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Natural Science - Year II

Unit 62: Cannon and Stellar Types

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Unit 62: Annie Cannon and the Classification of Stars

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The Classification of stars

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.

The Sun's Spectrum

Read about Frauenhofer's spectrum observations.

  • Click on the picture of the spectrum to enlarge it. What lines are in the visible range (4000-7000Angstronms)?
  • What element causes the line at 5900 A (line D)?
  • How could these lines be used to determine the composition of the sun? of what part of the sun?

Stellar Classes

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.

  • How many stars did Cannon classify?
  • How did Cannon build on the earlier classification systems developed by her predecessors?
  • How did her work help more women gain recognition and opportunities to pursue scientific and engineering careers?

The Temperature of a Star

Read about Cecilia Payne's work.

  • Where did Payne do most of her observing?
  • What did she add to Cannon's classification system?
  • Why wasn't her theory that stars are mostly hydrogen originally accepted by astronomers?

Study/Discussion Questions:

Further Study/On Your Own