OCTOBKE 1, 1915] 



SCIENCE 



441 



ical statistics has been given by Professor 

 Sebwarzsehild. His investigations are of 

 the greatest value in showing the exact de- 

 pendence of the density, luminosity and 

 velocity laws on the statistical facts which 

 can be collected from observation. The 

 many interesting statistical studies which 

 have been made are liable to be rather be- 

 wildering without the guidance furnished 

 by a general mathematical survey of the 

 whole position. 



When the proper motions are considered 

 in relation to the spectral types of the stars, 

 the small average velocities of the hydrogen 

 stars and still smaller ones of the helium 

 stars found from line-of -sight observations 

 are confirmed. If stars up to a definite 

 limit of apparent magnitude, say, to 6.0 m., 

 or between certain limits, say 8.0 m. and 

 9.0 m., are considered, then the solar stars 

 are found to be much nearer than either the 

 red or the blue stars. Thus both red and 

 blue stars must be of greater intrinsic lu' 

 minosity than the solar stars. As regards 

 blue stars, this agrees with results given by 

 parallax observations. But the red stars 

 appear to consist of two classes, one of great 

 and one of feeble luminosity, and it does 

 not seem that a sufficient explanation is 

 given by the fact that a selection of stars 

 brighter than any given apparent magni- 

 tude will include the very luminous stars 

 which are at a great distance, but only 

 such stars of feeble luminosity as are very 

 near. 



The significance of these facts was 

 pointed out by Professor Hertzsprung and 

 Professor Russell. They have a very im- 

 portant bearing on the question of stellar 

 evolution, a, subject for discussion at a later 

 meeting this week. From the geometrical 

 point of view of my address these facts are 

 of importance in that they help to classify 

 the extraordinarily large range found in 

 the luminosities of stars. Putting the 



matter somewhat broadly, the A stars, or 

 hydrogen stars, are on the average intrin- 

 sically 5 magnitudes brighter than the sun, 

 whilst the range in their magnitudes is such 

 that half of them are within | magnitude 

 of the mean value. The stars of type M, 

 very red stars, are of two classes. Some of 

 them are as luminous as the A stars, and 

 have a similar range about a mean value 5 

 magnitudes brighter than the sun. Others, 

 on the contrary, have a mean intrinsic 

 brightness 5 magnitudes fainter than the 

 sun and with the same probable deviation 

 of f magnitude. Between the types M and 

 A there are two classes the distance apart 

 of which diminishes as the stars become 

 bluer. The facts in support of this conten- 

 tion are very forcibly presented by Pro- 

 fessor Russell in Nature in May, 1914. If 

 this hypothesis is true, and it seems to me 

 there is much to be said in its favor, then 

 the apparent magnitude combined with the 

 type of spectrum will give a very fair ap- 

 proximation to the distances of stars which 

 are too far away for their proper motions 

 to be determinable with accuracy. 



In dealing with the proper motions of the 

 brighter stars, the sky has been considered 

 as a whole. Now that the direction and 

 amount of the solar motion are known, we 

 may hope that, as more proper motions be- 

 come available, the different parts of the 

 sky will be studied separately. In this way 

 we shall obtain more detailed knowledge of 

 the streaming, and also of the mean dis- 

 tances of stars of different magnitudes in 

 all parts of the sky, leading to a determina- 

 tion of how the density of stars in space 

 changes in different directions. A second 

 line of research which may be expected to 

 give important results is in the relationship 

 of proper motions to spectral type. There 

 is in preparation at Harvard College by 

 Miss Cannon, under Professor Pickering's 

 direction, a catalogue giving the type of 



