PERIODIC STARS. 



We shall first notice some of the most remarkable discoveries 

 respecting individual stars, and shall afterwards explain those 

 which indicate the arrangement, dimensions, and form of the 

 collective mass of stars which compose the visible firmament, 

 and the results of those researches which the telescope has enabled 

 astronomers to make in regions of space still more remote. 



PEKIODIC STABS. 



41. The stars in general, as they are stationary in their appa- 

 rent positions, are equally invariable in their apparent magnitudes 

 and brightness. To this, however, there are several remarkable 

 exceptions. Stars have been observed, sufficiently numerous to 

 be regarded as a distinct class, which exhibit periodical changes 

 of appearance. Some undergo gradual and alternate increase 

 and diminution of magnitude, varying between determinate 

 limits, and presenting these variations in equal intervals of time. 

 Some are observed to attain a certain maximum magnitude, from 

 which they gradually and regularly decline until they altogether 

 disappear. After remaining for a certain time invisible, they 

 re- appear and gradually increase till they attain their maximum 

 splendour, and this succession of changes is regularly and perio- 

 dically repeated. Such objects are called PERIODIC STARS. 



42. The most remarkable of this class is the star called 

 Omikron, in the neck of the Whale, which was first observed by 

 David Fabricius, on the 13th August, 1596. This star retains its 

 greatest brightness for about fourteen days, being then equal to a 

 large star of the second magnitude. It then decreases continually 

 for three months until it becomes invisible. It remains invisible 

 for five months, when it re-appears, and increases gradually for 

 three months until it recovers its maximum splendour. This is 

 the general succession of its phases. Its entire period is about 

 332 days. This period is not always the same, and the grada- 

 tions of brightness through which it passes are said to be subject 

 to variation. Hevelius states that, in the interval between 1672 

 and 1676, it did not appear at all. 



Some recent observations and researches of M. Argelander, 

 render it probable that the period of this star is subject to a 

 variation which is itself periodical, the period being alternately 

 augmented and diminished to the extent of twenty-five days. The 

 variations of the maximum lustre are also probably periodical. 



The star called Algol, in the head of Medusa, in the constella- 

 tion of Perseus, affords a striking example of the rapidity with 

 which these periodical changes sometimes succeed each other. 

 This star generally appears as one of the second magnitude ; but 

 an interval of seven hours occurs at the expiration of every sixty- 



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