Space and Interstellar Matter

Our Galaxy is one of billions of similar stellar systems which together make up the observable universe. The total mass of our galaxy is 1.8 x 1011 M . About 10% of that mass is in the form of interstellar matter whose principal constituents are gas and fine dust particles. The dust and gas appear to be 

Element Abundance relative First ionization potential Second ionization potential  

Elemental abundances with exception of He are average values from Unsold (1972, private communication). Ionization potentials as given in Allen, 1963 (Astrophysical quantities, 2nd edit. The Athlone Press, University of London 1963). 

The stars are concentrated to a flat disk of diameter about 40 kpc and width about 500 pc, and to a spherical bulge of diameter about 2 kpc in the central region (see Fig. 3). The plane of symmetry of the disk is called the 

The principal features of the spiral structure are believed to be maintained by a pattern of density waves. This pattern rotates about the galactic center like a rigid body with an angular velocity that is lower than that of most of the material. Thus, the material of the Galaxy (i.e. both stars and gas) flows through the pattern and as it enters the density wave, the gas is strongly compressed. There are also less important elements of the spiral pattern that rotate with the material these are called material arms, as against density-wave  

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