cover of this report and as Plate 2.11. The universe is filled with more distant galaxies, each containing billions of stars and gas in orbit about its center. It takes our sun about 250 million years to complete one orbit about the center of the Milky Way. Galaxies come in a variety of shapes. Some are nearly spherical, while others, like the Milky Way, are flattened disks with a bulge in the middle. Individual galaxies are separated from one another like atolls in a Pacific archipelago; clusters of galaxies are like individual island chains scattered across the vast ocean of nearly empty space.
It was only in the 1920s that astronomers realized that many of the fuzzy patches revealed by their telescopes were indeed distant assemblages of stars, that is, galaxies. For many years after their discovery, galaxies were assumed to be fixed and unchanging, but by the 1970s, astronomers realized that galaxies should, in fact must, evolve. Stars alter the chemical composition of interstellar gas in a one-way process that builds heavier and heavier atoms. Thus the chemical composition, color, and luminosity of a galaxy should all change in time. In addition, galaxies can evolve dynamically as giant galaxies cannibalize their smaller companions.
Finding direct evidence for the evolution of single galaxies is not easy. But, fortunately, light travels at a finite speed. Since the distances in space are large, we can use this effect to observe evolution directly. When we take a picture today of the Andromeda Galaxy, 2 million light-years away, we see that galaxy as it was 2 million years ago. When we look at a galaxy in the Virgo cluster of galaxies, 50 million light-years away, we see light that was emitted 50 million years ago. Looking deeper into space is looking further back in time. Telescopes are time machines. With larger telescopes, we can see more distant galaxies at earlier stages of evolution.
Unfortunately, the light from distant galaxies is faint. To detect such feeble light, astronomers need large telescopes equipped with sensitive detectors. With new telescopes and more sensitive electronic cameras, astronomers have begun to study galaxies at much greater distances and thus at much earlier stages of evolution. For example, some observational evidence suggests a systematic color change of galaxies with age, as theoretically predicted.
For the 1990s, astronomers are building several large, visible-light and infrared telescopes with diameters ranging from more than 300 in. (8 m) to nearly 400 in. (10 m), far larger than the 200-in. telescope at Palomar Mountain, California, which was completed in 1949. In the 1990s astronomers hope to see more distant galaxies, at a much earlier stage of evolution than any galaxies previously seen. What types of stars inhabit young galaxies? While individual stars are born and die, how does the bulk population of stars in a galaxy age in time? Does the shape of a galaxy change in time, or is it determined completely