Spectrum of the Pleiades. Visible light spectrum of stars in the Pleiades open cluster. The spectrum is created by allowing light exiting a telescope to pass though a prism, a technique called objective prism spectroscopy. This allows spectra of many stars to be recorded on a single frame, allowing astronomers to compare the elemental composition of stars and other objects. The Pleiades (also Messier 45, NGC 1432) is a naked-eye object in the constellation Taurus, about 400 light years from Earth. Although known as the Seven Sisters, most people may only distinguish the brightest six stars unaided.