320 



DISCOVERY 



the fact that they were moving comparatively quickly ; 

 it was therefore concluded that their motion was 

 apparent because these particular stars were nearer 

 to us, just as the motion of a man walking at a distance 

 of a hundred yards is more easUy observed than one 

 walking at a distance of a mile. It was found that it 

 was not necessarily the bright stars which exhibited 

 the greatest motion, so that the assumptiom made by 

 the earlier astronomers was proved to be untenable. 



The first successful attempt to determine the distance 

 or " parallax " of a star was accomplished by Bessel 

 in the year 1837. He showed that the star 61 Cygni, 

 a star of about the fifth magnitude, had a parallax 

 of about one-third of a second of arc. This is a very 

 small quantity to measure, but it is really large as 

 regards the parallax of a star. The method he adopted 

 was exactly similar in principle to the processes of 

 surveying land at the present day. As stars are 

 immense distances away from us, a short base-line is 

 of no avail, so he employed the largest one possible. 

 This consisted of a length equal to a line joining the 

 positions of the Earth at opposite points of its orbit, 

 that is a base line of 186 millions of miles. The process 

 of observation consisted in determining at a certain 

 time the exact position of the star in question and then 

 making similar measurements six months later. The 

 apparent change of position due to the translation of 

 the observer gave a value of its distance. This method 

 ' is known as the " absolute method " and may be 

 considered one of the most difficult operations in the 

 whole field of practical astronomy. 



The method of " parallactic motion," which is the 

 apparent movement of the stars due to the velocity 

 of the solar system as a whole in space, is another 

 means of obtaining trustworthy values of the average 

 distance of a group of stars, though not of individual 

 members. 



At a later date another process for determining 

 stellar distances was evolved, called the " differential 

 method." For this the position of the star to be 

 studied was measured in relation to neighbouring 

 stars at different times of the year. 



The application of photography to astronomical 

 problems has greatly facilitated this method, because 

 by photographing the star region at the specified 

 times, the position of the star in question in relation 

 to the other stars on the plate can be measured at 

 leisure. 



In spite of the great length of the base-line, the values 

 of the paraUaxes are so small that they are extremely 

 difficult to measure. Very great care has not only to 

 be exercised in the observations themselves, but every 

 possible source of personal and instrumental error 

 has to be taken into account and eliminated as far as 

 possible. As the observations have to be spread over 



at least six months, the determination of every single 

 parallax is a lengthy process. 



The New Spectroscopic Method 



In the year 1914 the spectroscope was first applied 

 to the determination of stellar distances, and this new 

 method, now coming into more general use, was 

 originated and developed by Professor W. S. Adams 

 and other astronomers at the Mount Wilson Observa- 

 tory, in California. By its means parallaxes can be 

 obtained with comparative ease and rapiditj'. 



The spectrum of a star is obtained by passing the 

 light of a star through a prism or train of prisms 

 attached to a telescope. A star's light is thus spread 

 out into a long band of light having the colours of a 

 rainbow, extending from violet at one end through 

 indigo, blue, green, yellow, orange to red at the other 

 end. This band is the analysis of the light of the star 

 and is termed its " spectrum." 



Throughout this spectrum lines or bands can be 

 observed in different parts of it, and these by their 

 positions indicate the presence in the star of particular 

 substances such as gases, metals, etc. 



The spectra of many thousands of stars have 

 been photographed, and, astonishing to relate, these 

 spectra can be classified into so few as approximately 

 fifteen different types. The type of spectrum is 

 indicative of the temperature of the star. It is now 

 known that stars which give gaseous lines in their 

 spectra are hot, those that exhibit metallic lines are 

 cooler, and those which indicate bands are cooler still. 



Now the life of a star consists first in increasing its 

 temperature or getting hotter ; then a maximum 

 temperature is attained, after which it cools and the 

 star eventually becomes invisible. It wiU be seen, 

 therefore, that if two stars have the same temperature, 

 it is quite possible that one star may be getting hotter 

 while the other is becoming cooler. This is a vcr}' 

 common occurrence. 



The peculiarity about stars at the beginning of 

 their careers is that they are of great size or volume 

 and have very small 'density : the older they get the 

 smaller becomes their volume and the greater their 

 density. Thus young stars are very much bigger than 

 old stars, and this had led astronomers to call the 

 former " giants " and the latter " dwarfs." 



Now it is quite easy to understand that if a " giant " 

 star and a " dwarf " star of the same temperature 

 were placed at equal distances from us, the former 

 would outshine the latter on account of his greater 

 size, or in other words the " giant " would have a 

 greater " intrinsic " brightness. 



If aU the stars had the same individual intrinsic 

 brightness, then the nearer stars would be the brightest 



