OCTOBEB 1, 1915] 



SCIENCE 



437 



consideration of these velocities supplies 

 complementary data which can not be ob- 

 tained from proper motions, and confirms 

 other results obtained by their means. For 

 distances greater than 1,000 parsecs our 

 knowledge is generally very vague. "We 

 have to rely on what can be learned from 

 the amount and color of the light of the 

 stars, and from their numbers in different 

 parts of the sky. 



FARAULAX 



Let us begin with the portion of space 

 nearest to us, within which the parallaxes 

 of stars are determinable. The successful 

 determination of stellar parallax by Bessel, 

 Struve and Henderson in 1838 was a land- 

 mark in sidereal astronomy. The distances 

 of three separate stars were successfully 

 measured, and for the first time the sound- 

 ing line which astronomers had for cen- 

 turies been throwing into space touched 

 bottom. The employment of the heliometer 

 which Bessel introduced was the main 

 source of our knowledge of the distances of 

 stars until the end of the nineteenth cen- 

 tury, and resulted in fairly satisfactory 

 determination of the parallaxes of nearly 

 one hundred stars. For the part of space 

 nearest to us this survey is sufficiently com- 

 plete for us to infer the average distances 

 of the stars from one another — 2^ to 3 

 parsecs. The parallax determinations of 

 double stars of known orbits lead to the 

 result that the masses of stars have not a 

 very great range, but lie between forty 

 times and one tenth of the mass of the sun. 



When the absolute luminosities of the 

 stars the distances of which have been 

 measured are calculated, it is found that, 

 unlike the masses, they exhibit a very great 

 range. For example, Sirius radiates forty- 

 eight times as much light as the sun, and 

 Groombridge thirty- four only one hun- 

 dredth part. This does not represent any- 



thing like the complete range, and Canopus, 

 for example, may be ten thousand times as 

 luminous as the sun. But among the stars 

 near the solar system, the absolute luminos- 

 ity appears to vary with the type of spec- 

 trum. Thus Sirius, of type A, a blue 

 hydrogen star, is forty-eight times as lu- 

 minous as the sun; Procyon of type Fj — 

 bluer than the sun, but not so blue as Sirius 

 — ten times; a Centauri, which is nearly of 

 solar type, is twice as luminous. 61 Cygni 

 of type Kg — redder thau the sun — one 

 tenth as luminous; while the still redder 

 star of type Ma, Gr 34, is only one hun- 

 dredth as luminous. In the neighborhood 

 of the solar system one third of the stars 

 are more luminous and two thirds less lu- 

 minous than the sun. The luminosity de- 

 creases 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 accu- 

 rate determinations of stellar parallax. So 

 soon as the parallax is known, all the other 

 observational data are immediately utiliza- 

 ble. At the commencement of the present 

 century the parallaxes of perhaps eighty 

 stars were known with tolerable accuracy. 

 Happily the number is now rapidly increas- 

 ing by the use of photographic methods. 

 "Within the last year or two, the parallaxes 

 of nearly two hundred stars have been 

 determined and published. This year a 

 committee of the American Astronomical 

 Society, under the presidency of Professor 

 Schlesinger, has been formed to coordinate 

 the work of six or seven American and one 

 or two English observatories. The com- 

 bined program 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 



