A spectrometer is an instrument for measuring light across a spectrum. The spectrum can be created by putting the light through a prism, and looks much like a rainbow. By measuring what specific parts of the spectrum are bright or dim, analysis of the light can be done, and this can tell us a good deal about the source of the light. For example, specific elements, when they are heated to glow, give off spectra that have bright lines located at specific places in them. There are, for example, spectral lines associated with hydrogen, others associated with helium, carbon, and so on. By putting the light from a star through a spectrometer, and observing the lines, we can know what elements are in the star. For more technical detail please write to us.