NEWS FEOM THE STAES — ABBOT. 



165 



they seem to us to be, and the second as they would seem if all were 

 equally distant. A difference of a magnitude means about 2^ fold 

 in brightness, and five magnitudes 100 fold. Thus a star of sixth 

 magnitude, which can just barely be seen by the naked eye, under 

 best conditions, is 100 times fainter than stars of first magnitude, 

 like Aldebaran, which is among the brightest. On this scale our 

 sun is of — 26.5 magnitude. 



But on the scale of absolute magnitudes our sun is only an aver- 

 age star. If removed to Aldebaran's distance the sun would seem 

 a fifth magnitude star. Some bright stars like Rigel, Canopus, and 

 Deneb give thousands, perhaps himdreds of thousands or millions, 



Fig. 2. — Light-curve of R R Draconis at eclipse (Sears). From Astrophysical Journal, 

 vol. 36. Vertical scale, days ; horizontal scale, magnitudes. 



of times as much light as does the sun. On the other hand a vast 

 number of stars give less light than the sun. 



Measurement of brightness is called photometry. A very large pro- 

 gram of stellar photometry has been done under Director Pickering at 

 Harvard College Observatory. Many stars are found to be of vari- 

 able brightness. It has been shown lately by the Smithsonian observ- 

 ers that even the sun is variable through a range of about 10 per cent. 

 But most of the known variable stars vary much more widely than 

 this. The cause of the variation is now known to be, in many but not 

 all cases, the presence of a companion star so near the primary star 

 as to be indistinguishable by the telescope, but discoverable by spec- 



73839°— SM 1916 12 



