TABLE 666.— GASES IN INTERSTELLAR SPACE* 210 



629 



The gases that have been detected are listed together with the means of detection and approxi- 

 mate abundances. Both the observations and the application of ionization theory introduce con- 

 siderable uncertainty in the determination of abundances. Values given are the best current 

 estimates. In general, the composition of the interstellar gas appears to be the same as for the 

 stars. 



The interstellar gas is strongly concentrated in clouds as evidenced by the multiplicity of 

 interstellar absorption lines. Stromgren suggests density between clouds is about 1% of that 

 in clouds. 



* Prepared by B. Donn. 



210 Adams, Astrophys. Journ., vol. 109, 1949; Publ. Astron. Soc. Pacific, vol. 60, p. 354, 1948; Dunham, Proc. 

 Amer. Philos. Soc, vol. 81, p. 277, 1939. Ledoux, Pop. Astr., vol. 49, p. 513, 1941. Stromgren, Astrophys. 

 Journ., vol. 108, p. 242, 1948. Struve, Journ. Washington Acad. Sci., vol. 31, p. 217, 1941; Astrophys. Journ., 

 vol. 89. p. 517, 1939. 



t Values for apparently abnormally dense cloud. 



TABLE 667.— THE ABUNDANCE OF CERTAIN ELEMENTS IN THE NEBULAE 211 



(Given as the exponent of 10) 



Abun- Abun- 



dance Element dance 



8 Sc <6+ 



7+ Ti <7— 



7 V <8 



6+ Cr <7 



7- Mn <7 



Fe 7+ 



211 Bowen and Wyse, Lick Obs. Bull., vol. 19. p. 1. 1939. 



TABLE 668.— MATTER IN INTERSTELLAR SPACE* 212 



The interpretation of the interstellar absorption curve and of absorption by dark clouds 

 requires the presence of small grains with radii ranging around 10" 5 cm. Polarization of 

 starlight indicates that some, if not all, grains are elongated. Composition, from absorp- 

 tion curve and scattering appears to be mainly dielectric. 



Density of matter 



Solid grains : 



Uniform region, abs 0.5 m/kpc 10"" g/cm 3 



Large cloud, abs 1 mag ( 10 m/kpc) 10 * g/cm s 



Dense condensation, abs 5-10 m ( 1000 m/kpc) 10" 23 g/cm 3 



Mean density, gas and grains 3X10" g/cm 



Oort limit (Max density, stars plus diffuse matter) 6Xl0 _2 |g/cm 3 



Mean space density of stellar matter 3X10 g/cm 



* Prepared by B. Donn. , .~.. , r j 



212 Greenstein, Harvard Circ. 422, 1938. Spitzer, Astrophys. Journ., vol. 93, p. 369, 1941. Van de 



Hulst, Rech. Astron. de l'Obs. d'Utrecht, vol. 11. pt. 1, 1946, pt. 2, 1949. Schalen, Publ. of Uppsala 



Observatory, 1930 on. Oort, Astron. Inst. Netherlands Bull. No. 283, 1932. 



SMITHSONIAN PHYSICAL TABLES 



