Light: Message system of the Universe
Until about a century ago astronomers were almost exclusively concerned with the positions and motions of celestial bodies. They knew little or nothing about the physical'makeup or nature of these bodies and were really not able to find out. Yet today concern with the physical nature of celestial bodies, which is the field of astrophysics, is one of the most important topics in astronomy. This change in orientation is primarily the result of two developments in the growing knowledge of the nature of light.
The first was the invention of the spectroscope, a device capable of breaking down white light from a distant source into its component colors. In his Opticks Newton described how one saw a rainbow of colors when sunlight was passed through a prism, but it was actually William Wollaston (1766-1828) in England and Joseph Fraunhofer (1787-1826) in Bavaria who were primarily responsible for developing the spectroscope.
The second development was the recognition that each different chemical element emits a specific set of colors that is peculiar to it, much like the fingerprints of an individual. As early as the 1830s this fact was suggested in connection with the presence, identity, and abundance of different elements in ores. The real beginning of the field of spectroscopy was made in the last half of the nineteenth century by chemist Robert Bunsen (1811-1899) and physicist Gustav Kirchhoff (1824-1887) at the University of Heidelberg.
From his experimental work in the laboratory Kirchhoff was able to formulate three empirical laws of spectroscopic analysis. These laws describe the physical conditions under which matter will produce light having one of three different spectra of colors. The most important astronomical application was the potential of determining the chemical composition of the sun and stars.
By 1864 the English astronomer Sir William Huggins (1824-1910) had identified nine elements in the bright star Aldebaran in the constellation of Taurus. Sir Norman Lockyer (1836-1920) in 1868 detected an element in the solar spectrum that was unknown on the earth. It was later found in natural gas, but it still carries its solar name-helium. In the years following, more elements were identified in stars, along with the discovery that the spectra of colors in the white light coming from stars are not the same for all stars. However, in the spectra of stars sufficient similarities existed so that the stars could be arranged into broad spectral classes.
By the beginning of this century an important relationship had been established between the researcher in the laboratory and the astronomer in the observatory. From it new means of viewing the universe appeared, which have fundamentally altered our concepts of the universe. The extension of spectrum analysis to radiation in parts of the electromagnetic spectrum other than the visible and the ability to move above the earth's obscuring atmosphere to view radiation coming from the depths of the universe are the Rosetta stone of today's astronomy.
