Spectroscopic observations of the Sun

Elements and molecules emit and absorb photons with characteristic energies. As a result, measurements of stars, comets, or other luminous bodies with a spectrograph, which permits the output to be measured as a function of wavelength, reveal numerous emission or absorption lines (Fig. 4.1). These lines can be used to infer the compositions of the objects. The first spectroscopic measurements of the Sun, stars, and other luminous objects were made in the last half of the nineteenth century. However, it wasn t until the late 1920s that relatively accurate elemental abundances for the Sun and the stars were determined (see Box 4.1). 

It is now relatively easy to obtain detailed spectra of the Sun, but determining the Sun s chemical composition from such spectra requires a realistic model of the stellar atmosphere and a detailed understanding of how the lines are formed and interact to give the observed spectrum. The elemental abundances are not observed, but are derived from 

Payne was born in 1900 in Wendover, England, to an upper-class family. Her early education was in botany, physics, and chemistry, but her passion was astronomy. In 1922, she heard a lecture by Harlow Shapley, the director of the Harvard College Observatory. The lecture inspired her to seek admission to Harvard, and with strong recommendations from her mentors in England, including Sir Arthur Eddington, she was admitted and arrived on campus in 1923. Two years later, in 1925, she became the first student, male or female, to earn a Ph.D. from the Harvard College Observatory. 

When Payne began her work in the 1920s, stellar spectroscopy was a very active area of research. Numerous elemental and molecular lines had been identified in stellar spectra. The lines observed in each star varied with the inferred temperature of the star, which was understood to mean that the elemental abundances varied with temperature. This body of data was the basis for the spectral typing of stars ( , B, A, F, G, , M, L). However, the power source for stars was not understood and it was not clear why the composition of a star should be related to its temperature. In the 1920s, it was also widely believed that the Sun had the same composition as the Earth models considered the Earth to have formed from the outer layers of the Sun. Payne used the new guantum mechanical understanding of atomic structure to show how and why the spectral lines of the different elements varied as a function of stellar spectral type. She demonstrated how the temperature of the stellar surface controls the spectral lines that are observed. Her analysis led to the conclusion that the chemical 

Payne (later Payne-Gaposchkin) went on to have a brilliant academic career. Although she suffered many years of gender discrimination, she eventually became Harvard s first female full professor and chair of the Astronomy Department. 

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