NEW, VARIABLE, AND COMPOUND STARS. 



171 



It changes from the sixth to the eleventh degree of magnitude, and consequently at its 

 brightest is only visible to the naked eye under favourable circumstances. Halley 

 observes respecting it : " We watched, as the absence of the moon and the clearness of 

 the weather would permit, to catch the first beginning of its appearance in a six-foot 

 tube : that, bearing a very great aperture, discovers most minute stars. On June the 15th 

 last (1715), it was first perceived like one of the very least telescopical stars ; but in the 

 rest of that month, and in July, it gradually increased, so as to become, in August, visible 

 to the naked eye, and so it continued all the month of September. After that, it again 

 died away by degrees, and on the 8th of December, at night, was scarcely discernible by 

 the tube, and, as near as could be guessed, equal to what it was at its first appearance on 

 June 15; so that this year it has been seen, in all, near six months, which is but little 

 less than half its period, and the middle, and consequently the greatest brightness 

 falls about the 10th of September." Its maximum brightness, however, does not seem to 

 be uniform, as, according to Cassini, it was scarcely perceptible in the years 1699, 1700, 

 1701, at those periods when it ought to have been most conspicuous. Its cycle consists of 

 396 days, 21 hours. 



The most remarkable of the versatile stars in our hemisphere are stated in the following 

 table, with their changes of magnitude, and periods of variation : 



There are altogether nearly forty stars ascertained to be variable, and others suspected to 

 belong to the class. 



Various conjectures have been hazarded to account for these cases of periodic stellar 

 change. If we suppose the varying brightness of the bodies in question to be caused by 

 varying distance, the same hypothesis as that which has been mentioned in connection 

 with the temporary stars, it is singular that their position should be wholly unaltered 

 during their respective changes. This appears fatal to the idea of orbital motion being 

 the cause of their versatile appearance. Another surmise is, that dark bodies revolve 

 around these stars, which, periodically intervening between them and us, temporarily cut 

 off their light. Mr. Goodricke proposed this theory respecting the star Algol; but it is 

 open to the objection that it requires us to assign a magnitude to the revolving body in 

 relation to that which must be deemed its primary, which is out of all proportion to that 

 which we are led to believe belongs to dependent orbs. The subservient bodies in our 

 system are immensely inferior to their primary, the sun, and owing to this, to a distant 

 observer of our part of the universe, the transit of the mighty globe of Jupiter across the 

 sun's disk would produce no perceptible effect. Our knowledge of planetary motion is 

 also adverse to that rigorous uniformity which is so marked a feature of the stellar 

 changes. 



It may appear a gratuitous assumption to take the solar system as a miniature 

 picture of others ; but we can only reason concerning what we know not from what we 



