THE MEASURE OF STELLAR SPACES 337 



as the determinations of Henderson and Bessel. We should 

 allow always for a possible error of a few million years. Our 

 sun would appear a star of the first magnitude to them, as does 

 alpha Centauri to us. 



At about twice the distance of alpha Centauri is No. 61 

 Cygni. It is of the fifth magnitude. Clearly, then, unless it 

 be of an utterly different constitution, it is very much smaller 

 than alpha Centauri or our sun. Its actual brilliancy appears 

 to be not more than one-tenth that of the sun ; it may not be 

 one-third the latter' s diameter. 



At about the same distance, between eight and nine light- 

 years, is the great Dog-star. Sirius is nearly three times as 

 bright as alpha Centauri, ten times as bright as 61 Cygni. The 

 light it gives is perhaps thirty times that of our sun. Its bulk, 

 if its temperature were the same, would therefore be more than 

 a hundred times the bulk of our sun or of our nearest neighbour 

 star. 



Again, outshining any other star in the northern heavens, is 

 huge Arcturus. It is fifteen or twenty times more distant than 

 the Dog-star, and still more vast. The light it sheds would be 

 equal to that of perhaps one thousand three hundred of our 

 suns. It is apparently a solar type of star ; its bulk, therefore, 

 must be three or four thousand times the volume of Sirius, perhaps 

 forty or fifty thousand times the volume of our luminary. 



Apparent magnitude is therefore but little index of the reality. 

 This conclusion is still further borne out by the amazing example 

 of Canopus, the great star of prehistoric Egypt. It is the 

 brightest of the southern heavens, second only to Sirius. Recent 

 careful determination reveals the fact that it has no appreciable 

 parallax, as it has no appreciable proper motion. The present 

 limits of error are in the neighbourhood of a few hundredths 

 of a second. Sir David Gill sets the distance of Canopus at a 

 minimum of two hundred and ninety-six light-years. It is 

 apparently situated among stars of the eighth magnitude, if 

 not farther. In order to shine at its present brilliancy from 

 this distance, it must have ten or fifteen thousand times the 

 luminosity of our sun. At this distance our sun would long 

 have ceased to be visible to the naked eye. 



Canopus is, again, a solar type of star ; its density and 

 temperature is, then, somewhere near the sun's. Consider the 

 inference. At the same temperature the amount of light shed 



