450 



KNOWLEDGE & SCIENTIFIC NEWS. 



[June, igo6. 



Way, the real reason is that the stars are not there, and 

 that in this region, at least, there is a real " thinning 

 out " of the stars at a certain distance from the earth. 

 It is clear that if absorption of liifht by the ether had 

 any real, or, at lea.st, appreciable, effect, it would have 

 the same effect in the direction of the Milky Way a.s in 

 that of the Galactic poles. We must, therefore, con- 

 chide that the paucity of stars near the poles of the 

 Milky Way indicates that the stars are really few in 

 number in that direction, and that here, at least, the 

 visible universe of stars is limited. And it seems hisjhly 

 [irobablc, and, indeed, we may say certain, that even 

 in the direction of the Milky Way itself the stars thin 

 out beyond a certain distance, and do not extend in- 

 definitely into space, for if they did, the Milky Way 

 would be much brighter than it is. 



Now let us consider what is the probable extent of 

 the visible universe. The faintest stars visible on photo- 

 graphs are probably about the i8th magnitude; that is, 

 about one magnitude fainter than the faintest visible in 

 the Yerkes telescope. .Assuming this magnitude, and 

 taking the sun's stellar magnitude as — 26.5, I find 

 that to reduce the sun's brightness to that of these 

 faint stars it should be removed to a distance repre- 

 sented by about 12,500 years of light travel. The sun, 

 if placed at the distance of Sirius (parallax = o".37), 

 would shine as a star of about 2.22 magnitude, or 3.8 

 magnitudes fainter than Sirius appears to us. From 

 this it follows that Sirius is about 33 times brighter than 

 the sun. Sirius might, therefore, be removed to 5.75 

 times (v 33) its present distance and still shine as a star 

 of 2.22 magnitude; and to reduce it to a star of the i8th 

 magnitude it should be removed to 8,241 times its pre- 

 sent distance. This would represent a light journey of 

 about 72,000 years. If, therefore, any of the i8th 

 magnitude stars in the Milky Way are suns similar to 

 Sirius, that is, of the same size and intrinsic luminosity, 

 they may lie at a distance of 72,000 years of light travel 

 from the earth. That is, provided that light suffers no 

 extinction in craversing this vast distance. .\nd if 

 similar to our sun, they may be at a distance of over 

 12,000 years' journey for light. 



There seems to be evidence, however, that the greater 

 portion of the light of the Milky Way does not come 

 from these faint stars, but from stars considerably 

 brighter. Mr. C. Easton finds from an examination of 

 a photographic plate of a very brilliant region of the 

 Milky Way to the south of the bright star 7 Cygni (a 

 region including 25 Cygni), that about half the total 

 light of the Milky Way in this region comes from stars 

 of the gth to the i2lh magnitude. From this he con- 

 cludes that neither the bright telescopic stars {6th to 

 qth magnitude) nor the very faint stars (12th to 14th 

 magnitude) have any great influence in producing the 

 light of the Galaxy. From an investigation of a much 

 fainter portion of the Milky Way he finds the same 

 result.* This agrees with my own computations of 

 the total brightness of starlight, which show that the 

 maximum amount of light comes from stars of the gth 

 to I2lh magnitude, t From further investigation 

 Easton thinks it " extremely probable that the great 

 majority of the fainter stars of the Milky Way — so far 

 as their existence is revealed to us by photography or 

 direct vision — are not much more distant from us than 

 the stars of the gth or 10th magnitude, at least, in the 

 regions to which our researches have extended." { 

 Professor Newcomb thinks that there is evidence to 



show that the stars of the Milky Way are probably 

 situated at a distance between 100 million and 2co 

 million times the sun's distance from the earth. These 

 distances correspond to i,57g and 3,i5g years of light 

 travel. Placed at the greater of these distances, I find 

 that the sun would be reduced in brightness to a star of 

 the 15th magnitude. 



There seems to be evidence that the faint stars of the 

 Milk-y Way have spectra of the Sirian type. Sup- 

 posing, with Easton, that the fainter stars of the Milky 

 Way are of the 12th magnitude, and, further, that they 

 are comparaijie with .Sirius in size and brightness, I 

 find that their distance would be represented by about 

 4,600 years of light travel. But Sirius is, perhaps, a 

 larger body than the average Galactic star. Its mass 

 is about two and a half times the mass of the sun, and 

 its brightness about 33 times greater. Possibly the 

 stars of the Milky Way may be much smaller. Pro- 

 fessor Kapteyn finds from an investigation of the 

 probable distances and brightness of a number of stars 

 of various magnitudes that in a volume of space con- 

 taining two millions of stars of the same luminosity as 

 the sun there would probably be about half a million 

 brighter than the sun, and about 12^ millions of smaller 

 luminosity; that is, out of a total of 15 millions of stars, 

 about 12^ millions would be smaller than our sun. 



To reduce the sun to the brightness of a star of the 

 1 2th magnitude it should be removed to a distance of 

 about 7go "light years." To reduce the following 

 stars to the 12th magnitude they should be removed to 

 the distances represented by the light years given in 

 the fifth column : — 



* " Knowledge." July, 1903. 

 " K.NOWLEDGE," August, iSgj. 



t "Knowledge," August, 1901. 



Omitting the last two stars, which seem to be smiill 

 bodies comparatively near the earth, we see that the 

 distances of the others would range from 278 to 9,300 

 years, if all were reduced in light to a star of the 12th 

 magnitude. The average of these is 2,g2i light years. 

 As these stars are of various sizes and brightness — 

 their " relative brightness " compared with the sun 

 ranging from 0.122 to 128.33*— we may, perhaps, as- 

 sume that they represent nearly all classes of stars, and 

 that the average distance of 12th magnitude stars is 

 about 3,000 light years. 



If we assume that the stars of the Milky Way are 

 much smaller than Sirius, say one-half the mass of the 

 sun, or one-fifth of the mass of Sirius, I find that the 

 distance of 12th magnitude stars would be — if of the 



• See my paper on The licUtive Brightness of Stars in Monthly 

 Notices, R.A.S,, Janii.-iry, 1905. 



