National Geographic : 1956 Dec
probably the most colossal underestimate ever made by man. In the Milky Way certain peculiar stars that pulsate steadily can be seen, waxing and waning in brightness. From years of study of these important stars, the Cepheid variables, astronomers had calculated their true brightness. They found also that their rate of pulse, the time it takes them to change from bright to dim and back again, corresponds closely to their maximum brightness. Fortunately, Cepheids can be seen in some other galaxies as well, such as the famous Andromeda Galaxy. By observing their pulse rate, astronomers could assign them a true brightness, and from that could compute the distance to the galaxy. Shortly after the survey began, Dr. Walter Baade of Palomar found with the 200-inch telescope that the real or absolute brightness of the Cepheids in Andromeda was four times greater than had been thought previously. Astronomers, checking the Cepheids in our own system with modern techniques, found that they, too, were four times brighter than had been believed. Thus the basic yardstick of space expanded in length: All galaxies, not Andromeda alone, were twice as far away as had been originally computed. At one stroke distances in the universe were doubled, and the volume of visible space became eight times greater. Even today the problem of cosmic distances is not completely cleared up. The correction to the most distant galaxies seems closer to three times than twice. Thus the Sky Survey, which was thought to reach out to something more than 300 million light-years when it began, now gives measurements of more than a billion light-years to remote clusters. Survey Offers Clues to Creation For ages men have questioned: How did the universe begin? Here again we expect the Sky Survey to help with the basic problem. There are two conflicting theories that seek to explain the behavior of all that we see in the heavens. One holds that all matter was once closely packed together in a "super dense" state. Then some tremendous ex plosion started this primeval matter flying outward in space. The alternative theory holds that the uni verse is in continuous creation. As galaxies rush away in the expanding universe, others 785 Mount Wilson and Palomar Observatories Spiral Galaxies: Blazing Star Systems Pinwheel formations, their arms trailing rotating hubs, comprise about 80 percent of known galaxies. Above: In the face-on portrait of galaxy NGC 5194, Palomar's 200-inch mirror shows stars never before seen as individuals. Satellite galaxy shines below it. + Seen edge-on, galaxy NGC 4565 reveals a dark band of cosmic dust rimming the wheel.