Some spectra also have broader bands produced by molecules. Since each atom, ion, or molecule has a unique and characteristic set of lines or bands, the chemical composition of stars can be determined. Stars can be classified using criteria based on the appearance of their spectra. The Draper catalog, published in 1890, was the initial effort of Harvard University astronomers to classify stars based on the varied patterns of lines and bands in stellar spectra. One of the greatest women astronomers of the time, Williamina Fleming, made monumental contributions to the development of the Draper catalog, which contained spectral classifications for 10,351 stars.
Most stars have very similar chemical compositions.
They are composed primarily of hydrogen with some helium and very little of the remaining elements. The variation in the appearance of stellar spectra is therefore not due to large differences in chemical composition but rather results from a variation in the temperatures of stellar atmospheres, where the absorption spectrum is formed. The horizontal axis of the graph in Figure 10.4 is labeled with the letters astronomers call spectral classes. Each class is subdivided into 10 subclasses designated by a number. For example, A0, A1, A2, . .