September 9, 1915] 



NATURE 



43 



of sidereal astronomy may seem to have gone on 

 slowly for a time. The more rapid progress of recent 

 years arises from the accumulation of data, for which 

 we are indebted to generations of astronomers, and 

 from the gradual increase in power and perfection of 

 our instruments. 



The first insight into the stars as a whole naturally 

 came from the survey of their numbers and distribu- 

 tion ; and Herschel, who constructed the first great 

 telescopes, explored the heavens with untiring skill 

 and energy, and speculated boldly on his observations, 

 is justly regarded as the founder of sidereal astronomy. 

 In his great paper "On the Construction of the 

 Heavens," Herschel gives the rules by which he was 

 guided, which I should like to quote, as they may well 

 • rve as a motto to all who are engaged in the ob- 

 servational sciences : — 



" But first let me mention that if we would hope to 

 make any progress in an investigation of this delicate 

 nature we ought to avoid two opposite extremes of 

 which I can hardly say which is the most dangerous. 

 If we indulge a fanciful imagination and build worlds 

 of our own, we must not wonder at our going wide 

 from the path of truth and nature ; but these will 

 vanish like the Cartesian vortices, that soon gave way 

 when better theories were offered. On the other hand, 

 if we add observation to observation, without attempt- 

 ing to draw not only certain conclusions but also con- 

 jectural views from them, we offend against the very 

 end for which only observations ought to be made. 

 I will endeavour to keep a proper medium ; but if I 

 should deviate from that I could wish not to fall into 

 the latter error." In this spirit he discussed the "star 

 gauges " or counts of stars visible with his great re- 

 flector in different parts of the sky, and concluded 

 from them that the stars form a cluster which stretches 

 to an unknown but finite distance, considerably greater 

 in the plane of the Milky Way than in the per- 

 pendicular direction. He gave this distance as 497 

 times that of Sirius. He did not hesitate to advance 

 the theory that some of the nebulae were similar 

 clusters of stars, of which that in Andromeda, judging 

 from its size, was the nearest. Herschel had no 

 means of telling the scale of the sidereal system, 

 though he probably supposed the parallax of Sirius 

 to be of the order of i". 



Though some of the assumptions made by Herschel 

 are open to criticism, the result at which he arrived 

 is correct in its general outline. I shall attempt to 

 give a brief account of some of the principal methods 

 used to obtain more definite knowledge of the extent 

 and constitution of this " island universe." The stars 

 of which most is known are, in general, those nearest 

 to us. If the distance of a star has been measured. Its 

 co-ordinates, velocity perpendicular to the line of sight, 

 and luminosity are easily found. In the case of a 

 double star the orbit of which is known the mass may 

 also be determined. But only a very small proportion 

 of the stars are sufficiently near for the distance to be 

 determinable with any accuracy. Taking the distance 

 corresponding to a parallax of 1" or the parsec as 

 unit — i.e. 200.000 times the distance of the earth from 

 the sun — fairly accurate determinations can be made 

 up to a distance of 25 parsecs, but only rough ones 

 for greater distances. 



For much greater distances average results are ob- 

 tainable from proper motions, and the mean distances 

 of particular classes of stars — for Instance, stars of a 

 given magnitude or given type of spectrum — can be 

 found with confidence up to a distance of 500 parsecs, 

 and with considerable uncertainty to twice this dis- 

 tance. The density of stars In space as a function of 

 the distance, the percentage of stars within different 

 limits of luminosity, the general trend of the move- 



NO. 2393, VOL. 96] 



ments of stars and their average velocities can also be 

 found, within the same limits of distance^ 



For all distances, provided the star is sufficiently 

 bright, its velocity to or from the earth can be 

 measured. The general consideration of these veloci- 

 ties supplies complementary data which cannot be ob- 

 tained from proper motions, and confirms other results 

 obtained by their means. For distances greater than 

 1000 parsecs our knowledge Is generally very vague. 

 We have to rely on what can be learned from the 

 amount and colour of the light of the stars, and from 

 their numbers in different parts of the sky. 



Varallax. 



Let us begin with the portion of space nearest to 

 us, within which the parallaxes of stars are determin- 

 able. The successful determination of stellar parallax 

 by Bessel, Struve, and Henderson In 1838 was a land- 

 mark In sidereal astronomy. The distances of three 

 separate stars were successfully measured, and for the 

 first time the sounding line which astronomers had for 

 centuries been throwing into space touched bottom. 

 The employment of the heliometer which Bessel Intro- 

 duced was the main source of our knowledge of the 

 distances of stars until the end of the nineteenth 

 century, and resulted in fairly satisfactory determina- 

 tion of the parallaxes of nearly one hundred stars. 

 For the part of space nearest to us this survey is 

 sufficiently complete for us to Infer the average dis- 

 tances of the stars from one another — 2\ to 3 parsecs. 

 The parallax determinations of double stars of known 

 orbits lead to the result that the masses of stars have 

 not a very great range, but lie between forty times and 

 one-tenth of the mass of the sun. 



When the absolute luminosities of the stars the dis- 

 tances of which have been measured are calculated, it 

 is found that, unlike the masses, they exhibit a very 

 great range. For example, Sirius radiates forty-eight 

 times as much light as the sun, and Groombrldge 34 

 only one-hundredth part. This does not represent 

 anything like the complete range, and Canopus, for 

 example, may be ten thousand times as luminous as 

 the sun. But among the stars near the solar system, 

 the absolute luminosity appears to vary with the type 

 of spectrum. Thus Sirius, of type A, a blue hydrogen 

 star, is forty-eight times as luminous as the sun ; Pro- 

 cyon of type F5 — bluer than the sun, but not so blue 

 as Sirius — ten times ; a Centaurl, which Is nearly of 

 solar type, is twice as luminous. 61 Cygnl of type 

 K5 — redder than the sun — one-tenth as luminous ; 

 while the still redder star of type Ma, Gr 34, is only 

 one-hundredth as luminous. In the neighbourhood of 

 the solar system one-third of the stars are more 

 luminous and two-thirds less luminous than the Sun. 

 The luminosity decreases as the type of spectrum 

 changes from A to M, i.e. from the blue stars to the 

 red stars. 



These three results as to the density in space, the 

 mass, and the luminosity have been derived from a 

 very small number of stars. They show the great 

 value of accurate determinations of stellar parallax. 

 So soon as the parallax is known, all the other 

 observational data are Immediately utilisable. At the 

 commencement of the present century the parallaxes 

 of perhaps eighty stars were known with tolerable 

 accuracy. Happily the number is now rapldlv Increas- 

 ing by the use of photographic methods. Within the 

 last year or two, the parallaxes of nearly two hundred 

 stars have been determined and published. This vear 

 a committeee of the .American Astronomical Society, 

 under the presidency of Prof. Schleslnger, has been 

 formed to co-ordinate the work of six or seven 

 American and one or two English observatories. The 

 combined programme contains iioo stars, of which 



