ASTRONOMY 



temperature of the outside layers which radiate 

 the heat and light which we receive. Of course 

 the temperature in the interior must be very much 

 greater. At the centre it is probably of the order 

 of one million degrees. 



In 1 9 14 H. N. Russell constructed a diagram 

 in which he plotted the stars according to their 

 absolute magnitude and spectral type. The 

 absolute magnitudes are defined to be the mag- 

 nitudes which they would appear to have were 

 they all at a distance of 32^ light-years. I have 

 constructed a similar diagram, using other data 

 which have been published more recently (Fig. 8). 

 Altogether some six hundred stars are repre- 

 sented. It is seen at once that the stars from A to 

 M fall into two very distinct groups. In the first 

 place we have a group running horizontally, the 

 absolute magnitudes lying between 1 and +2, 

 and then there is a second group running diagonally 

 downwards. The M stars are thus separated into 

 two sets entirely distinct from one another. The 

 K stars also form two sets, though the separation 

 is not so definite, whereas the G, F, and A stars 

 are intermingled. The B stars form a group by 

 themselves, and their absolute magnitudes show 

 that their luminosity is very great. The position 

 which our sun occupies in this diagram is marked 

 by a large star. 



Now if we ask how it happens that there should 



65 



