About 200 billion stars whirling in a great wheel-like system; the sun is ~ 8.5 kpc from the galactic center (Fig. 15-1,2) Fig. 15-1 The Milky Way galaxy seen face-on and edge-on illustrates the shape and location of the disk, halo, and nucleus. Note the position of the sun. Fig. 15-2 A map of our galaxy at 21-cm radio waves shows traces of a spiral pattern in the distribution of cool hydrogen. Disk component contains almost all of the gas and dust in the galactic plane. The interstellar medium consists of thinly spread hydrogen and helium with denser clouds. Young, O, B stars and dense clouds lie within ~ 100 pc of the galactic plane, but sun-like stars are much less confined. Spiral arms: Long spiral patterns of bright stars, star clusters, gas and dust. Dust clouds obscure visible light. the spiral structure can be mapped by using radio telescope Associations: Groups of 10-1000 stars widely scattered, but moving together in space; the stars in an association may formed from a single gas cloud and have not wandered apart. Spherical component contains all matter in the galaxy scattered in a spherical distribution around the center. Halo: Thin scattering of old, lower mass main-sequence stars and giants, globular star clusters; almost no gas and dust. Nuclear bulge: The most crowded part of spherical component around the galactic core; the center is obscured at visual wavelengths and requires radio or infrared observations. Stars rotate about the galactic center. The sun is moving at a speed ~ 200 km s-1 in the direction of Cygnus; it takes ~ 240´106 years to make one cycle Stars in the outer parts are moving quickly, deviating from Keplerian motions our galaxy must have appreciable mass in its outer parts the halo of our galaxy extends much farther into space? Stellar populations Population I stars: Most belong to the disk component, they relatively young and rich (~ 2-3 %) in "heavy elements"(elements heavier than hydrogen and helium) e.g., the sun is a population I star; young stars in Orion nebula M42 are extreme population I stars. Population II stars: most belong to spherical component, relatively old and poor (~ 0.1%) in "heavy elements"(Fig. 15-3) Fig. 15-3 Part of the spectra of population II(top) and population I (bottom) stars of similar spectral type show hydrogen lines of equal strength (top arrows), but the lines of heavier atoms are weaker (bottom arrows) in the population II star's spectrum. Nucleosynthesis(element-building cycle):Apart from hydrogen and helium, which formed at the beginning of the universe, all 81 stable elements on Earth were made by stars Elements lighter than iron are made in stars by nuclear fusion Elements heavier than iron are only made in the short-lived violence of supernova explosions gold, silver, uranium, etc. are rare (Fig. 15-4) Fig. 15-4 The abundances of the elements. Because elements heavier than iron are made only during supernova explosions, they are rare. First generation of massive stars died, some as supernovae enriched interstellar medium with "heavier elements" successive generations of stars contain more "heavier elements", becoming population I stars First generation of lower mass main-sequence stars have longer lives still exist as old stars with less "heavier elements", becoming population II stars