History Weblecture for Unit 62
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Nineteenth century chemists Robert Bunsen and Gustav Kirchhoff discovered that hot materials glow or burn with characteristic spectra, photon patterns emitted by excited electrons changing orbits. Bohr explained this mechanism with his work on atomic structure in the early twentieth century, but astronomers had already begun exploring the spectra of near and distant objects, comparing these to see what information could be gleaned about the composition of the sun, the atmospheres of other planets, and distant stars and nebulae.
Read about Frauenhofer's spectrum observations.
In the late nineteenth century, surveys of stars in different parts of the sky suggested that stars fell into distinct classes. The members of the Harvard University Observatory received funds from the widow, Anna Draper, of a noted amateur astronomer, Henry Draper, to create a catalog of stars down to apparent magnitude +9, and to conduct a disciplined study of these stellar observations, including spectral analysis, in order to better understand whether types of stars existed and could be classified so they could be recognized and compared. Much of the actual work was carried out by a team of women under the leadership of first Williamina Fleming and later Annie Cannon. Cannon took two different classification systems proposed by others on the team and managed to negotiate a compromise, resulting in the OBAFGKM classification system now in use.
Read about Annie Cannon at the San Diego Supercomputer Center site.
Read about Cecilia Payne's work.
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