<< Spectroscopy is one of an astronomer’s favourite tools to help understand the Universe. Planets, stars and galaxies are just too far away to be analysed in a laboratory. Fortunately, very important information about these distant bodies is written in the light we detect with a telescope.

But the light is not an open book. To be read, light must be split into its different colours (or wavelengths), in the same way that rain droplets disperse the light to form a rainbow. Newton called this rainbow of colours a spectrum, the Latin word for “image”.

The first astronomical application of spectroscopy was in the analysis of sunlight by Fraunhofer and Kirchhoff, in the early 19th century. It was expected that the white light emitted from the Sun would produce a clean rainbow when passing through a prism. But, for the very first time, a pattern of dark lines was also noticed. These unexpected lines were the “fingerprints” imprinted in the light by the different chemical elements interacting with it and are called absorption lines.

The beauty of this interaction is that each chemical element or molecule produces a unique signature in the spectrum, a sort of barcode that unequivocally identifies one element from another. By decoding these barcodes, spectroscopy can reveal important properties of any body which emits or absorbs light.

The spectrum of a star or any astronomical object not only reveals the presence of certain chemical elements, but also informs about the prevailing physical conditions, such as temperature and density. Spectra can also tell us about motion: by using the Doppler effect, the speed of a star or a galaxy with respect to the Earth can be measured. This effect is used to discover extrasolar planets, and a similar effect allows astronomers to measure the distances to galaxies. Spectra also contain information on the magnetic field present in the object, the composition of the matter and much more.

The next generation of spectrographs, like those planned for the European Extremely Large Telescope (E-ELT), will go beyond anything we can currently achieve. Among the things we cannot do today, astronomers expect to be able to look for possible traces of life in the atmospheres of exoplanets similar to Earth. If signs of life are ever discovered on another planet, it’s most likely that the instrument involved will be a spectrograph. >>

Source: http://www.eso.org/public/teles-instr/te...