Journey into Spectroscopy




University of Texas at Austin
McDonald Observatory

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McDonald Observatory



Introduction: Astronomy fundamentally changed after Henry Draper first photographed the spectrum of the star Vega in 1872. Soon thereafter, astronomers began surveying the stars and photographing their spectra. The resulting flood of stellar spectra was a blessing and a challenge. The bewildering variety of stellar spectra demanded a simple, adaptable, and meaningful organization system. By 1924, Annie Jump Cannon had revised the stellar spectra organization system and personally classified over two hundred thousand stars. Her basic system is still in use today. In this activity, students begin their exploration by following in Cannon’s footsteps to organize a few stellar spectra. Later they learn that the absorption features in the spectrum change according to the temperature of the star. Given this information, they may revise their organization scheme and order the stars by temperature. In this way, they come to understand the code for temperature in a star’s spectrum. In the explanation section, students learn the history of how Cannon and others dealt with this same problem

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