title: Galactic spiral structure.
authors: B J
authors: BJ
authors: Bok
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D005118:Extraterrestrial Environment / Q000737:chemistry*; D005740:Gases; D006859:Hydrogen / Q000032:analysis* / Q000737:chemistry; D008962:Models, Theoretical
chemical_list: D005740:Gases; D006859:Hydrogen