360 TRANSACTIONS OF SECTION A. 



as luminous. In the neighbourhood of the Solar system one-third of the stars 

 are more luminous and two-thirds less luminous than the Sun. The luminosity 

 decreases as the type of spectrum changes from A to M, i.e., from the blue 

 stars to the red stars. 



These three results as to the density in space, the mass and the luminosity 

 have been derived from a very small number of stars. They show the 

 great value of accurate determinations of stellar parallax. As soon as the 

 parallax is known, all the other observational data are immediately utilisable. 

 At the commencement of the present century the parallaxes of perhaps 80 stars 

 were known with tolerable accuracy. Happily the number is now rapidly 

 increasing by the use of photographic methods. Within the last year or two, 

 the parallaxes of nearly two hundred stars have been determined and pub- 

 lished. This year a Committee of the American Astronomical Society, under 

 the presidency of Prof. Schlesinger, has been formed to co-ordinate the work 

 of six or seven American and one or two English observatories. The combined 

 programme contains 1,100 stars, of which 400 are being measured by more 

 than one observatory. We may expect results at the rate of two hundred a 

 year, and may therefore hope for a rapid increase of our knowledge of the 

 stars within our immediate neighbourhood. 



Velocities in the Line of Sight. 



The determination of radial velocities was initiated by Huggins in the early 

 'sixties, but reliable results were not obtained till photographic methods were 

 introduced by Vogel in 1890. Since that time further increase in accuracy has 

 been made, and the velocity of a bright star with sharp lines is determinable 

 (apart from a systematic error not wholly explained) with an accuracy of 

 ^ kilometre per second. As the average velocities of these stars are between 



10 and 20 kilometres a second, the proportional accuracy is of a higher order 

 than can be generally obtained in parallax determinations or in other data 

 of Sidereal Astronomy. A number of observatories in the United States and 

 Europe, as well as in South America, the Cape, and Canada are engaged 

 in this work. Especially at the Lick Observatory under Prof. Campbell's 

 direction, the combination of a large telescope, a well-designed spectroscope, 

 and excellent climatic conditions have been utilised to carry out a bold pro- 

 gramme. At that observatory, with an offshoot at Cerro San Christobal in 

 Chile, for the observation of stars in the Southern Hemisphere, the velocities 

 of 1,200 of the brightest stars in the sky have been determined. Among the 

 results achieved is a determination of the direction and amount of the Solar 

 motion. The direction serves to confirm the results from proper motions, but 

 the velocity is only obtainable accurately by this method. This quantity which 

 enters as a fundamental constant in nearly all researches dealing with proper 

 motion, is given by Campbell at 19'5 kilometres per second, or 4'1 times the 

 distance of the Earth from the Sun per annum, though there is some uncer- 

 tainty arising from a systematic error of unknown origin. 



Variation of their radial velocity shows that a large proportion of stars 

 are spectroscopic binaries, and the results have been discussed by Campbell 

 from the point of view of the genesis of the double stars by fission. It would 

 be somewhat outside the scope of my address to speak further of this. I have 

 already drawn attention to the fact that we derive from spectroscopic binary 

 stars a considerable part of our somewhat scanty knowledge of the masses of 

 stars. 



The observations of radial velocities have shown within what limits the 

 velocities of stars lie and have given a general idea of their distribution. The 

 most important result, and one of a somewhat surprising character, is that 

 the mean velocities of stars, the motion of the Sun being abstracted, increase 

 with the type_ of spectrum. Thus the stars of type B, the helium stars, the 

 stars of the highest temperature, have average radial velocities of only 6'5 kilo- 

 metres per second; the hydrogen stars of type A have average velocities of 



11 kilometres per second ; the Solar stars of 15 kilometres per second ; while 

 for red stars of types K and M it has increased slightly more to 17 kilometres 

 per second. Further, the few planetary nebulae — i.e., condensed nebuh-e with 

 bright line spectra — have average velocities of 25 kilometres per second. There 



