July, 1902.] 



KNOWLEDGE 



155 



Professor Seeliger, of Munich, set himself to again try the 

 "statistical method," that which concerns itself with the 

 apparent distriljution of the stars in order to infer from 

 this their real distribntion in space.* Seeliger utilised 

 for this the enumeration which he himself made of the 

 stars observed by Argelander. Krueger and Sclioenfeld 

 (as far as mai;nitude 95 or 1(1), beside a research of 

 Celoria, present Director of the Milan 01>servatory, on 

 fainter stars, then of the gauges of the two Hers-'hels, 

 and of a work ■which the author of this paper had 

 published some years before on the distribution of stars 

 of different mairnitudes in a portion of the Milky Way. 

 There is no need for me to dwell at length on these 

 matters (see Knowledge for August, 1895). I will con- 

 line myself to giving as succinctly as possible the cou- 

 clusions to which the German astronomer has arrived iu 

 his researches. 



We know tliat the stars, taken as a whole, become more 

 numerous in iiroportion as we approach the Milky Way, 

 from which it follows that the phenomenon of the Galaxy 

 is intimately ciuinected with the distribution of the other 

 stars. Seeliger first shows that this pnjgressive increase 

 towards the plane of the Milky Way is not due to the 

 systematic en-ors in the brightness of stars, which have 

 been established in the Durchmusterung of Argelander. 

 He also shows that this progression is not regular, that 

 the stars of very feeble brightness — fainter than magni- 

 tude 11 or 11 5 — are distributed quite differently from the 

 more brilliant stars ; we find these faint stars iu only 

 relatively small numbers near the poles of the Milky 

 Way; within the Galactic zone, on the contrary, they 

 are exceedingly numerous, and the same appears to 

 hold good for stars of a still feebler brightness, such 

 as Herschel's telescope scarcely sufficed to show. All 

 those stars, which apipear to us as minute light points, 

 are, for the most part, really confined to the Milky Way. 

 Thus the stellar universe does not seem to be indefinitely 

 extended in the Galactic plane. We can imagine it to be, 

 according to these researches, of a fairly thick lens shape, 

 filled with stars which are much more densely congregated 

 near the edges than near the centre of the lens. Our sun 

 is not j)recisely at the centre, but neither is it very far 

 removed from it. The soutliern zone of the Milky Way is 

 probably less rich in stars than the northern zone. These 

 conclusions are not new, but they rest on a large amount 

 of material, and on rigid mathematical development. 

 Seeliger estimates the distance between our sun and the 

 internal border of the zone of stellar condensation (that 

 is to say the Milky Way properly so called) as 500 times 

 the distance of Sirius — the annual parallax of this star is 

 about 0"-:i~, which places it at 88 light-years, or 500,000 

 semi- diameters of the orbit of the earth — the external 

 border as 1100 times the distance of Sirius. There lie 

 the limits of the stellar system to which the sun belongs. 

 This system reckons probably (according to Seeliger) from 

 2" to 41 millions of stars down to the magnitude just 

 visible in Herschel's telescope, a magnitude that we cannot 

 fix, but which we can estimate to be of the 13th or 14th. 



As for determining more exactly the relative ])ositious of 

 the most crowded regions on the borders of the stellar 

 system — they must not be confused with clusters properly 

 so called, agglomerations of a more intimate nature — 

 Professor Seeliger makes no effort to do so. " It may be 

 a rather vague expression," he says, " to speak of an 

 annular Milky Way, but these words indicate fairly well 

 the general distribution of the stars, but nothing hinders 

 us from esteeming certain portions of the Milky Wiiy to 



•Seeliger: " Betrachtungen ueber die raumliche Vertheilung dor 

 FiMtcrne." Miinchen. 189S. 



be neai'er to us, and others more remote, than the avenige 

 stars of magnitude 95 (Argelander), whose distribution 

 in certain parts of the zone presents a remarkable analogy 

 to the situation of tlie brilliant and dark spots of the 

 Milky Way." 



Seeliger docs not admit that an ap])reciable extinction 

 of light can be made sensible in our stellar system. Such 

 an extinction, however, might easily prevent our pei'ceiving 

 other galaxies besides our own, whose existence Seeliger 

 does not presume to deny, although we have no undoubted 

 direct indication of it. 



A Russian astronomer, W. Stratonoff, of the Observatory 

 of Tashkent, in Central Asia, has taken a different method 

 of placing in evidence the apparent distribution of the 

 stars.* He follows the cartogi-aphic method in continuation 

 of the work of the Italian astronomer, Giovanni Sshiaparelli. 

 It was Richard Proctor who especially demonstrated the 

 use of charting iu astronomy, and who drew from it im- 

 portant conclusions. Schiaj)arelli and Stratonoff not only 

 insert on their charts the stars themselves, but for each order 

 of brightness (magnitude — 6 or 6 5, 66 — 7, 7"1 — 75, 

 itc.) the relative densities of the stars contained in Arge- 

 lander's catalogue. Then they join by lines portions of 

 equal density, and they obtain charts resembling topo- 

 graphic charts on the earth where the contour-lines mark 

 out the greater or less elevation of the surface. The 

 |)ortions of the sky comprised between these lines are 

 therefore regions where the stellar density is very nearly the 

 same, and for greater clearness these regions are tinted in 

 colours of a depth the more pronounced in proportion as 

 the density is greater ; so that the region of the deepest 

 tint has, for a certain stellar class, an extreme density, 

 whilst, on the other hand, regions where the densities are 

 not up to (he mean density are left quite white. 



It goes without saying that such charts are very useful 

 for showing at a glance where the stars of a certain mag- 

 nitude are more or less strongly condensed, and that a 

 comparison of the charts will teach us whether there ia 

 anything systematic in the manner iu which stars are con- 

 densed iu certain regions — as, for example, in the Galactic 

 zone, according as we take into consideration stars of a 

 feeltler brightness. Schiaparelli, in a work published in 

 1889, took into consideration both hemispheres, but ho 

 did not go below the magnitude t5'5 — that is to say, almost 

 the limit of stars visible to the naked eye. Stratonoff goes 

 much farther, down to9'5th magnitude of Argelander, but, 

 on the other hand, he has had to confine himself to the 

 northern heavens. The results of his research are not the 

 less interesting. 



We will enter now into the details of the stellar distri- 

 bution of which Seeliger alone has studied the general 

 features. An inspection of Stratonoff's charts shows us 

 at once that the stars do not cluster towards the entire 

 zone of the Milky Way continuous and uninterrupted as 

 we see it with the naked eye, but principally towards 

 certain parts in this bright band. It is as if the stars 

 were attracted particularly towards this region of the 

 Galaxy, and were repelled from that region. For the 

 brilliant stars these places of condensation in the Milky 

 Way are not constant ; thus the chart of the stars of the 

 class 0^65 show other maxima than those of the class 

 66 — 7-0; but soon— after about passing the 8tli magni- 

 tude — the condensation of stars of an intermediate 

 brilliancy preserves in general the same configuration up 

 to the limit of Argelander's catalogue. We find for stars 

 between the 8th and 10th magnitude, maxima of con- 

 densation in Auriga and Monoceros, and a minimum in 



• W. Stratonoff : Public, de I'Obs. de Tashkent, II. ; " Etudes sur 

 la Structure dc I'Univcra." I. 1900. 



