FREEMAN D MILLER / SCIENCE PHOTO LIBRARY FREEMAN D MILLER / SCIENCE PHOTO LIBRARY
Star spectroscopy, Optical image of the spectra of the stars in the Hyades open cluster. It was obtained by placing a thin prism at the end of a telescope. With this technique, known as objective prism spectroscopy, the image of every object in the field is turned into a small spectrum. Hundreds of spectra are obtained on a single photographic plate, although at low resolution. The thin dark lines in the spectra are absorption lines which are caused by atoms of various elements present in the outer layers of the stars. This technique is mainly used for detecting different types of objects, notably quasars, in surveys of large regions of the sky.
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