16 POPULAR SCIENCE MONTHLY. 



The cluster which, according to Pickering, may he called the finest 

 in the sky, is oj Centauri. It lies just within the border of the Milky 

 Way, in right ascension, 13h. 20.8m., and declination — 46° -47'. There 

 are no bright stars near. To the naked eye it appears as a hazy star 

 of the fourth magnitude. Its actual extreme diameter is about 40'. 

 The brightest individual stars within this region are between the eighth 

 and ninth magnitudes. Over six thousand have been counted on one 

 of the photographs and the whole number is much greater. 



The most remarkable and suggestive feature of the principal clusters 

 is the number of variable stars which they contain. This feature has 

 been brought out by the photographs taken at the Harvard Observa- 

 tory and at its branch station in Arequipa. The count of stars and the 

 detection of the variables was very largely made by Professor Bailey, 

 who, for several years past, has been in charge of the Arequipa station. 

 The proportion of variables is very different in different clusters. In 

 the double cluster, 869-884, only one has been found among a thousand 

 stars. The richest in variables is Messier, 3, in which one variable 

 has been detected among every seven stars. It might be suspected that 

 the closer and more condensed the cluster the greater the proportion of 

 variables. This, however, does not hold universally true. In the 

 great cluster of Hercules only two variables are found among a thou- 

 sand stars. 



Very remarkable, at least in the case of go Centauri, is the shortness 

 of the period of the variables. Out of one hundred and twenty-five 

 found, ninety-eight have periods less than twenty-four hours. On the 

 subject of the law of variation in these cases, Pickering says: 



"The light curves of the ninety-eight stars whose periods are less 

 than twenty-four hours may be divided into four classes. The first is 

 well represented by No. 74. The period of this star is 12h. 4m. 3s. and 

 the range in brightness two magnitudes. Probably the change in 

 brightness is continuous. The increase of light is very rapid, occupy- 

 ing not more than one-fifth of the whole period. In some cases, pos- 

 sibly in this star, the light remains constant for a short time at mini- 

 mum. In most cases, however, the change in brightness seems to be 

 continuous. The simple type shown by No. 74 is more prevalent in 

 this cluster than any other. There are, nevertheless, several stars, as 

 No. 7, where there is a more or less well marked secondary maximum. 

 The period of this star is 2d. llh. 51m. and the range in brightness 

 one and a half magnitudes. The light curve is similar to that of well- 

 known short-period variables, as 3 Cephei and // Aquilae. Another 

 class may be represented by No. 126, in which the range is less than a 

 magnitude and the times of increase and decrease are about equal. 

 The period is 8h. 12m. 3s. No. 24 may perhaps be referred to as a 

 fourth type. The range is about seven-tenths of a magnitude and the 



