August 16, 1907] 



SCIENCE 



203 



and other systems may be regarded as the 

 supreme achievement of practical astron- 

 omy. So great are the diificulties of the 

 problem, so minute the angles involved, 

 that it is but in comparatively recent years 

 that any approximate estimate could be 

 formed of the true parallax of any fixed 

 star. Bradley felt sure that if the star y 

 Draconis had a parallaji of 1" he would 

 have detected it. Henderson by "the 

 minute sifting of the numerical results" 

 of his own meridian observations of a Cen- 

 tauri, made at the Cape of Good Hope in 

 1832-33, first obtained certain evidence of 

 the measurable parallax of any fixed star. 

 He was favored in this discovery by the 

 fact that the object he selected happened 

 to be, so far as we yet know, the nearest 

 sun to our oavu. Shortly afterwards 

 Struve obtained evidence of a measurable 

 parallax for a Lyrse and Bessel for 61 

 Cygni. Astronomers hailed with delight 

 this bursting of the constraints which our 

 imperfect means imposed on research. 

 But for the great purposes of eosmieal as- 

 tronomy Avhat we are chiefly concerned to 

 know is not what is the parallax of this 

 or that particular star, but rather what is 

 the average parallax of a star having a 

 particular magnitude and proper motion. 

 The prospect of even an ultimate approxi- 

 mate attainment of this knowledge seemed 

 remote. The star a Lyrse is one of the 

 brightest in the heavens; the star 61 Cygni 

 one that had the largest proper motion 

 known at the tirae ; whilst a, Centauri is 

 not only a very bright star, but it has also 

 a large proper motion. The parallaxes of 

 these stars must therefore in all probability 

 be large compared with the parallax of the 

 average star; but yet to determine them 

 with approximate accuracy long series of 

 observations by the greatest astronomers 

 and with the finest instruments of the day 

 seemed necessary. 



Subsequently various astronomers in- 

 vestigated the pai'allaxes of other stars 

 having large proper motions, but it was 

 only in 1881, at the Cape of Good Hope, 

 that general research on stellar parallax 

 was instituted.^- Subsequently at Tale 

 and at the Cape of Good Hope the work 

 was continued on eosmieal lines with larger 

 and improved heliometers.^^ By the intro- 

 duction of the reversing prism and by other 

 practical refinements the possibilities of 

 systematic error were eliminated, and the 

 accidental errors of observation reduced 

 within very small limits. 



These researches brought to light the im- 

 mense diversity in the absolute luminosity 

 and velocity of motion of different stars. 

 Take the following by way of example: 



Our nearest neighbor amongst the stars, 

 a, Centauri, has a parallax of 0".76, or is 

 distant about 4% light-years. Its mass is 

 independently known to be almost exactly 

 equal to that of our sun; and its spectrum 

 being also identical with that of our sun, 

 w« may reasonably assume that it appears 

 to us of the same magnitude as would our 

 sun if removed to the distance of oa 

 Centauri. 



But the average star of the same ap- 

 parent magnitude as a, Centauri was found 

 to have a parallax of only 0".10, so that 

 either a, Centauri or our sun, if removed 

 to a distance equal to that of the average 

 fixed star of the first magnitude, would 

 appear to us but little brighter than a star 

 of the fifth magnitude. 



Again, there is a star of only 8^ magni- 

 tude^* which has the remarkable annual 

 proper motion of nearly 8f seconds of arc 

 —one of those so-called runaway stars — 



^Mem. R.A.8., Vol. XL VIII. 



"Annals of the Cape Observatory, Vol. VIII., 

 part 2, and Trans. Astron. Ohservatory of Yale 

 University, Vol. I. 



"Gould's Zones, V" 243. 



