128 Dr. Frank Watson Dyson [April 24, 



niugnitude is 2. It may be remarked incidentally that the distance 

 of the pole star has been actually measured ; it is 20 parsecs or 

 4 million times the distance of the sun from the earth, and if it 

 were at the same distance as the sun it would appear to be 100 times 

 as bright. Then there are about 20 stars which are visible to the 

 naked eye. The following table gives the actual number of stars of 

 difiFerent magnitudes (photographic). 



Table III. — Number of Stars of Different Magnitudes (Phot.) 



18. Then again, we may separate the stars into groups according 

 to the physical characteristics revealed by the spectroscope. The 

 researches of Kapteyn, Campbell, Boss and othei's have shown, at any 

 rate for the brighter stars, remarkable relationships between the 

 distances and velocities of the stars, and the type of spectrum which 

 they manifest. It is therefore desirable to examine the proper 

 motions of stars of different spectral types separately. The spectra 

 of many thousands of stars have been determined at Harvard College, 

 under Prof. Pickering's direction, by Miss Cannon. The different 

 classes are indicated in the Harvard Classification by the letters B, A, F, 

 G, K, M, with further subdivisions. The B stars are characterized by 

 the presence of hehum ; the A stars by series of broad hydrogen 

 lines. In the F stars the hydrogen lines are thinner and fine 

 metallic lines are shown. The G- stars are very like the sun, full of 

 metallic lines, and with broad lines due to calcium. In the K stars 

 the two calcium lines are still broader and there are many fine 

 metalhc lines. The M stars are characterized by broad absorption 

 bands. A further characteristic is that, taking the stars in the order 

 M to B, their spectra extend more and more towards the violet. This 

 classification is a regular progression and places the stars in the order 

 of their temperatures — the B stars are the bluest and hottest, and the 

 M stars the reddest and coolest. 



19. The character of the spectra of about 800 of the stars in 

 Carrington's catalogue is given by the Harvard observations. For 

 the fainter stars the spectra have not been determined, but they can 

 be inferred in another way. As the blue stars are more active 

 photographically than the red stars, if a red and a blue star have 

 the same visual magnitude, the magnitudes estimated from the 

 images on a photograph will differ considerably, and this difference 

 is an index of the colour, and thus of the type of spectrum. This is 



