66 THE UNIVERSE A VAST SYSTEM OF PARTS every star that is visible to the unaided eye even on the clearest night. It would take about fifty years, counting two to a second without stopping, day and night, to count this number of stars. Each of these stars is a bright sun, pouring forth much more light in many cases than does our own sun (S. as our sun), and yet some of Doradus emits 500,000 times as much light them are so far distant that the light gathered in by the 100-inch reflector becomes visible only by taking time exposures of several nights' length. So far distant are the stars that a unit of measurement, the light-year, has been devised. This is about 6,000,000,000,000 miles, or the distance light travels in a year at the rate of 186,272 miles per second. Light takes 1.3 seconds to travel from the moon to the earth, and 499 seconds, or 8.3 minutes, to travel from the sun to the earth. It takes light 4.16 years to come to the earth from the nearest star, Alpha Centauri. Only fifteen known stars are within a distance of a dozen light-years. The light from the most distant objects visible with present instruments is now estimated to have started on its way to us about three hundred million years ago. The 200-iiich Reflecting Telescope Ranks as One of the Greatest Technical Achievements of This Generation. The 200-inch reflecting telescope is the result of the efforts of George Ellery Hale, who died in 1938 before it became a reality. It was George EUery Hale's dream of a 200-inch telescope that brought the $6,000,000 gift from the International Education Board, financed principally by John D. Rockefeller, Sr. This giant telescope is located on Mount Palomar, in southern California, and is operated by the California Institute of Technology and the Carnegie Institution of Washington, D. C. The plans for this gigantic instrument have been progressing since 1929. The most intense radiations from the very hot blue stars are in the ultraviolet portion of the spectrum, while the most intense light from the cool red stars is in the infrared.^ It was found that silver used on the mirrors of modern telescopes will not reflect the ultraviolet rays, so the technique of coating telescopic mirrors with aluminum, which will reflect the ultraviolet rays, has been perfected. /. Strong, of the California Institute of Technology, has already coated the 36-inch reflector of the Lick Observatory ^ and all of the Mount Wilson reflector instruments as a sort of preliminary exercise. There are plans to give an aluminum coat to the 200-inch mirror also. The light-gathering ability of the 200-inch reflector will be 640,000 times that of the human ' The different types of stars are described in Section 3 of this Unit. * Do not confuse the 36-inch reflector with the 36-inch refractor at the Lick Observatory.
TELESCOPES 67 eye, as compared with 160,000 times for the Mount Wilson 100-inch telescope. The aluminum coating should also greatly increase the total reflected light to which the much-improved photographic emulsions are sensitive, so it is hoped that the new telescope will penetrate three times as far into space as formerly, opening for investigation an unexplored sphere about thirty times the volume now within the range of the Mount Wilson 100-inch reflector. The 200-inch mirror for this telescope was made of Pyrex glass, borosilicate glass, and weighs 20 tons. It was cast from a batch of 65 tons of glass after heating it in a furnace for forty days. It was placed in an oven to cool on December 2, 1934, and removed on December 8, 1935. Then followed many tests to determine whether it would be satisfactory or whether another mirror would have to be cast. The mirror was then shipped to California on April 10, 1936, to be polished to an accuracy of "two millionths of an inch." In the final stages, polishers could not work on the mirror more than an hour each day, because it was necessary to test so frequently. Places on the surface that were too high had to be lowered, for there was no means of changing those that were too low, and the lowering could not go too far. Often two or three minutes' work was all that could be done without testing to see whether or not enough had been done. In order to make these tests, the surface had to be cleaned off and dried and then set up for the test. In this process temperature changes were inevitable; and hence, with a large mirror, several hours elapsed before a reliable test reading could be made. The construction of the mounting for the 200-inch mirror has been a tremendous engineering problem. Five hundred tons of steel had to be fitted together to tolerances in some places as close as two millionths of an inch. The whole mounting is floated on oil and can be moved by exerting only 1/650,000 of a horsepower. Many Other Giant Telescopes Are in Use. Important as this large telescope is, it must be remembered that there is a tremendous number of observations to be made and that all observations must be checked as many times as possible by different observers using different instruments located at different places. Other great instruments now in u.se are: a new 76-inch reflector at the University of Toronto, Canada; a 72-inch reflector at Victoria, British Columbia; a 69-inch reflector at the Perkin Observatory at Delaware, Ohio; an 85-inch reflector at Base Lake, Michigan; and an 80-inch reflector in the $840,000 McDonald Observatory on Mount Locke, Texas. There are at least nineteen other giant reflectors, ranging from a