The greatest innovation in astronomy was the invention of the spectroscope.
During the seventeenth century, Isaac Newton had shown that a glass prism could spread sunlight into a rainbow band of colours, or spectrum. In the early years of the nineteenth century, Joseph Fraunhofer, in Germany, and William Wollaston, in England, experimented with passing light through a narrow slit before it entered the prism and, by 1814, Fraunhofer had been able to construct a device, called a spectroscope, which used lenses to form a focused image of the emerging band of colours.
Using this instrument, Fraunhofer found that the Sun’s spectrum was crossed by hundreds of dark lines, the nature of which was a mystery. Then, in 1859, Gustav Kirchhoff and Robert Bunsen proved that each chemical element produces its own ‘fingerprint’ pattern of lines, and thereby opened up the possibility of discovering the chemical composition of the Sun and stars.
Subsequently, astronomers came to realise that they could extract from the spectrum of a celestial object a host of information, not only about chemical composition, but also about factors such as temperature, pressure, density, and the speeds at which these objects were approaching, receding or rotating. The spectroscope transformed astronomy from the rather dry study of positions and brightness into astrophysics - the investigation of the physical nature and evolution of planets, stars, gas clouds and galaxies - and led directly to the discovery that the entire universe is expanding.
Dr Iain Nicholson is a writer and lecturer in the fields of astronomy and space science, visiting fellow of the University of Hertfordshire, and contributing consultant to the magazine Astronomy Now. He is the author of more than 20 books on astronomy and space science.
Book suggestion: Dark Side of the Universe (buy this book from Amazon(UK))