Variable Stars. 79 



energy, while the other side was wrapped in comparative gloom 

 and darkness. We do not perceive how those antithetical con- 

 ditions could be so adjusted to sharp and fixed lines of demarkation. 



Prof. Young, of the Princeton Observatory, inclines to the 

 opinion that variables of the Algol type — those of short and exact 

 periods — are produced by partial eclipse, resulting from the revolu- 

 tion of some dark body round the luminous star and periodically 

 shutting off a portion of its light. This theory is particularly appli- 

 cable to those variables of short period where the light curve bends 

 downward for a brief interval. 



The same authority imagines that the stars of the Mira type — 

 those of long, irregular periods — may be subject to violent out- 

 bursts of hydrogen, which, flaming up with unwonted energy for a 

 short time, soon spends itself, and then subsides to its normal con- 

 dition. This theory applies to those variables whose light curve 

 takes a sharp bend upward and then returns to its ordinary level. 



But neither of these theories is free from objections. Algol is a 

 very large star, probably much larger than our sun. It is difficult to 

 imagine a world large enough to cut off five-sixths of his light, 

 revolving around him in so short a period as three days, without 

 swaying him perceptibly from side to side. The companion of 

 Sirius is only about one tenth of his giant mass, and yet it pro- 

 duced perturbations in that great star that were noticed and meas- 

 ured before the telescope detected its existence. 



Besides, we have seen that the light of Algol diminishes more 

 rapidly just before the minimum than it recovers after the min- 

 imum. Now, if a round dark body should pass across the face of a 

 round luminous body, the light of the latter would diminish and 

 recover in precisely the same time. 



An approximate estimate of the size of Algol, of his companion, 

 and of the orbits in which they move, has been made by Royal 

 Hill. He assumes that the diameters of Algol and his dark com- 

 panion are respectively 6}i and 5 times greater than that of our 

 own sun. The two great worlds, thus nearly balanced in size, 

 revolve about a common center; the smaller at a distance of 

 26,000,000 miles, with a velocity of 327 miles per second, and the 

 larger at a distance of 8,000,000 miles, with a velocity correspond- 

 ingly less. The following diagram will represent their respective 

 Hiotions : 



