a smaller variation of 

 brilliancy within the larger period. After its mi- 

 nimum brilliancy, it passes in 3d. 5h. to its first 

 maximum, and then in 3d. 3h. to its second minimum. 

 In 3d. 2h. it again rises to its second maximum, and 

 after this, in 3d. 12h., to its former minimum. The whole 

 period, according to Argelander, is 12d. 21h. 46m. 40s., 

 in which period it performs all the preceding changes. 

 This celebrated astronomer is of opinion that the period 

 is diminishing, and that it is now some hours shorter 

 than when first discovered. Even from recent obser- 

 vations it is apparent that there is a decrease in the 

 period. 



Previous to the time of Sir W. Herschel, it was only 

 stars of very long and very short periods which were 

 known to be variable. The period of a Herculis, dis- 

 covered by him, proved, however, that there was an in- 

 termediate class. This star is remarkable for its deep 

 orange-red colour, which is very strikingly contrasted 

 with its small dark blue companion. The period was 

 considered by Herschel to be 63 days ; but the change of 

 lustre ia very slight, being only from the third to the 

 third and a-half magnitude. There is still some doubt 

 respecting its period, some observers considering that it 

 ia is much as ninety-five days ; but from seven years' 

 observations, it would appear that a period of sixty-six 

 days would agree better with the changes which it under- 

 goes. The observer Heis, who has particularly attended 

 to this species of phenomena, is of opinion that the 

 variations of light are best satisfied by a period of 184-9 

 days with two maxima and two minima. Since the time 

 of Herschel, various other stars of medium period have 

 been added to that of a Herculis, nearly all of which 

 are, like it, stars of considerable magnitude. Thus a 

 Cassiopeia; changes from the second to the third mag- 

 nitude in 79d. 3h., according to Biot ; but it is very 

 difficult to determine the exact period, the change of 

 brilliancy being very slight, and apparently irregular. 

 This is equally the case with a Orionis, which, at its 

 minimum, becomes slightly inferior to stars of the first 

 magnitude ; and, according to Argelander, the period of 

 increase of light takes place in ninety-one days and 

 a-half, whilst it continues to decrease in brilliancy during 

 a hundred and four days and a-half. The period of 

 o Hydrse is supposed by some to be performed in fifty- 

 five days ; but the change of brightness being minute, 

 as in other instances, it is somewhat doubtful. Sir. I. 

 Herschel considers it to be performed in twenty-nine or 

 thirty days. The star commonly designated as I in the 

 constellation of Sobittki't Shield, is remarkable for the 

 great variation of its magnitude, particularly at its 

 minimum, when it varies between the sixth and ninth 

 magnitudes. At its maximum it only varies between 

 the fifth and sixth. Among those of shorter period may 

 be counted that of ft Pegasi, which varies between the 

 second and second and a-half magnitude, but whose 

 ]*riod is not more than forty days. At the present 

 time, we thus know stars of almost every period from 

 2d. 20h. to 406 days. There are ten whose period varies 

 l>etween 1 and 100 days ; two between 100 and 200 ; 

 eight from 300 to 400 ; and two above 400. It is curious 

 that there are none at present known whose period is 

 between 200 and 300 days. 



Herschel considered that, by introducing a medium 

 period between those which were very long and very 

 uliort, he proved the rotation of those objects on an axis, 

 by which means such portions of their surfaces as were 

 dark and obscure came into sight at stilted times, and 

 hence their apparent faintness. To the astronomers of 

 the 16th and 17th centuries by many of whom the 

 rotation of the earth was doubted, and that of the other 

 planets unknown this method of accounting for it would 

 have appeared strange. It was attempted, however, by 

 Bouilland to explain th changes seen in o C'eti on this 



principle : the larger portion of the surface of this star 

 was, he considered, non-luminous, which rendered it 

 invisible for the greater part of its period. Another sup- 

 position in respect to those changes was, that they were 

 due to the interposition of opaque bodies, by which their 

 light was more or less eclipsed at regular intervals. 

 Maupertius considered that, among the many stars which 

 were scattered through space, there were some whose 

 figure was not regularly spherical, but greatly flattened 

 at the polar regions, and that the variations of light were 

 due to the thin edges of those being sometimes presented 

 to view, and at other times the large surface of the 

 flattened disc. There is yet no experimentum cruets 

 known, by which it can be said whether either or any of 

 these hypotheses are correct. 



CLUSTERS AND NEBULA. The most striking and mag- 

 nificent objects which the sidereal heavens present to 

 view, are the clusters and nebulae, where, in a telescopic 

 field of view of a few minutes in diameter, we may per- 

 ceive a crowd of stars collected together, and forming a 

 small patch of light, the individuals composing which 

 must be some thousands in number. To those wherein 

 the stars are plainly seen, have been given the name of 

 dusters ; but in others, which are unresolved, the terms 

 nebula and unresolved nebulosities have been applied. 

 The nebulfe and clusters which are at present known are 

 of every degree of resolvability ; and though some of those 

 which are generally called nebulae have been resolved by 

 the most powerful instruments, yet there are others which 

 have baffled all the attempts of opticians to separate them 

 into individuals. They are of every conceivable shape and 

 size, from the regular and minute form of the planetary 

 nebula of a few seconds in diameter, to the amorphous 

 and large surface of the great nebula in Orion. Numerous 

 clusters of stars are visible to the naked eye, as the 

 Pleiades, Hyades, the Presepe of Cancer, and that in the 

 sword-handle of Perseus ; and in the southern hemisphere 

 we have the Magellanic Clouds, which cover a space of 

 some degrees square. 



The most magnificent of the clusters which have been 

 resolved by the telescope, is that in the constellation of 

 Hercules (Fig. 78), which can be detected on a clear night 

 by the naked eye, and 

 which, in the tele- 

 scope, presents one 

 of the finest instances 

 of dense isolated clus- 

 tering in the whole 

 of the heavens. The 

 stars are from the 

 tenth to the fifteenth 

 magnitudes, and are 

 so densely congrega- 

 ted round the centre, 

 that it presents the 

 appearance of a blaze 

 of light in that part. 

 The stars are pretty 

 evenly distributed in 

 the interior, but are rather irregular at the edges, where 



they are formed into 

 curvilinear branches. 

 Sir J. Herschel is of 

 opinion that there are 

 many thousands of 

 them, some of which 

 are the faintest the 

 telescope can show. 

 This object is only 

 seven or eight minutes 

 in diameter, and, with 

 the telescope which 

 Sir J. Herschel em- 

 ployed, no appearance 

 of a nucleus was per- 

 ceptible ; although, 

 with a smaller one, 

 such might be suspected. 



The fifth object in Messicr's catalogue, is another very 



Fig. 78. 



fig. 79. 



