Our galaxy. The Milky Way is a spiral galaxy with four major arms containing young bright stars, gas and dust. The mass of the Milky Way is estimated to be between 4 x 10^11 and 10^12 solar masses. The luminous diameter of our galaxy is some 80,000 light-years with our Sun orbiting about 25,000 light-years from the galactic center. However, the large-scale rotation of galactic matter suggests an even larger diameter extending beyond 120,000 light-years. This data indicates that galactic mass is not centrally located in the core; rather, the bulk is spread out beyond the Sun's galactic orbit. Perhaps only 10 percent of the galaxy glows as stars; the remainder is nearly invisible. 


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