Tables 843-845 64-1 



TABLE 843.— The Galaxy, its Center and Rotation 



The center of the galaxy lies apparently among the dense clouds in Sagittarius 40,000 

 light-years (13,000 parsecs). About this center the sun revolves with a period of about 

 250,000,000 years, an orbital speed of 200-300 km/sec. Amount of matter within sun's 

 orbit must have mass about 200 billion times our sun's. In following table based partly on 

 Redman, M.N. 92, 113, 1931, r = mean distance in parsecs from center of objects. A = 

 about .01 7 km/sec. /parsec.yo longitude galactic center. The sun is about +33 parsecs from 

 galactic plane (Gerasimovic, Luyten, Proc. Nat. Acad. Sci., 1927). 



* These values seem consistent on the supposition that the Ca is more or less evenly distributed between the 

 stars and us, so that r for Ca should be yi that for the stars. 



Lindblad (Scientia, 61, 325, 1932) gives a more recent summary of various workers. With Plaskett's A = 

 +0.0155 km/sec./parsec, time of revolution = 200,000,000 years; /o = 327 . With a linear speed of 275 km/sec. 

 our distance to center is 9,400 parsecs (about 30,000 light-years). Total mass of stellar system = 16 X io 10 

 solar masses. 



TABLE 844. — Transmission of Light Across Space ; Theoretical 



(Russell, Proc. Nat. Acad. Sci., 8, 115, 1922; Nature, no, 81, 1922.) 



Let radius of particle = r' , density, p, (random distribution); quantity of matter per unit 

 vol. = d. The extinction of a beam of light will be e stellar magnitudes per unit distance 

 where e = 0.814 qd/pr. 5 is a numerical factor independent of physical units, taking account 

 of complications when 2r becomes near the wave length, X, of the light; when 2r = 2 or 3\, 

 q = sensibly unity, q increases for small particles to a max. 2.56, when circumference = 

 1. 12 X X; then rapidly decreases, = nearly (14/3) X 2irr/\ i for particles less than 1/2 this 

 diameter, q/r is max. 2.42, when circumference = X. 



Clouds, same mean density, d, opacity reaches sharp max. when particles of this size, at 

 the same time becomes selective (1/X 4 ). Visual max. when r = o.o86p. A cloud of this size 

 dust, (density 2.7), absorbs 1 magnitude if 1/86 mg./cm 2 regardless of cross section. If 1/2 

 this or smaller, selective absorption almost as complete as for a gas. Best size particles for 

 opaqueness also best for light pressure. 



Rayleigh's formula for gas is / = I e~ k 



k = — ~rr — —, where n is the index of refraction, N, Loschmidt's number. 



3N X 4 



TABLE 845. — Transmission of Light Across Space; Observed Estimates 



Kapteyn, 1904. . .0.0016 mag./parsec 



Seeliger, 191 1 0003 



Halm, 1917 0030 



Van Rhijn, 1928. . . .0.000035* 



Shalen, 1929 0005 



Lundmark, 1925. . . .00000007 

 Shapley, 1929 000000007 



mag./parsec 



* Equivalent to 4.7 X 10-" g/cm 2 . Bull. Astron. Soc. of Netherlands, 4, 123, 1928. Eddington computed 

 0.00007 per 100 parsecs as scattering coefficient. For Ca Gerasimovic, 1929. obtained I.I X io-» as the scat- 

 tering coefficient. v 



Absorption and space reddening in the Calaxy as shown by the colors of globular clusters. Steb- 

 bins, Proc. Nat. Acad. Sci. 19, 222, 1933. 



Smithsonian Tables 



