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At the end of 19th century, physicists knew there were electrons inside atoms, and that the wiggling of these electrons gave off light and other electromagnetic radiation. Each type of atom gives off a unique set of colors. The colored lines (or Spectral Lines) are a kind of "signature" for the atoms. This technique is so reliable that scientists can tell what elements they are looking at just by reading the lines. Spectroscopy is the science of using spectral lines to figure out what something is made of. That's how we know the composition of distant stars, for instance.