10 



KNOWLEDGE. 



[Januaby 1, 1895. 



brighter stars have spectra of the Sirian type. Most of the 

 stars in the Pleiades have a very similar proper motion, 

 both in direction and amount, and there can be no 

 doubt that they form a connected system. The superior 

 brilliancy of the stars composing the llyades would 

 indicate that they are nearer to the earth than the Pleiades 

 group, and they may possibly form members of the " solar 

 cluster.'' 



Assuming that the distances are inversely proportional 

 to the proper motions, Prof. Kapteyn computes the relative 

 volumes of the spherical shells which contain the stars 

 with different proper motions (from one-tenth of a second 

 to one second of arc, and more). Comparing these volumes 

 with the corresponding number of stars, we arrive at an 

 estimate of the density of star distribution at various 

 distances. The result of this calculation shows that the 

 distribution of stars of the Sirian type approaches uni- 

 formity when a large number of the faint stars (the ninth 

 magnitude! are considered. With reference to stars of 

 the second type, however, the larger the proper motion 

 the greater the number of the stars, or, in other words, 

 the second type or solar stars are crowded together in the 

 sun's vicinity. Evidence in favour of this conclusion is 

 afibrded by the fact that of eight stars having the largest 

 measured parallax (and whose spectrum has been deter- 

 mined), I find that seven have spectra of the solar type. 

 The exception is Sirius, which is evidently an exceptional 

 star with reference to its brightness and comparative 

 proximity to our system, no other star of the first magni- 

 tude having nearly so large a parallax. Indeed, the 

 average distance of all the first magnitude stars in the 

 heavens has been found to be over forty times the distance 

 of Sirius. 



Prof. Kapteyn finds that the centre of greatest con- 

 densation of the solar type stars lies near a point situated 

 about ten degrees to the west of the great nebula in 

 Andromeda, and that this centre nearly coincides with the 

 point which, according to Struve and Herschel, represents 

 the apparent centre of the Milky Way considered as a ring. 

 This would indicate that the sim and solar system lie°a 

 little to the north of the plane of the Milky Way, and 

 towards a point situated in the northern portion "of the 

 constellation of the Centaur. The fact is worth noting 

 that the nearest fixed star to the earth. Alpha Centaurf, 

 lies not very far from this point. Possibly there may be 

 other stars in this direction having a measurable parallax. 

 The southern portion of the heavens has not yet been 

 thoroughly explored. 



Prof. Kapteyn finds that, for stars of equal brightness, 

 those of the Sirian type are on an average about two and 

 three-quarter times further from the earth than those of 

 the solar type. As light varies inversely as the square of 

 the distance, this would imply that the' Sirian stars are 

 intrinsically over seven times brighter than those of the 

 solar type. This conclusion is confirmed by the great 

 brilliancy of Sirius, and other stars of the same type, in 

 proportion to their mass. I have sho'ira elsewhere that 

 Sirius is about forty times brighter than the sun would be 

 if placed at the same distance, although its mass is only 

 twice the mass of the sun, as computed from the orbit of 

 its satellite. 



The general conclusions to be derived from the above 

 results seem to be that the sun is a member of a cluster 

 of stars possibly distributed in the 'form of a ring, and that 

 outside this ring, at a much greater distance from us than 

 the stars of the solar cluster, lies a considerably richer 

 ring-shaped cluster, the light of which, reduced to nebulosity 

 by immensity of distance, produces the Milky Way gleam 

 of our midnight skies. 



THE NEW SOLAR RECORDS. 



By E. Walter Maunder, Hnn. Sec, B.A.S. : Fresident, 

 British Astionoiiiicdl Association : Superintendent of the 

 Pliysical ]>epartment, Roi/iil Obsereatori/, (rreenwich. 



FltOM the point of view of astronomical physics, no 

 celestial body demands so much study as the sun, 

 and none repays it better. A star amongst the 

 stars, it reveals to us at a comparatively near view 

 the lines upon which general stellar anatomy is 

 constructed. The most vigorous and active member of the 

 solar system, it supplies hints to us of what in the distant 

 past was the condition of our own world and of the other 

 members of the planetary family. And as the great source 

 and maintainer of nearly all the energy of the solar system, 

 it would be an object of surpassing interest, even if it had 

 nothing to reveal to us of any other body in the universe. 

 And so, from the very first invention of the telescope, 

 solar physics have formed an important department of 

 astronomy. But the present century has been specially 

 rich in persevering and devoted observers. Schwabe, who 

 discovered the periodicity of the sunspots ; Wolf, who 

 carried Schwabe's record onward, and followed it backward, 

 so as to extend tlie sunspot curve over two centuries and a 

 half; Carringtou, who determined the solar rotation period 

 and its change with the latitude, and corrected the elements 

 of the solar axis ; Spoerer, who brought out the relation 

 of the distribution in latitude of sunspots to the phases 

 of the sunspot cycle ; these stand out as leading names 

 amongst those who used the visual method of observing, 

 or who discussed the results obtained by that method. 



To Warren De la Rue we owe the important advance 

 of the employment of photography for obtaining records of 

 the solar surface. Great as is the skill to which many 

 observers have -attained in drawing sunspots, and though 

 the best drawings still outstrip all but two or three very 

 exceptional photographs as to the delicacy of the detail 

 they show, still the photographs much surpass the drawings 

 as to the fulness and the accuracy of their record. 



These earlier photographs, and those still taken at 

 Greenwich and elsewhere, were of course taken by means 

 of the general light of the sun — chiefiy by means of the 

 blue and violet rays. It is substantially a picture of the 

 photosphere that they give us, precisely as eye drawings do. 

 But the ingenuity of Mr. Hale and ]M. Deslandres has 

 recently supplied us with a new class of solar photographs — 

 photographs by monochromatic light : photographs which 

 no longer give us a record of the solar photosphere, of that 

 incandescent cloud surface from which the white light 

 of the sun proceeds, but of certain specific luminous gases 

 in the solar atmosphere. 



The principle upon which the instruments for obtaining 

 these new records are based is sufficiently simple, the 

 necessary mechanical adjustment much more dilMcult. 

 Substantially it amounts to this : A spectroscope is 

 arranged, having not merely a slit in the focus of its 

 collimator, but another in the focus of its viewing telescope. 

 The second slit can be so arranged as to correspond to the 

 place of any selected line in the spectrum. Now, if a 

 photographic plate be placed behind the second slit, it will 

 receive from the sun light only of the wave-length 

 selected. 



In the ordinary form of photo-heliograph the exposure is 

 usually given by drawing a narrow slit rapidly across the 

 image of the sun in the primary focus, and the various 

 portions of the sun, exposed in this way in rapid succession, 

 build up a complete image on the sensitive plate. In the 

 spectro-heliograph, as just said, the light of one particular 



