42 



SECTIONAL ADDRESSES. 



Betelgeuse, diameter '051". Next to it comes Antares, '043". Other 

 examples are Aldebaran '022", Arcturus "020", Pollux '013". Sirius 

 comes rather low down with diameter '007". The following table may 

 be of interest as showing the angular diameters expected for stars of 

 various types and visual magnitudes: — 



Prohahle Angular Biametefs of Sfni-s. 



However confidently we may believe in these values, it would be 

 an immense advantage to have this first step in our deductions placed 

 beyond doubt. If the direct measurement of these diameters could be 

 made with any accuracy it would make a wonderfully rapid advance 

 in our knowledge. The prospects of accomplishing some part of this 

 task are now quite hopeful. We have learnt with gi'eat interest this 

 year that work is being carried out by interferometer methods with the 

 100-inch reflector at Mount Wilson, and the results are most promising, 

 At present the method has only been applied to measuring the separation 

 of close double stars, but there seems to be no doubt that an angular 

 diameter of "05" is well within reach. Although the great mirror is 

 used for convenience, the interferometer method does not in principle 

 require great apertures, but rather two small apertures widely separated 

 as in a range-finder. Prof. Hale has stated, moreover, that success- 

 ful results were obtained on nights of poor seeing. Perhaps it would 

 be unsafe to assume that ' poor seeing ' at Mount Wilson means quite 

 the same thing as it does for us, and I anticipate that atmospheric 

 disturbance will ultimately set the limit to what can be accomplished. 

 But even if we have to send special expeditions to the top of one of the 

 highest mountains in the world the attack on this far-reaching problem 

 must not be allowed to languish. 



I spoke earher of the radiation-pressure exerted by the outflowing 

 heat, which has an important effect on the equilibrium of a star. It is 

 quite easy to calculate what proportion of the weight of the material 

 is supported in this way ; it depends neither on the density nor opacity, 

 but solely on the star's total mass and on the molecular weight. No 

 astronomical data are needed ; the calculation involves only fundamental 

 physical constants found by laboratory researches. Here are the 

 figures, first for average molecular weight 3'0: — 



For mass i x Sun, fraction of weight supported by radiation- 

 pressure = '044. 



For mass 5 x Sun, fraction of weight supported by radiation- 

 pressure ='457. 



For molecular weight 5'0 the corresponding fractions are •182 and 

 '645. 



