ASTRONOMY. 



[BKICUTNESS or RTAXS. 



CHAPTER VI. 

 STELLAR ASTRONOMY. 



t* no other Milijecl to which Uie mind of man can be 

 directed, i tho grandeur so imposing or inexhaustible as 

 ui that relating to the distant regions of space known as 

 the sidereal heavens. This is not merely made manifest 

 by the help of gigantic telescopes, and other artificial 

 appliances which the intellect of man has devised ; even 

 with the naked eye, we can perceive how multitudinous 

 are the brilliant specks of light so lavishly scattered in 

 every direction, and congregated in such numbers. In 

 the constellations of Orion or Taurus, however, in the 

 Hyadee, and, above all, in tho great southern Magellaniu 

 Clouds, or the wondnrful zono of the Milky Way, flw 

 stars ore so numerous that they cannot be separately seen 

 by unassisted vision. But although the prospect is so 

 magnificent, and apparently so boundless, even in the 

 most simple and universal view, it becomes infinitely 

 more so when the depths of the heavens are sounded by 

 means of the telescope. When we consider, furthermore, 

 that our sun is but one out of those myriads of objects, 

 shining by their own light, and that each is performing 

 in its sphere the same important functions to worlds re- 

 ceiving its radiated heat and light, which the sun per- 

 forms for us, we become more and more deeply impressed 

 with the boundless extent and variety of celestial objects. 

 Viewing the heavens with tho unassisted eye, we per- 

 ceive that the store are of very different degrees of 

 brightness ; and, for the purposes of classification, as- 

 tronomers have chosen to divide those visible to the 

 naked eye into six degrees of lustre ; the faintest, or 

 those just seen by persons of ordinary eyesight, being 

 termed stars of the sixth magnitude. This classification, 

 however, gives but a rude approximation to the truth, 

 as it would be difficult to find a number of stars of exactly 

 the same degree of brilliancy, while it would be easy to 

 count twenty or thirty stars of perceptibly different de- 

 grees of brightness. With the help of the telescope, ten 

 additional degrees of magnitude each ranging from the 

 seventh to the sixteenth magnitude have been described 

 by astronomers. Stars of the seventh magnitude can be 

 perceived by persons of keen eyesight ; but those of tho 

 fifteenth and sixteenth magnitudes are only visible in 

 tin largest telescopes : even in the gigantic reflectors of 

 Herschel, of twenty feet focal length and two feet aper- 

 ture, they are very faint objects. The satellites of 

 Uranus, and the faintest of Saturn's moons, are estimated 

 to be of this degree of brightness. When the magnitudes 

 of the fixed stars are expressed in this manner, it should 

 be premised that their apparent magnitudes are under- 

 stood by the expression, and not their intrinsic and abso- 

 lute value. The relative intensity of the light of tho stars 

 is still a matter of considerable doubt ; they have not yet 

 been determined phonometrically with any exactitude. 

 Sir W. Herschel endeavoured to- compare the light of a 

 star of the sixth magnitude with that emitted by Sirius, 

 by covering the speculum of the telescope, when pointed 

 at the latter object, with a disc having a circular opening ; 

 and the aperture was so diminished that Sirius appeared 

 as a star of the sixth magnitude as seen with the full 

 opening. When tho magnitudes were thus rendered ar- 

 tificially equal, he found that Sirius, viewed with a cir- 

 cular aperture of one inch in diameter, was reduced to 

 the same intensity of light as a star of the sixth magni- 

 tude when viewed with the full aperture of eighteen 

 inches ; and he concluded from this, that if the light of 

 the latter be supposed equal to unity, that of Sirius would 

 b eighteen time* eighteen, or three hundred and twenty- 

 four. As the light of Sirius is fully three times that of 

 a star of the ordinary first-class magnitude, he considered 

 that, in general, the light of a star of the first, in prc- 

 portoou to that of one of the sixth magnitude, was as a 

 hundred to one. If, in the intermediate classes, the 



brightness is supposed to bo inversely as the square* of 



tho distances, we nave 



1st magnitude . . the brightness = 100 

 2nd J J- 25 



3rd *J- 12 



4th w- <; 



And in the two remaining classes, he concluded, with- 

 out reference to this law, 



5th magnitude . . tho brightness = 2 

 6th = 1 



This determin tion of the relative intensities <>f the 

 brightness of stars, rests, however, on too narrow a basis 

 to be regarded as more than an approximation ; and it is, 

 perhaps, impossible to determine the brightness of one 

 class of magnitudes in fractional parts of that of another. 

 All that astronomical observers have hitherto accom- 

 plished on this subject, has been to arrange and catalogue 

 them in the order of brightness, which, for the tirst six 

 magnitudes, is best done with the naked eye. When 

 they are estimated by means of the telescope, great dis- 

 crepancies have occurred between different observers ; and 

 the fault of our celestial globes and charts has been, that 

 a number of faint telescopic stars are inserted as among 

 those visible to the naked eye ; whilst others, equally 

 bright, are completely omitted, their magnitudes being 

 set down at the time their positions were first deter- 

 mined, and, perhaps, wrongly estimated from the state 

 of the atmosphere, position of the moon, or other causes. 

 Those visible to the naked eye in the northern hemi- 

 sphere have been thoroughly revised by Argelandor, who 

 has subdivided the six classes formerly reckoned into 

 sixteen, inserting two new divisions between each of the 

 ancient classes ; thus, for instance, between the fourth 

 and fifth magnitudes he finds many stare a little fainter 

 than the fourth, which he terms the 4 '5 magnitude ; and 

 others a little brighter than the fifth (yet not so bright 

 as those of the 4 '5 class), which he denominates the 4 '8 

 magnitude. Sir John Herschel, during his sojourn at 

 the Cape, has instituted a further approximation to cor- 

 rectness in the relative brightness of stars, which will be 

 of the utmost importance to future ages in arriving at a 

 knowledge of the constancy of their brightness. This 

 method of seqntttccs, as he terms it, consists in choosing 

 one of the brightest stare in any region of the heavens ; se- 

 condly, one just inferior to it in lustre ; thirdly, one imme- 

 diately inferior to that of the supposititious Kfand magni- 

 tude ; and so on for twenty or thirty times, until a catalogue 

 is formed for this particular region, with the stars ranged 

 merely in regard to their brightness. On any subsequent 

 opportunity this is repeated with a new set of stars, in- 

 troducing, however, as many as possible of the former 

 series. By this means, the relative intensity of all such 

 stare as are visible to the naked eye, may be determined 

 with the most rigorous accuracy. A valuable catalogue 

 of such objects, and various proofs of the almost abso- 

 lute certainty of the process, may be seen in Sir J. 

 Herschel's Remit of Obterrationt, made at the Cape. 

 The want of such a class of observations has led many 

 astronomers to conjecture that great changes have taken 

 place in the brightness of stare since the time of Bayer, 

 at the commencement of tho seventeenth century. 

 Among others, Sir W. Hersohel was of opinion, that at 

 least one out of every thirty stare observed and mapped 

 by Bayer, had diminished or increased in brightness in 

 the two centuries which intervened between the end of 

 the sixteenth and commencement of the nineteenth. It 

 was supposed that Bayer's practice was to call the 

 brightest star of any constellation by the first letter of 

 the Greek alphabet, a ; the next brightest, ft ; tho third, 7 ; 

 and so on. But .Sir W. Herschel found that, instead of 

 the stare in the constellation of Cygnus preserving this 



