In the image above , the stellar spectrum is the fuzzy horizontal band in the center, crossed by dark vertical absorption lines. These lines have known positions, and so they form a reference scale that permits measurements of line wavelengths in the stellar spectrum. A spectrometer produces a spectrum in the form of a chart or graph, as in the lower portion of the figure below. Wavelength is shown along the bottom, and the vertical direction represents the intensity of the light. Absorption lines appear as dark vertical lines on an image of the spectrum and as notches or valleys on a graph.
Emission lines appear as bright vertical lines or as sharp peaks.
The spectral lines appear as dark vertical lines in the color plot and as deep valleys in the scan. In 1872, Henry Draper first photographed stellar spectra. Astronomers could then directly record, compare, and measure the spectra of thousands of stars. Such massive amounts of data required a classification scheme, and in the 1880s a system of spectral classes based on the number and appearance of spectral lines was invented. The sequence of classes is O, B, A, F, G, K, and M. The M stars are the coolest .