Most fully developed galaxies have a vivid spiral structure, but the formation and evolution of the spiral structure are still an enigma in astrophysics. In this paper, according to the standard Newtonian gravitational theory and some observational facts, we derive an idealized model for spiral galaxy, and give a natural explanation to the spiral structure. We solve some analytic solutions to a spiral galaxy, and obtain manifest relations between density and speed. From the solution we get some interesting results: (I) The spiral pattern is a stationary or static structure of density wave, and the barred galaxy globally rotate around an axis at tiny angular speed. (II) All stars in the disc of a barred spiral galaxy move in almost circular orbits. (III) In the spiral arms, the speed of stars takes minimum and the stellar density takes maximum. (IV) The mass-energy density of the dark halo is compensatory for that of the disc, namely, it takes minimum in the spiral arms. This phenomenon might reflect the complicated stream lines of the dark halo.
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