Light Curve of S. Velorum. 25 



once more for 5 days and 7 hours S. Velorum is like the great 

 majority of stars, unvarying, unchanging. At the end of the 

 5 days and 7 hours, the descending phase begins again, and for 

 another fifteen hours — four hours descending, six and a half hours 

 at its minimum, and four and a half hours ascending— the star is a 

 variable. It is to be noticed that the increasing period is somewhat 

 longer than the decreasing, indicating slight eccentricity. 



When observations have been secured over a long period of time 

 it is possible to determine with great exactness the total period of 

 variation, that is the time from any point on, say, the descending 

 curve to a similar point on the next descending curve. The usual 

 method adopted is to compare the light curves at the beginning and 

 end of the series. 



In determining the period of S. Velorum from the Lovedale 

 measures, the method adopted was to reduce the various observations 

 at minimum to an expression of the form, 



Mag. = a. -f /3t + y t2 _j_ dt 3 



where «> /3, y, d, &c., are constants for each minimum. From the 

 numerical values of these constants for different minima the period 

 is readily obtained. 



The resulting period was : 



5 days 22 hrs. 24 mins. 22 sees, -f- 2 sees. 



Having secured our data, the next step is to interpret them. The 

 simple explanation of such variation as we have described is to 

 consider it due to eclipse. S. Velorum is a close binary star, the two 

 components revolving round one another in the period already given. 



Inasmuch as that during 6^- hours the light from the system is 

 constantly at 9*25 magnitude, the light of one of the stars must be 

 completely eclipsed during this period ; that is, the central star is 

 a large, faint star round which a brighter, but smaller, companion 

 revolves in a little less than six days. Such another system have 

 we on a much grander scale in Procyon, only here the existence of 

 the great non-luminous globe round which Procyon revolves is only 

 made evident through the perturbations of Procyon. 



The magnitude during the constant period (7*75) when compared 

 with the magnitude at minimum (9-25) gives us the relative bright- 

 ness of the two stars. For \ 



let m = light ratio between two magnitudes. 

 \ = light of central dark star. 

 1 2 = light of bright satellite. 

 then : ^ + ^ 



(9-25—7-75) log. m. = log. — r- 



