August i, 1907] 



NA TURE 



325 



I star giving from 100,000 to 10,000 



26 stars ,, 10,000 ,, 1,000 I «J^ 1- 



iiiOO >, ., 1,000 ,, 100 -"o 3 



22,000 „ ,, 100 ,, 10 S JZ ^ 



140,000 ,, ,, 10 ,, I .§ 1=" o 



430,000 ,, ,, I ,, OT H 



650,000 ,, ,, O'l ,, O'OI.' 



The Density of Stellar Distrihidioii at Different Distances 

 from our Sun. 

 Consider, lastly, the distribution of stellar density, that 

 is. the number of stars contained in the unit of volume. 



We cannot determine absolute star-density, because, for 

 example, some of the stars which we know from their 

 measured paralla.\es to be comparatively near to us are 

 in themselves so liule luminous that if removed to even 

 a few light-years greater distance they would appear 

 fainter than the ninth magnitude, and so fall below the 

 magnitude at which our data at present stop. 



But if we assume that intrinsically faint and bright 

 stars are distributed in the same proportion in space, it 

 will be evident that the comparative richness of stars in 

 any part of the system will be the same as the com- 

 parative richness of the same part of the system in stars 

 of a particular luminosity. Therefore, as we have already 

 found the arrangement in space of the stars of different 

 degrees of luminosity, and consequently their number at 

 different distances from the Sun, we must also be able 

 to determine their relative density for these different 

 distances. 



Kapteyn finds in this way that, starting from the Sun, 

 the star-density (i.e., the number of stars per unit volume 

 of space) is pretty constant until we reach a distance of 

 some 200 light-years. Thence the .density gradually 

 diminishes until, at about 2500 light-years, it is only about 

 one-fifth of the density in the neighbourhood of the Sun." 

 This conclusion must, however, be regarded as uncertain 

 until we have by independent means been enabled to 

 estimate the absorption of light in its course through 

 interstellar space, and obtained proof that the ratio of 

 intrinsically faint to bright stars is constant throughout 

 the universe. 



Thus far Kapteyn "s researches deal with th.- stellar 

 universe as a whole ; the results, therefore, represent only 

 the im-iii! conditions of the system. The further develop- 

 ment of our knowledge demands a like study applied to 

 the several portions of the universe separately. This will 

 require much more extensive material than we at present 

 possess. 



As a first further approximation the investigation will 

 have to be applied separately to the Milky Way and the 

 parts of the sky of higher gal.actic latitude. The velocity 

 and direction of the Sun's motion in space may certainly 

 be treated as constants for many centuries to come, and 

 these constants may be separately determined from groups 

 of stars of various regions, various magnitudes, various 

 proper motions, and various spectral types. If these con- 

 stants as thus separately determined are different, the 

 differences which are not attributable to errors of observ- 

 ation must be due to a common velocity or direction of 

 motion of the group or class of star to which the Sun's 

 velocity or direction is referred. Thus, for example, the 

 Sun's velocity as determined by spectroscopic observations 

 of motion in the line of sight appears td be sensibly 

 smaller than that derived from fainter stars. The ex- 

 planation appears to be that certain of the brighter stars 

 form part of a cluster or group of which the Sun is a 

 member, and these stars tend to some extent to travel 

 together. For these researches the existing material, 

 especially that of the determination of velocities in the 

 line of sight, is far too scanty. 



Kapteyn has found that stars the proper motions of which 

 exceed o"o5 are not more numerous in the Milky Way 

 than in other parts of the sky ; - in other words, if only 

 the stars having proper motions of o"o5 or upwards were 

 mapped there would be no aggregation of stars showing 

 the existrnce of a Milky Way. 



The proper motions of stars of the second spectral type 

 are, as a rule, considerably larger than those of the first 

 ^ Publications AsTon, T ab.. Oroningen, No. 11. 

 - Ver'. Kn. Akad. Amster'am, January. 1893. 



NO. 1970. VOL. 76] 



type ; but Kapteyn comes to the conclusion that this 

 difference does not mean a real difference of velocity, 

 but only that the second-type stars have a smaller 

 luminosity, the mean difference between the two types 

 amounting to 2^ magnitudes.' 



The Future Course of Research. 

 In the last Address delivered from this Chair on an 

 astronomical subject, Sir William Huggins, in 1891, dealt 

 so fully with the chemistry of the stars that it seemed 

 fitting on the present occasion to consider more especially 

 the problem of their motion and distribution in space, as 

 it is in this direction that the most striking advances in 

 our knowledge have recently been made. It is true that 

 since 1891 great advances have also been made in our 

 detailed knowledge of the chemistry of the Sun and 

 stars. The methods of astro-spectrography have been 

 greatly improved, the precision of the determination of 

 motion in the line of sight greatly enhanced, and many 

 discoveries made of those close double stars, ordinarily 

 termed spectroscopic doubles, the study of which seems 

 destined to throw illustrative light upon the probable 

 history of the development of systems from the original 

 nebular condition to that of more permanent systems. 



But the limitations of available time prevent me from 

 entering more fully into this tempting field, more especially 

 as it seems desirable, in the light of what has been said, 

 to indicate the directions in which some of the astro- 

 nomical work of the future may be most properly 

 systematised. There are two aspects from which this 

 question may be viewed. The first is the more or less 

 immediate extension of knowledge or discovery ; the second 

 the fulfilment of our duty, as astronomers, to future 

 generations. These two aspects should never be entirely 

 separated. The first, as it opens out new vistas of re- 

 search and improved methods of work, must often serve 

 as a guide to the objects of the second. But the second 

 is to the astronomer the supreme duty, viz., to secure 

 for future generations those data the value of which grows 

 by time. 



As the result of the Congress of Astronomers held at 

 Paris in 1887 some sixteen of the principal observatories 

 in the world are eng.aged, as is well known, in the 

 laborious task, not only of photographing the heavens, 

 but of measuring these photographs and publishing the 

 relative positions of the stars on the plates down to the 

 eleventh magnitude. .\ century hence this great work 

 will have to be repeated, and then, if we of the present 

 day have done our duty thoroughly, our successors will 

 have the data for an infinitely more complete and thorough 

 discussion of the motions of the sidereal system than any 

 that can be attempted to-day. But there is still needed 

 the accurate meridian observation of some eight or ten 

 stars on each photographic plate, so as to permit the 

 conversion of the relative star-places on the plate into 

 absolute star-places in the heavens. It is true that some 

 of the astronomers have already made these observations 

 for the reference stars of the zones which they have 

 undertaken. But this seems to be hardly enough. In 

 order to coordinate these zones, as well as to give an 

 accuracy to the absolute positions of the reference stars 

 corresponding with that of the relative positions, it is 

 desirable that this should be done for all the reference 

 stars in the sky by several observatories. The observ- 

 ations of well-distributed stars by Kustner at Bonn pre- 

 sent an admirable instance of the manner in which the 

 work should be done. Several observatories in each hemi- 

 sphere should devote themselves to this work, employing 

 the same or other equally efficient means for the elimin- 

 ation of sources of systematic error depending on magni- 

 tude, &c., and it is of far more importance that we should 

 have, say. two or three observations of each star at 

 three different observatories than two or three times as 

 many observations of each star made at a single observ- 

 atory. 



The southern cannot boast of a richness of Instrumental 

 and personal equipment comparable with that of the 

 northern hemisphere, and consequently one welcomes with 

 enthusiasiTi the proposal on the part of the Carnegie 

 Institute to establish a meridian observatory in a suitable 

 1 /-/■/,f.,Ayri', iSc2. 



