390 PERIODICAL CHANGES OF LUSTRE. SECT. XXXVI. 



The great gulfs that separate the stars from the sun, and 

 probably from one another, no doubt maintain the stability of 

 the stellar system, in the same manner that in the solar system 

 the distances of the planets from the sun and the satellites from 

 their primaries are so arranged as to preserve their mutual dis- 

 turbances within due limits. The stars supposed to be nearest 

 the sun are probably in a great zone which crosses the Milky 

 Way between rj Argus and a Crucis. It comprises the bright 

 stars of the constellations Orion, Canis Major, the Southern Cross, 

 Centaur, Lupus, and Scorpio. The axis of the zone is inclined 

 at an angle of 20 to the medial line, or circle, passing through 

 the centre of the Milky Way. 



A very great number of stars undergo periodical changes of 

 lustre, varying in some cases from complete extinction to their 

 original brilliancy, strongly suggesting the idea that they are 

 temporarily obscured, and sometimes completely hid, by opaque 

 bodies revolving round them in regular periodic times, as the 

 planets do about the sun. 



The star Mira, or o> Ceti, which was first noticed to be periodical 

 by Fabricius, in 1596, appears about twelve times in eleven 

 years, or in periods of 331 d 8 h 4 m 16 8 ; it remains at its greatest 

 brightness about a fortnight, being then on some occasions equal 

 to a large star of the second magnitude ; then it decreases during 

 about three months, till it becomes completely invisible to the 

 naked eye, in which state it remains about five months ; after 

 that it continues increasing during the remainder of its period. 

 Such is the general course of its changes ; but it does not always 

 return to the same degree of brightness, nor increase and dimi- 

 nish by the same gradations, neither are the successive intervals 

 of its maxima equal. From the observations and investigations 

 of M. Argelander, the mean period given is subject to fluctuation, 

 embracing 88 such periods, and having the effect of gradually 

 lengthening and shortening alternately those intervals to the 

 extent of 25 days one way and the other. The irregularities in 

 the degree of brightness attained at the maximum are probably 

 also periodical. For four years previous to 1676 it did not 

 appear at all ; and on October 5, 1839, it exceeded a Ceti, and 

 equalled Aurigae, in lustre. These irregularities may be occa- 

 sioned by periodical perturbations among opaque bodies revolving 

 about the star. Algol, or ft Persei, is another very remarkable 



