How was the light – year discovered?

We know that just by studying the spectrum an astronomer can tell you what a star billions of miles away is made of and the elements present, take the star’s temperature, figure out how fast it is moving, and whether that motion is toward earth or away from it! The spectrum consists of the lines into which white light is broken up when it is bent, as when it goes through a prism. All across the spectrum, in addition to the shadings of color, there are hundreds of parallel lines. They are known as Fraunhofer lines, in honor of their discoverer. Each chemical element in a gaseous or vapor state has its own pattern of lines occupying its own place in the spectrum. The lines stand for the colors taken up from the light by the element when it is heated so that it glows. This means that a scientist can find out what materials are present in any substance, no matter how far removed.