When a beam of white light passes through a prism it is broken up into many different colours which form the white light. This is called the spectrum. In addition the rainbow colours, there are hundreds of parallel lines all across the spectrum. A German scientist, J. Fraunhofer first discovered and studied these lines in the sun’s spectrum and these lines are called Fraunhofer lines in his honour.
With the help of these lines, physicists and astronomers can tell amazing things - things such as what a star billions of miles away is made of, what is its temperature, how fast it is moving, and whether it is moving away from
us or towards us.
Scientists can find out these facts because each chemical element present in the universe in a gaseous or vapour state has its own pattern of line occupying a specific place in the spectrum. The lines represent the colours taken up from the light by the element when it is heated till it glows. Even if the material is very far away this rule applies. Each element leaves its ‘dark line’ (or absorption spectrum) which is different from any other element. Just as no two finger prints of two different persons can be same, no two absorption spectrum of two different elements will be same.
The positions of lines in the spectrum change with the temperature of the element, so astronomers can tell a great deal about the temperatures of stars even billions of miles away. Also, when a star is moving away from us, the lines are shifted towards the red end of the band; when it is coming towards us, the lines are shifted towards the violet end.