126 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



THE MODERN METHOD OF ATTACK 



The stars fall into clearly defined categories. As an unassorted 

 whole, they exhibit an enormous range in candlepower, but all the 

 stars in any one category are of approximately the same candle- 

 power, so that we can obtain a reasonably good estimate of a star's 

 distance by considering its apparent brightness in conjunction with 

 the category into which it falls. For the majority of stars, the cate- 

 gory is determined mainly by the star's spectrum, but in the case 

 of variable stars, the period of variabilit}'' is even more important 

 than the spectrum, and leads to results of far greater precision. 



To take an instance of the simplest kind, the star Sirius, which 

 looks the brightest in the whole sky, is one of those nearer stars whose 

 distance can be determined by ordinary trigonometrical methods — 

 methods which are the same in principle as those which the sur- 

 veyor uses to determine the distance of an inaccessible mountain peak. 

 The whole process is, of course, conducted on an enormously larger 

 scale; the surveyor takes a base-line a few miles long on the earth's 

 surface and finds his mountain is a few miles distant, while the as- 

 tronomer takes as his base-line the diameter of the earth's orbit round 

 the sun — a base-line 186,000,000 miles long — and finds that his star is 

 many millions of millions of miles distant. In this way he finds 

 that the distance of Sirius is 51 million million miles, or 8.65 light- 

 years. Knowing this, we can estimate the distance of all stars which 

 belong to the same category as Sirius; for example, a similar star 

 which looks 100 times less bright must be 10 times as distant, because 

 light falls off as the square of the distance. 



Variable stars provide a more reliable method of estimating 

 astronomical distances. For example, the star 8 Cephei is found by 

 the ordinary surveyor's method to be about 60 times as distant as 

 Sirius, All stars which have the same period of variability as S 

 Cephei are found to have about the same candlepower, so that again 

 their distance can be estimated from their faintness. As these 

 variable stars are enormously bright, they can be seen to immense 

 distances — hence their special value as indicators of astronomical 

 distance. 



We can test these methods in various ways. The obvious one is 

 to find a group of stars which are already known to be all at the 

 same distance, and see whether each of the stars tells the same story 

 as to the distance of the group. Such groups of stars are to be 

 found in the globular clusters, the Magellanic clouds, and in the 

 nearer of the extragalactic nebulae. 



In these last objects, even the vivid Cepheid variables look very 

 faint. Nevertheless they are visible, and their feeble brightness can 

 be measured with considerable accuracy in a large telescope. In this 



