M i 



ASTRONOMY. 



[VARIABLE BTAIUL 



accurate raulta which have been obtained as vet, are due 

 to observation* with the naked eye, compared with star* 

 in ita VK-imty ; by thil meani the interval between the 

 faintest and brightest phases hare been found with far 

 mure accuracy than would be imagined. 



There are numerous instances of the appearances of 

 temporary stars recorded in the Chinese annals ; but the 

 two most remarkable of those phenomena have occurred 

 in comparatively modern times, and have been minuU-ly 

 described by the two great contemporaries, Tycho Brah6 

 and K.-1'U-r. The first appeared in th.- yc:ir 1T.7-', in the 

 constellation of Cassiopeia*. On the llth of Novemlx r < ( 

 that year, when seen for the first time by Tycho, it sur- 

 jNWsed in brilliancy both Sinus and Jupiter, the most 

 luitrous objects in tin- heavens. In the following mouth 

 it had diminished slightly in brightness, but was still 

 equal to Jupiter. At the beginning of 1573, it was in- 

 ferior to that planet in brightness ; and by the end of 

 March it was not brighter than the principal star in 

 Taurus, although still a good first magnitude. It con- 

 tinued gradually to decrease until the end of the year, 

 but remained visible to the naked eye until March, 1574. 

 If its period of increase were equal to its period of de- 

 crease, it must have been visible to the naked eye for 

 nearly three years, as it was not noticed until it had at- 

 tained its maximum brightness. For a description of the 

 second instance we are indebted to Kepler. The star was 

 perceived suddenly in the constellation Serpentarius, on 

 October 10th, 1604, and appears to have been nearly as 

 brilliant as it was formerly, though not so favourably 

 situated for observation. It remained visible to the 

 naked eye (the telescope had not yet been invented) fur 

 upwards of a year. A star of the third magnitude also 

 appeared suddenly in the constellation of Cyfjnus, in the 

 year 1670, which soon afterwards disappeared ; it again 

 made its appearance, and again disappeared, and has not 

 been since seen. It underwent several changes during 

 the two years in which it was observed. 



VARIABLE PERIODIC STABS. The appearance of tem- 

 porary stars is as rare as the phenomena observed are 

 extraordinary. Of a similar nature, however, are the 

 claw of periodic variable stars, whose changes of lustre 

 are equally as decided and curious as those of the objects 

 just mentioned ; being more known, however, they excite 

 less attention. The must singular of those objects is 

 that of o Ceti, whose variability was first discovered by 

 Holwarda in 1639, and which has a period of about eleven 

 months. Although at its maximum brightness it reaches 

 to the second magnitude, it does not appear to have been 

 at all noticed by any observer previous to 1596 ; but this 

 may be partly in consequence of the length of its period ; 

 for if the maximum intensity of light fall in the summer 

 months, it may sometimes remain invisible for three or 

 four years together. During the winter months this is 

 not the case ; but, as happened in February and December, 

 1847, two maxima may take place during the same year. 

 There U another circumstance which has been noticed, 

 and which may tend to explain the silence of ancient 

 authors and observers. This is, that the star does n>/t 

 alway ( tjfach the same brilliancy, sometimes approaching 

 in brightness to stars of the first or second magnitudes 

 at this period, whilst at other times it is not brighter 

 than the fourth maximum. It has generally been sup. 

 posed that it disappears entirely at the period of ita 

 minimum, even in the best telescopes ; but this is not 

 always the case, for at these times it has occasionally 

 been observed to be not fainter than stars of the eleventh 

 magnitude. What its variations of light may be when 

 visible only in the telescope U not very well known, as it 

 has not been closely observed at those times ; but it is 

 certain that, whilst visible to the naked eye, its 11' 

 ations of brightness are very remarkable. It is visible 

 to the naked eye, on an average, fur about two months 

 previous to the period of its maximum brightness ; but 

 the period of its diminution of brightness is generally 

 longer than that of its increase; and it has been visible 

 for three months after it* maximum, the average duration 

 being, however, only seventy days. It sometimes, but 

 rarely, happens that the interval which elapses between 



its coming into sight and its maximum brightness, is 

 greater than the time between its maximum and dis- 

 appearance. Some attempt has been made to reduce 

 the fluctuations which its light undergoes, and to establish 

 a law ; but the changes are too abrupt and irregular to 

 be dealt with in this manner, or to be foretold with any 

 accuracy. 



Among the other stars of long period, which are visible 

 to the naked eye for a length of time, and then entirely 

 disappear from sight, is x in the neck of Cygnus, which 

 is almost as remarkable as the preceding. Its light varies 

 between the fifth and eleventh magnitude. Its maximum 

 brightness, like that of o Ceti, i, nowever, very variable ; 

 sometimes it reaches to the fourth magnitude, and at 

 other times ita maximum U not more than the 6*7 mag- 

 nitude, when it is quite invisible to the naked eye. The 

 variability of the light of this star was discovered by 

 Gottfried Kirch ; but the period was first found by 

 Maraldi. The interval between its successive maj'im-i 

 and minima, as well as its intensity of light, at those 

 times, has since been discovered to be very irregular. 

 On some occasions it has been visible to unassisted vision 

 for a period of nearly three months ; but the average 

 duration, according to Argelander, is only fifty-two days, 

 being twenty days of an increase, and thirty-two of a 

 decrease. The longest period which has yet been re- 

 corded in the class of variable stars, occurs with the star 

 30 Hydrre, whose period has been determined, by Maraldi, 

 at four hundred and ninety-four days ; this period, bow- 

 ever, is very irregular. At its maximum brightness it 

 sometimes arrives at the fourth magnitude, and at other 

 times only at the fifth, and thence it decreases in bril- 

 liancy until its entire disappearance. The star 19 Leonis 

 is another with a very long period, the interval between 

 its successive maxima being three hundred and eleven 

 days. This period, however, is also somewhat irregular, 

 and the changes in its brilliancy occasionally abrupt. 

 At its maximum brilliancy, it is equal to stars of the 

 fifth magnitude, and is invisible at its minimum. These 

 are the only four stars hitherto discovered which are in- 

 visible to the naked eye at their maxima, and vanish out 

 of sight at their minima, except to the most powerful 

 telescopes. There are a few other stars discovered by 

 Harding, Schwerd, and Hind, with periods which appear 

 to be upwards of a year ; but, even at their maxima, these 

 stars are only visible in a telescope, and not the slightest 

 trace of them can be perceived in the best instruments 

 at the epoch of their minima. 



In others of the variable stars, the period is much 

 shorter ; but the change of light is not so considerable. 

 The most remarkable of those is the star ft Persei, in 

 which the interval between two successive maxima is only 

 2d. 20h. 48m. 58s., and is otherwise a very curious ob- 

 ject, from the abrupt and variable changes of brilliancy 

 which it exhibits. For the greater portion of its period 

 it remains at nearly its maximum brightness, or of the 

 two-and-a-half magnitude. In about four hours it sud- 

 denly decreases to the fourth magnitude, which is its 

 minimum brightness ; and in a like period of four hours 

 it regains its former brightness. There are other but 

 less remarkable changes in its light ; it is one of the best 

 determined of th variable class. The majority of the 

 periodic stars shine with a reddish light, but this shows 

 no sign of colour, being of a pure white. The star & 

 Cephei has a period of 5d. 8h. 37m., with a variation of 

 brilliancy from the third to the fifth magnitude. This 

 star exhibits a great regularity in its successive changes 

 of brilliancy, which have been determined with con- 

 siderable exactness ; but the periods of increase and 

 decrease are very dissimilar, as it takes 3d. 18h. to pass 

 from its maximum to its minimum, and only Id. loh. to 

 return from its minimum to its maximum, according to 

 the careful observations of Argelander. During eight 

 hours of its decrease, it scarcely, if at all, changes ; and 

 for a whole day its diminution of brightness is scarcely 

 perceptible. Two other variable stars of short period 

 i/ Aijuilii; and /3 Lyree are known at present. Both of 

 those stars change from the three-and-a-half to the four- 

 and-a-half magnitude ; the former in 7'18 days, the 



