The study of the properties of the spectrum of different objects is called, as you may have already guessed, spectroscopy. It is the most important method for learning about remote planets, stars and galaxies.

Every chemical element has a unique spectral `fingerprint'. You will soon examine the spectra of a few different gases and see this effect for yourself. This makes spectral analysis a very powerful tool--when you see the fingerprint of a particular element, you know immediately that the gas producing the spectrum contains the element. Not only does each chemical element produce a unique spectral fingerprint, but ions (atoms missing one or more electrons) of a particular element produce fingerprints different from those of the neutral atoms. This fact can help determine the temperature of a hot gas. At higher temperatures, more highly charged ions will be present, so we can estimate the temperature by identifying the ions that are creating the spectral lines.


Spectral Classes for Main Sequence Stars

In 1872, Henry Draper first photographed stellar spectra. This represented a tremendous advance. Instead of sketching spectra astronomers could directly record, compare and measure them. Spectra of thousands of stars became available for precise analysis.

Such massive amounts of data required a classification scheme. This was begun by Edward Pickering and Williamina Fleming, Pickering's former house maid. Fleming invented a system that classified the stars based on the strength of the Balmer Lines and the number and appearance of other spectral lines. She is credited with categorizing more than 10,000 stars. Annie J. Cannon, a deaf woman who also worked for Pickering, later led a group of young women assistants known as "Pickering's Harem" who revised the classification system to reflect the temperature of the star. When Annie Cannon published the Henry Draper catalogue it contained spectral data on 225,320 stars and became the basis for all modern astronomical spectroscopy.

The sequence of spectral classes begins with the hottest stars, the O stars, and ends with the coolest stars, the M stars. This is summarized in the following table. The strong lines column describes the prominent line features in the spectrum.

Annie J Cannon and one of her photographic plates with stellar spectra