fig. 18-1 The red shifts in the spectral lines of distant galaxies increase with increasing distance. The indicated lines occur in the spectrum of calcium. Reference spectra are shown above and below each galactic spectrum. The red shifts are interpreted as doppler shifts that come about become of the expansion of the universe.

fig. 18-2 Graph of recession speed versus distance from the earth for the galaxies of Fig. 18-1. The speed of recession averages about 17 km/s per million light-years.

fig. 18-3 Two-dimensional analogy of the expanding universe. As the balloon is inflated, the spots on it become farther apart. A bug on the balloon would find that the farther away a spot is from it, the faster the spot seems to be moving away. This is true no matter where the bug is.

fig. 18-4 If the universe is "open," its expansion will continue forever. If it is "closed," the universe will eventually stop expanding and then collapse. In that case, a new big bang may conceivably occur that would start a new expansion.

fig. 18-5 Snapshots from the youth of the universe.

fig. 18-6 (a) The solar system began as a spinning cloud of gas and grains of ice (frozen gases) and rock. (b) When the protosun began to shine, the ice grains near it were heated and vaporized. (c) The rock and ice grains collided and stuck together to form planetesimals, which were rocky near the sun and a mixture of rock and ice farther away. (d) The planetesimals themselves collided and stuck together to form the planets. The solar wind blew away the gas and other material that had not become incorporated in the planets.

fig. 18-7 (a) When the earth was young, its interior melted. Under the influence of gravity, heavy materials then migrated inward to form the core and lighter ones were forced outward to form the mantle and crust. (b) Eventually the separation into core, mantle, and crust was complete, the outer parts cooled and hardened, and the early oceans and continents came into being.