184 



KNOWLEDGE. 



[AuctsT 1, 1896. 



half a magnitudo or over. For the brighter stars it would 

 seem that the uuassisted eye is still the best photometer. 

 Then from the iifth to the seventh ma^'iiiUide the 

 binocular or field-glass is a very excellent means of 

 research, while for stars below the seventh magnitude the 

 telescope must be called into use. 



Hence this is a study peculiarly applicable to amateurs, 

 many of whom take it up to become practically professional 

 observers. This has bi'en especially the case in America. 

 The writer can speak to the enjoyment and pleasure 

 obtained in such observations, for he has devoted his 

 leisure hours to the variables for the past eleven years. 



A photometer provides the means of bringing the images 

 of two stars close together for study and comparison ; but 

 the naked eye can do this by a slight movement of the 

 observer's head without the intervention of any lenses or 

 prisms whatever. Again, the binocular commands a large 

 field, and one or more comparison stars are generally in- 

 cluded in it ; but it also can, if necessary, be swiftly shifted 

 a few degrees right or left, up or down, and a good estimate 

 of relative brightness of two stars formed. In the telescope 

 we are of necessity tied down to one field alone, and for 

 the brighter stages of a variable star it is not always 

 possible to include in it a suitable comparison star. 



The meridian photometer of Pickering is of course, 

 theoretically, a more accurate method of comparing the 

 light of two stars ; and so should photography be also. 

 When, however, we come to practice, it is easy to see that 

 the instrumental or photographic results are by no means 

 in rigid agreement, inter ^r. Hence we are of opinion that, 

 for some time yet, the ordinary method of direct eye 

 estimates of brightness will hold its own against the other 

 methods. 



Photography, as a record, is more or less perfect, and we 

 see how fruitful the study of plates has already become 

 when we read of Mrs. Fleming announcing a batch of 

 fourteen new long-period variables at once." 



In 1886, Mr. Gore published a very interesting paper on 

 variable stars, t and with a slight modification we shall 

 adopt his classification ; one hundred and eighty-seven 

 stars were then dealt with. 



Taking now the second catalogue of variable stars by 

 Dr. Chandler, together with the revised supplement to the 

 same, which brings our information up to June, 1895, we 

 find three hundred and forty-three variables contained 

 therein. Those discovered since that publication are not 

 here referred to. 



The variables may be divided into the following classes : 



Class I. Temporary or new stars (eleven). 



Class II. Stars with large variations and periods of 

 one hundred days and upwards in length (two hundred 

 and thirty-eight). 



Class III. Irregularly variable stars (thirty-two). 



Class IV. ^'ariable stars with periods of less than one 

 hundred days (forty-seven). 



Class V. Algol-type stars (sixteen). 



This catalogue of Chandler's is quite a mine of 

 information on the subject. 



Going into a few statistics: — As to Class I., the text- 

 books give two stars which are not included in the eleven, 

 probably on account of insufficient data as to position, etc., 

 viz., the star of b.c. 134 (Hipparchus), and that of a.d. 

 329, in Aquila. So that the number of temporary stars 

 is just thirteen, which, however, has been augmented since 

 by photographic discoveries. 



Class II. comprises the great bulk of the variables. An 



* Sarvartl Colleqe Observatory Circular, Xo. 6. 

 t Jcurnnl of Lirfrpool Axlronomical Society, Vol. V., p. 23. 



analysis of the two hundred and thirty-eight stars is given 

 in the following table : — ''■'■ 



Leiigtli of Period. 

 Daj». 



100 to 

 120 „ 

 145 „ 

 175 „ 

 200 „ 

 225 „ 

 250 „ 

 275 „ 

 300 „ 

 325 „ 

 350 „ 

 375 „ 

 4(K) ., 

 425 „ 

 4.50 „ 

 475 „ 

 500 „ 

 600 „ 

 Over \ 



120 

 145 

 175 

 200 

 225 

 250 

 275 

 300 

 325 

 3.50 

 375 

 400 

 425 

 450 

 475 

 500 

 600 

 700 

 00 



No. of 

 Stars. 



Nil. 



6 

 12 



8 

 16 



9 

 17 

 17 



18 

 14 

 23 

 14 



7 



6 



2 



3 



2 



1 



1 



Variation in HagDitudes, 



It will be seen that up to three hundred and twenty-five 

 days there is a marked tendency for the amount or ampli- 

 tude of variation to increase with the period. This is not 

 so marked from three hundred and twenty-five days to 

 four hundred and seventy-five, although the variation is 

 still considerable. Higher than this the stars are so few 

 in number that the " mean " is not reliable. 



Class III., the " irregulars," may be further subdivided 

 into (n) those with small variation, (/') considerable 

 variation. We thus get : — • 



Variation. 



No. of Stars. ' G-reatest. Least. Mean. 



III. (a) 20 1-9 0-4 1-0 



III. {h) 12 7-2 1-9 4-1 



Class IV., or short-period variables of less than one 

 hundred days' period, comprises forty-seven stars, analysed 

 as under : — 



Here again, as in Class 11. , can be seen a tendency for 

 the amount of variation to increase with the period. One 

 deduction, however, from the table is plain : the range or 

 amplitude of variation in these short-period variables is 

 much less than in the case of Class II., the stars of long 

 period. 



Class V. comprises the rare Algol stars — only sixteen in 

 number up to the date of the catalogue. The star with 

 the longest period and greatest amount of variation is S 

 Cancri, viz., 9d. llh. 37m. and 1-6 magnitude; that 



* The author regrets that an error as to the total number of stars 

 has crept into this table, and also the table of Class IV., which he is 

 unable to correct in time for press owing to not having access to his 

 original notes. The general eonclusions are, however, in no way affected. 



