SECTION XIII. SIDEREAL ASTRONOMY. NEW STARS-VARIABLE STARS-DOUBLE STARS-COLOURED STARSTRIPLE AND MULTIPLE STARS-THE MILKY WAY-GROUPS AND CLUSTERS OF STARS-DIFFERENT CLASSES OF NEBULE. 140. THE bodies composing our system we have seen to be, in many respects, similar to our earth; some of them exceeding it, while others fall below it in magnitude. The distances of these bodies the mind can scarcely comprehend; yet those are as nothing when compared with that of the least remote of the fixed stars. When we are informed that each of the thousands of the stars may be equal in magnitude and brilliancy to our sun, and is probably accompanied by attendant planets, the grandeur of the universe thus disclosed overwhelms the mind, and its powers fail to comprehend the immensity of space filled with system after system in apparently endless succession. Without entering upon any disputed point, or alluding to hypotheses, which, however brilliant and ingenious, are hardly founded upon severe philosophical reasoning, it will be our endeavour, in this part of the work, to exhibit such facts respecting the fixed stars as have been firmly established by repeated and laborious observations. To the immortal Sir William Herschel, whose labours have been followed up by his no less illustrious son, Sir John F. W. Herschel, this branch of astronomy is under the deepest obligations. The information communicated by these renowned astronomers in the form of memoirs, may be found scattered throughout the Philosophical Transactions of the last seventy years. Sir J. F. W. Herschel's observations at the Cape complete the survey of the entire heavens by one individual. From these sources two writers have of late presented popular views of sidereal astronomy: Dr. Nichol in The Architecture of the Heavens, and Dr. Dick in his Sidereal Heavens. Both of these works will repay perusal, and will exhibit the nature of the labours of the Herschels-especially with respect to nebulæ and double stars. says the Rev. R. Sheepshanks, "may have measured and noted very laudably, but philosophical views and practical details are almost wholly due to them." "Others," The book which, above all others, ought to be in the hands of the amateur astronomer is The Celestial Cycle of Captain W. H. Smyth, R. N. In the second volume, the "Bedford Catalogue" contains a full and particular account of 850 remarkable objects-double stars, multiple stars, clusters, and nebula of each object are given, 1st, the designation and synonyme, with its apparent place; 2d, the position and distance of the components of the double or multiple star; these are followed by a general description of each individual, telescopic views of the most remarkable, and the most authentic details of the history of all. 141. The distance of the fixed stars is immeasurable; observed even with the best telescopes they present no apparent diameter, but only appear as lucid points. Though preserving the same relative distance from each other, it is supposed, and that not without reason, that the stars have a proper motion, exceedingly slow, and only to be appreciated by the most delicate observations continued through a series of ages. Hence the advan tage of extensive and elaborate catalogues, by which the position of the stars at any distant period may be compared with that which they occupied at the time of the formation of the catalogue, and any alteration may be perceived and registered. NEW STARS. 142. The disappearance of a star in the time of Hipparchus induced him to form the first catalogue. Since that period other stars then known have ceased to shine; while others, not before visible, have been called into existence. In the year 389 a new star appeared in the constellation of the Eagle, and disappeared after having shone with a brilliant lustre for three weeks. In 1572 a brilliant star made its appearance in the space between the constellations Cepheus and Cassiopeia, which excited the attention of the Danish astronomer Tycho Brahe, who carefully watched it till the month of March 1574, when it disappeared, after having exhibited varied colours in the light it sent forth, and attained a brilliancy surpassing that of the planet Venus. At a more recent date, 1670, a star of the third magnitude, which had appeared in the head of the Swan, could not be seen; it then re-appeared, and after exhibiting singular fluctuations of light for two years, became extinct, and has not since been observed. There is reason to suppose that many other stars found in old catalogues have shared the same fate. Sir John Herschel concludes, that the star designated 42 Virginis in the Astronomical Society's catalogue has disappeared since May 9th, 1828. 143. One of the most remarkable instances of the appearance of a new star in modern times occurs in the history of the nebula surrounding the star 0 Orionis. This was the first object to which Sir W. Herschel directed his famous forty-feet reflector in February 1787. The star was then pronounced to be quadruple. The star has now become decidedly sextuple, even to instruments of very much inferior power; and the legitimate conclusion to which astronomers have arrived is, that the two additional members are of late creation, having made their appearance since the year 1826. On April 28th, 1848, Mr. Hind discovered, at Mr. Bishop's observatory in the Regent's Park, a star of between the fourth and fifth magnitude, in a line joining n and 20 Ophiuchi, where none was noticed on April 5th its light in the telescope was remarkably vivid, but it became extinct before the end of the year. : VARIABLE STARS. 144. Variable stars offer phenomena well deserving of the closest attention; they exhibit a periodical increase and diminution of light, and some even disappear altogether for a time. One of the most remarkable of these is o Ceti, R. A. 2 hours 8 minutes 33 seconds, dec. 3° 57′ 25′′ s., which has been the subject of observation since the year 1596. This star passes through all the gradations of light and magnitude, from the second to the sixth or seventh magnitude, until it becomes invisible to the naked eye, and can only be traced with the telescope. It remains at its greatest brightness for a fortnight, decreases in brilliancy for three months, remains invisible for five months, when it again becomes visible, and for the remaining three months of its period gradually increases in brilliancy, attaining not always, however, the same degree. Sir W. Herschel makes its period 331 days 10 hours 19 minutes, during which it goes through all its gradations. 145. Another remarkable variable star is Algol, in Caput Medusa. Its changes are gone through in 2 days 20 hours 49 minutes. During four hours it gradually diminishes in lustre; during the next four its brightness is recovered, and for the remainder of its period it retains its maximum of brilliancy. The explanations given of these changes have been. various some suppose that an opaque body revolves round the star, and intercepts its light; others, that large spots may exist on the surface, on that part which appears the least brilliant, and that these are presented to us as the star revolves on its axis; others, that the variable star may describe an orbit very elliptical, and that the variation in brilliancy may arise from the distance between us and the star increasing or decreasing. The following table gives the periods of the more remarkable of the variable stars : |