RECENT ADVANCES IN SCIENCE 535 



have an angular diameter of only o"-oo7, and a linear diameter 

 only twice that of the sun. Similarly, Canopus and a Centauri 

 have small angular diameters. In fact, if the diameter of a 

 star exceeds o"'Oi, we must have m — j < — 0*3, and no star 

 of solar or earlier spectral type satisfies this relationship. All 

 the stars with the largest apparent diameters must therefore 

 be sought amongst the red stars, which therefore become objects 

 of considerable interest. Russell's calculations indicate that 

 the stars which should have the largest angular diameters are 

 a Orionis (Betelgeuse), a Scorpii (Antares), 7 Crucis, a Tauri 

 (Aldebaran), and /S Gruis. For Betelgeuse he finds o"-o3i, 

 which is of the same order of magnitude, though smaller, than 

 the o"'045 measured at Mount Wilson. 



Similar conclusions have been reached by Wilsing at Pots- 

 dam, who has determined the temperatures of a number of 

 stars from the energy distribution in their spectra and so been 

 able to derive their surface brightness. Eddington, in his 

 Presidential Address to Section B of the British Association 

 (" The Internal Constitution of the Stars," Observatory , 43, 341, 

 1920), gave a theoretical determination of the angular diameter 

 of several stars, based on somewhat similar considerations ; 

 the following table summarises his values for the probable 

 angular diameters of stars of various spectral types and visual 

 magnitudes : 



The large values for the bright red stars (type M) will be 

 noticed. Eddington agrees with Russell in placing Betelgeuse 

 and Antares at the top of the list, finding "'051 for the former, 

 a somewhat larger value than given by Russell. Wilsing finds 

 for Betelgeuse "'042, the determination being based upon a 

 measurement of its effective temperature. The values are in 

 very close agreement with the observed value. 



The first result to be obtained confirms the theroetical basis 

 on which the calculated values are based, and is therefore of 

 the greatest value. It is probable that further results will be 

 announced before long, and it is to be hoped that the progress 

 in our knowledge in this direction will be as rapid as the recent 

 progress in our knowledge of the distances of stars. 



The following is a selection from recent papers : 



Jeffreys, H„ Tidal Friction in Shallow Seas, Proc. R.S., A. 221, 239, 1920. 

 Sampson, R. A., On Clock Errors and Wireless Time Signals, M.N., R.A .S., 

 81, 89, 1920. 



