644 Tables 846-848 



TABLE 846. — Amount of Matter in Interstellar Space 



(Eddington, Proc. Roy. Soc, A3, 424, 1926. Note also Table 848.) 



Whether or not matter exists in space is important in estimating absolute magnitudes 

 (Cepheids), and, as a resisting medium, for its dynamical effects. 

 Density at average point, io" 24 g/cm 3 (Eddington, dynamical reasons, star velocities). 

 (Gerasimovic, Struve, Gaseous substratum of galaxy, Astrophys. Journ., 69, 7, 1929.) 

 Interstellar density of Ca probably about p Ca = 3.6 X icf 32 g/cm 3 



For all gases io" 28 " 



Assuming matter of about atomic weight 20, doubly ionized so that there will be about 

 one free electron per cm, then 



Free path for ions, roughly 10 s km, duration 1 year. 

 Free path for electrons, roughly 5.2 X io 8 km, duration 10 days. 

 An ion encounters and deflects an electron once in 5 days. 

 Central density of typical diffuse nebula, estimated, io" 20 g/cm 3 . Hubble (Astrophys. 

 Journ., 1926) estimates if all matter within 100 light-years uniformly distributed, density of 

 order io" 31 g/cm 3 . Eddington (Nature, 128, 702, 1931), io 79 electrons and protons in 

 universe. 



TABLE 847. — Radii of Curvature of Space 



Radius of curvature of the finite universe of general relativity is of the order of 

 2."7 X io 10 parsecs (Hubble, Astrophys. Journ., 64, 1926). 

 Radius of Curvature of de Sitterian space time : 



460 stars 3.63 X io 11 astr. units = 5.74 X 10" light-years = 1.8 X io 6 parsecs 



29 Cepheids 3.0 " " " 



35 O stars 3.2 



(Silberstein, Nature, 9, 50, 1930.) 



TABLE 848. — Interstellar Gases (Calcium, Sodium) 



Since excited atoms are exceedingly rare, the only strong absorption lines will be the 



principal lines. Na, Ca, and Ca -f- have principal lines in the observable spectrum. If we 



take a 12000 K. temperature for the interstellar medium, ionization potential may be 



taken as 20 volts = \po. For electrons of ionization potential \p the fraction ionized is 



x/(i —x) =e ( ^~^ /RT . 'For Na, <A = 5.I v. and 30.35 v., T = 12000, RT = 10.3 v. 



wncncp 



x/(i — x) = 2 X io 6 for 1st ionization; io" 8 for 2nd ionization. 



Thus the Na -f (which is undetectable) is but one part in 2,000,000 of Na. For Ca with 

 ionization potential 6.1 and 11.8 v. we have nearly all Ca as Ca + -f, but one part in 3000 

 of Ca +, one in 2 X io 9 of Ca. 



Certain brighter stars show these lines of Ca (fewer those of Na), which when cor- 

 rected for solar motion indicate a stationary (relative to sun) absorber, whereas other 

 lines indicate a definite radial velocity for the star (Plaskett). Struve gives the following 

 table indicating definitely the increase of this absorption with the distance: 



Note. — Max. intensity K corresponds closely to outside boundary of local cluster (He B 

 stars). The interstellar Ca apparently shares in the rotation of the galaxy (see Table 843). 



Note added 1933. — Plaskett, Pearce consider best value of interstellar density of matter 

 as io~ 25 g/cm 3 . 



Smithsonian Tables 



