National Geographic : 1965 Nov
Newton unlocks the rainbow "- PROCURED ME a Triangular glass-Prisme...." I related Isaac Newton, "and made a small hole in my window-shuts, to let in... the Suns light." The 23-year-old English genius, at Cambridge University in 1666, thus launched the famous ex periments that split sunlight into its spectrum of colors and, in time, gave astronomers one of their most valuable instruments, the spectroscope. Having taken the rainbow out of sunlight and displayed it on the wall, Newton (left) and his roommate, John Wickins, turn it back to white light with a convex lens. The light beam's passage would ordinarily be invisible unless the air were full of dust or smoke. Newton's experiments with the spectrum gave man the key to the universe. A century and a half later, Joseph von Fraunhofer, a Bavarian optician, observed dark crosslines in the spectrum (below). Each chemical element produces its own combina tion of lines that can serve as an identifying finger print. Analyzing these lines, astronomers assay stars many thousands of trillions of miles away, learning that they are made of exactly the same elements we find on earth. use as the Astronomical Unit for measuring the solar system. Since earth's orbit is slightly elliptical, the actual distance varies from 91 to 94 million miles. When we compare earth's size with that of the sun, we find that the sun would hold some 1,300,000 earths, and that it contains nearly 330,000 times as much mass as the earth. Since gravitational pull depends directly on the mass, but decreases with the square of the distance from the center of the body, a man on the sun would weigh some two tons. "Intellectual Boring" Bares Sun's Secrets Having determined the sun's mass and diameter, the astrophysicist can then deduce the temperature, density, and pressure at all distances from the center to the surface, even though he is unable to see deeper than the 200 miles of the luminous photosphere. Sir Arthur Eddington described this deductive process as "intellectual boring." As a result of this boring, we have good 726 reason to believe that at the center of the sun, close to half a million miles deep, pressure reaches 100 billion atmospheres. (An "atmos phere" is 14.7 pounds per square inch, the weight of the column of air over a square inch of earth's surface at sea level.) To produce such great pressure, we know that gas must be heated to a temperature of about 16,000,000° C.* How hot is 16 million degrees? Sir James Jeans, in The Universe Around Us, calculates that a pinhead of material at the temperature of the sun's core would emit enough heat to kill a man a hundred miles away. Although the density at the center of the sun must be about 11.4 times that of solid lead, the sun remains gaseous everywhere. That is, the atoms are free to move about, un like those in a solid, which are fixed in a regu lar pattern. However, the atoms in the core are not normal. Most of their outer electrons *Astronomers, like other scientists, use centigrade. To convert to Fahrenheit, multiply by 9/5 and add 32.