84 ANNTJAL REPORT SMITHSONIAN INSTITUTION, 1960 



atmospheres. Computation by this new procedure of a niunber of 

 model atmospheres has begun, with the collaboration of Dr. Charles A. 

 "VATiitney. Dr. Krook continues his investigation of various problems 

 related to the dynamics of ionized gases, the structure of shock fronts 

 in the presence of magnetic fields, and the kinetic theory of gases. 



Dr. "VVhitney continued his study of stellar atmospheres, begun in 

 1956. Under his supervision, work on the theory of the formation 

 of spectral lines in moving atmospheres is virtually completed. With 

 numerical tecliniques developed for use with electronic computers, 

 he is constructing first approximations to precise models of stellar 

 atmospheres, and making preliminary calculations concerning their 

 dynamic behavior. 



Dr. Robert J. Davis completed a study of the spiral structure of the 

 Milky Way, and the relationship between neutral hydrogen and 

 stellar clusters and associations, by means of the 21-centimeter radia- 

 tion of neutral hydrogen. His observations confirm the fact that 

 the spiral arms are trailing and indicate that neutral hydrogen is 

 probably concentrated in some stellar aggregates. This work makes 

 a major contribution to our knowledge of galactic structure, and to 

 the study of the dynamics, origin and evolution of stars, stellar 

 clustei*s, associations, and the galaxy. 



Dr. J. Allen Hynek is compiling a revised catalog of stars with 

 composite spectra. In collaboration with Dr. Werner Kirschoff, using 

 the 74-inch reflecting telescope of the Eadcliffe Observatory at Pre- 

 toria, South Africa, he has obtained a series of 25 spectrograms of 

 a nimibcr of stars with composite spectra visible only in the skies of 

 the Southern Hemisphere to add to the catalog. 



Dr. Richard E. McCrosky, in collaboration with Dr. William Sinton 

 of the Lowell Observatory, has begun attempts to detect infrared 

 radiation from molecular hydrogen in interstellar space, by use of 

 infrared-sensitive detectors on the 42-inch telescope at the Lowell 

 Observatory. 



Dr. Paul W. Hodge began a study to determine the chemical com- 

 position, physical characteristics, ages, and distances of selected nearby 

 galaxies. The work requires exceedingly detailed and accurate 

 photometry of individual stars in the field and in clusters of certain 

 extragalactic nebulae, and wiU provide evidence on the true distance 

 scale of the universe. 



Upper atmosphere. — Dr. Luigi G. Jacchia continued his study 

 of the earth's upi^er atmosphere, basexi on analyses of the motion 

 of artificial satellites. This work, at the very frontier of knowl- 

 edge, has revealed startling new facts concerning solar-terrestrial 

 relationships. 



Dr. Jacchia has shown that at heights above 200 kilometers the 

 density of the atmosphere displays a diurnal variation. As the earth 



