141 



Koomledge & Seientifie Neuis 



A MONTHLY JOURNAL OF SCIENCE. 



Conducted by MAJOR B. BADEN-POWELL and E. S. GREW, M.A. 



Vol. II. No. 7. 



[NEW SERIES.] 



JULY, 1905. 



SIXPENCE. 



CONTENTS. See Page VIL 



Ovir StelloLr Universe. 



Bv T. E. Heath. 



It is related by Campbell that Sir William Herschel 

 told him he had observed stars the light of which 

 must take two million years to reach thi.s earth. Did 

 Herschel grasp what this meant? It is supposed his 

 reflector could show stars to the 15th magnitude; if so, 

 he might have seen a star the light of which had been 

 travelling for two million years, provided it gave 

 438,000 times as much light as our Sun. 



Professor Seeliger thinks our Stellar Universe is in 

 shape like a thick lens, and estimates the nearest parts 

 of the Milky Way are about 4,400 light-years distant, 

 and the most remote about 9,700. Other astronomers 

 appear to have very vague ideas as to its limits. Some 

 estimate its probable radius at from 10,000 to 30,000 

 light-years. Few seem to have checked their con- 

 clusions by considering what the sun-power of stars at 

 the supposed boundary would be. 



I have drawn a section of our Stellar Universe of the 

 shape which Professor Seeliger's statistics of stellar 

 density seem to require, but I have not adopted his 

 dimensions. The maximum size I think probable in 

 light-years is indicated by circles struck from what ap- 

 pears to be the most likely present position of the Sun. 

 The drawing pretends to no accuracy of stellar distribu- 

 tion, save that I have, very roughly, made the stars 

 congregate about the Galactic Zone and the nebulfe 

 about its poles. So far astronomers are agreed, but 

 for the rest some think the stars thin out, others that 

 they get more numerous towards the boundary. 

 Probably the Milky Way is very much what it appears 

 to be, a congregation of stars, clustering here, 

 separating there; irregular forms and masses such as 

 we see in Spiral Nebulfe; nearer to us in parts, more 

 distant elsewhere, but lying generally in a great circle 

 round us. There seems to be no reason for thinking 

 the thickness is greater than the lateral extension; in- 

 deed, the rifts and openings give the opposite im- 

 pression. 



I remember how thirty or forty years ago most of 

 the stars were supposed to be almost infinitelv distant, 

 but yet revolved in some mysterious way round Alcyone. 

 No one did more by star-charting and lecturing to 

 change the general opinion about the \'isible Universe 

 than Richard Proctor. His equal surface chart of 

 324,000 stars to about gth magnitude shows the chief 

 features of the Milky Way and even the wonderful 

 complexity of its interior structure. He savs it has on 



it 1,115 lucid stars, and it covers 9 per cent, of the sky; 

 the gaps in the Milky Way cover 1.6 per cent, and 

 have on them only 20 lucid stars, whilst the remaining 

 89.4 per cent, of the sky has on it 4,715 lucid stars. 

 He concludes the 640 stars in excess of normal density 

 which we see upon the Milky Way are actually in it. 

 Professor Newcomb has shown that the circle of the 

 Milky \yay can be found within 5° of its true position 

 by the clustering of lucid stars alone, and proved that 

 about 70 per cent, of stars up to 6th magnitude and 

 about 140 per cent, up to 7th magnitude, which appear 

 to be on the Milky Way, are in it. 



Possibly there are no stars in the Milky Way greater 

 than the 3rd or 4th magnitude, and but few of them; 

 of the 5th there would be more, of the 6th a consider- 

 able number, and of the 7th and 8th very many 

 thousands. According to Profe.ssor Kapteyn, in anv 

 group of 15,000,000 stars, 13J per cent, would be about 

 one sun-power each, 3I per cent, would be more, and 

 833 per cent, would be less. If this be true, from what 

 we know of the probable number of stars of each magni- 

 tude (see Mr. Gore's estimate, " Knowledge," 1901, 

 page 178), a star of one sun-power at the Milkv Wav's 

 average distance would appear about the 12th' or i '^th 

 magnitude. 



Assuming the Sun's magnitude is - 26.4, I worked 

 out his magnitude at different distances in light-years; 

 they come thus, at 479, nth magnitude; at 759,' 12th; 

 at 1,210, 13th; at 1,910, 14th; at 3,020, isth; at 4,790, 

 i6th; at 7,590, 17th; at 12,100, ^8th: and at 19,100, 

 19th magnitude. 



Now there are very few i8th magnitude stars, and it 

 is doubtful if there are any 19th, but it will be seen that 

 if the Sun were removed to Professor Seeliger's esti- 

 rnated mean distance of the Milky Way (viz., 7,550 

 light-years), he would be reduced to the 17th magni- 

 tude. There may be 20 or 30 per cent, of' the Milky 

 Way stars as small as this, but thev are certainly 

 nothing like 83 per cent, smaller. 



Profes.sor Newcomb writes that " the bluest and 

 most luminous stars are situate mainly in the regions 

 of the Milky Way," but apparently he comes to this 

 conclusion because he thinks the Milkv Way begins 

 beyond a distance of 1,300 light-years, and considera- 

 tions based upon proper motions lead him to place 

 these stars even beyond the sphere of 3,260 light-years 



Proctor thought the Milky Way stars are for the 

 most part small. 



It seems not unreasonable to suppose that 

 tor any considerable group of stars, such as the 

 Milky Way, we can make the best estimate of probable 

 distance by assuming that none of them are much more 

 luminous than the giants of the same class we have 

 measured, and that they contain a fair proportion of the 

 stars of all sizes we have discovered in the only space 

 sphere at all thoroughly surveyed (that of 15 light-years 

 radius). 



