August 12, 1922] 



NA TURE 



219 



The Determination of Stellar Distances. 



By Dr. William J. S. Lockyer. 



IN his presidential address delivered before the Royal 

 Astronomical Society, in connexion with the cele- 

 bration of that society's centenary (see Nature, June 

 24, p. 815), Prof. Eddington referred to six great land- 

 marks of astronomical progress during that century. 

 He pointed out that this was a record of advance which 

 was continuous, and not in great waves followed by 

 periods of exhaustion. As he further remarked, the 

 centre of most rapid progress has shifted from time to 

 time and the various branches of astronomy have had 

 their ups and downs. In this second category may 

 perhaps be placed the determinations of the parallaxes 

 or distances of the stars, because quite recently a very 

 great impetus has been given to this branch of astro- 

 nomy by the introduction of a rapid and effective new 

 method. 



So long ago as 1837 the first successful attempt to 

 determine the parallax of a star was accomplished by 

 Bessel, who made his result known in the last month of 

 1838, showing that 61 Cygni had a parallax of about 

 one-third of a second of arc. Since that date this 

 research has been carried on continuously and we have 

 now catalogues of the parallaxes of a large number of 

 stars. Among the observatories measuring trigono- 

 metrical parallaxes to-day, may be mentioned Alleg- 

 heny, Dearborn, Greenwich, McCormick, Mount Wilson, 

 Oxford (Radcliffe), Swarthmore, and Yerkes, and these 

 institutions secure material which provides about three 

 or four hundred parallaxes a year. 



It is interesting to note that in the early days it was 

 thought that the brightest stars were the nearest to us, 

 and therefore attempts were first made to determine 

 their distances. It was soon found, however, that 

 estimates of distance based upon apparent magnitude 

 were wholly futile, for the greater number of the larger 

 parallaxes determined were of stars of the fifth, sixth, 

 and fainter magnitudes. 



The work of measuring the parallax of a star may be 

 considered one of the most delicate operations in the 

 whole field of practical astronomy. There are three 

 methods available. The absolute method consists in 

 making meridian observations at different times of the 

 year and then studying the resulting places after all 

 known corrections have been made. The differential 

 method may be classed under two sub-heads. The 

 first consists in measuring the position of the star to be 

 studied in relation to neighbouring stars at different 

 times of the year. If the neighbouring stars in the 

 field of view of the telescope be close to the star under 

 examination, a wire micrometer is used, but if distant, 

 the heliometer is the more efficient instrument. The 

 second differential method utilises the sensitive plate 

 and consists in photographing a star region at different 

 times and eventually measuring the positions of the 

 star in question in relation to the neighbouring stars. 



It was not until the year 1914 that the spectroscope 

 was applied to the determination of stellar distances, 

 and the method now in use is that originated and 

 developed by Prof. W. S. Adams and other astronomers 

 at the Mount Wilson Observatory in California. It is 

 based on the fact that the intrinsic brightness of a star 



NO. 2754, VOL. I IO] 



has an appreciable effect on its spectrum. Thus, if 

 two stars have the same type of spectrum but differ 

 greatly in luminosity they will probably differ greatly 

 in size, density, and in depth of their surrounding 

 gaseous atmospheres. If this be so, then their spectra 

 should exhibit variations in the intensity and character 

 of such lines as are peculiarly sensitive to the physical 

 conditions of the gases in which they find their origin, 

 in spite of the general correspondence between the two 

 spectra. If, as Prof. Adams states, " such variations 

 exist and a relationship can be derived between the 

 intensities of these lines and the intrinsic brightness of 

 the stars in which they occur, we have available a means 

 of determining the absolute magnitudes x of stars, and 

 hence their distances." 



It has been found that certain lines in stellar spectra 

 do give indications of variation with absolute magnitude, 



SPECTRA TYPES F6 <* F>. UNES <r078 Sr* 407a Fe. 



4 3 2 1 



ABSOLUTE MAGNITUDE 



Fig. 



One of the fundamental curves formed from known parallaxes (black dots 

 cf stars of spectrum types F6 and F7. 



When the intensity-difference in any star of these types has been deter- 

 mined, the absolute magnitude can be read off the curve and the parallax 

 calculated. 



and the detection of them we owe to Hertzsprunu; and 

 Adams and Kohlschiitter. 



To determine the absolute magnitudes of stars any of 

 three different sources of data can be utilised, namely, 

 the trigonometrical parallaxes, parallactic motions, or 

 proper motions. The most serviceable of these is the 

 first, and reference to this alone will be made here. 



The first step in the process is to have available a 

 classification of star spectra based on detailed measure- 

 ments of line intensities instead of on the more general 

 eye estimations, estimations which have been extremely 

 valuable up to the present time for the general classifica- 

 tion of stars but are now superseded. Such a detailed 

 classification for many of the brighter stars has been 

 made and is being rapidly extended. 



It is next necessary to construct a series of reduction 

 curves for each type or class of spectrum or for small 

 groups of types (see Fig. i). These curves are based 



