#stellar spectra

[lindahall]

Mary Anna Palmer Draper – Scientist of the Day

Born to wealth and privilege on September 19, 1839, Mary Anna Palmer grew up in a New York City mansion on Madison Avenue.

read more...

[spaceplasma]

Spectroscopy and the Birth of Astrophysics

The 3D animation (above) depicts how the light of a distant star is studied by astronomers. The spectrum of the light provides vital information about the composition and history of stars. Now, let's look into the history of stellar spectroscopy.

In 1802, William Wollaston noted that the spectrum of sunlight did not appear to be a continuous band of colours, but rather had a series of dark lines superimposed on it. Wollaston attributed the lines to natural boundaries between colours. Joseph Fraunhofer made a more careful set of observations of the solar spectrum in 1814 and found some 600 dark lines, and he specifically measured the wavelength of 324 of them. Many of the Fraunhofer lines in the solar spectrum retain the notations he created to designate them. In 1864, Sir William Huggins matched some of these dark lines in spectra from other stars with terrestrial substances, demonstrating that stars are made of the same materials of everyday material rather than exotic substances. This paved the way for modern spectroscopy.

Since even before the discovery of spectra, scientists had tried to find ways to categorize stars. By observing spectra, astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral lines. Classification by spectral features quickly proved to be a powerful tool for understanding stars.

The current spectral classification scheme was developed at Harvard Observatory in the early 20th century. Work was begun by Henry Draper who photographed the first spectrum of Vega in 1872. After his death, his wife donated the equipment and a sum of money to the Observatory to continue his work. The bulk of the classification work was done by Annie Jump Cannon from 1918 to 1924. The original scheme used capital letters running alphabetically, but subsequent revisions have reduced this as stellar evolution and typing has become better understood.

While the differences in spectra might seem to indicate different chemical compositions, in almost all instances, it actually reflects different surface temperatures. With some exceptions (e.g. the R, N, and S stellar types), material on the surface of stars is "primitive": there is no significant chemical or nuclear processing of the gaseous outer envelope of a star once it has formed. Fusion at the core of the star results in fundamental compositional changes, but material does not generally mix between the visible surface of the star and its core. Ordered from highest temperature to lowest, the seven main stellar types are O, B, A, F, G, K, and M. Astronomers use one of several mnemonics to remember the order of the classification scheme. O, B, and A type stars are often referred to as early spectral types, while cool stars (G, K, and M) are known as late type stars.

Scientists assumed that the spectral classes represented a sequence of decreasing surface temperatures of the stars, but no one was able to demonstrate this quantitatively. Cecilia Payne, who studied the new science of quantum physics, knew that the pattern of features in the spectrum of any atom was determined by the configuration of its electrons. She showed that Cannon’s ordering of the stellar spectral classes was indeed a sequence of decreasing temperatures and she was able to calculate the temperatures.

More information: here

Credit: ESO, Jesse S. Allen

[spaceplasma]

Annie Jump Cannon

Oh, Be A Fine Girl--Kiss Me! This phrase has helped several generations of astronomers to learn the spectral classifications of stars. Ironically, this mnemonic device, still used today, refers to a scheme developed by a woman.

In a time when women rarely attended college, Annie Jump Cannon studied physics, mathematics, and astronomy, and in 1896 she was hired by Edward Charles Pickering at Harvard, as one of several women assigned to assist Pickering in the Henry Draper Catalog of Stellar Spectra. Cannon, however, found the system being used unworkable, and devised a different system, categorizing stars by letters of the alphabet. Now called the Harvard Spectral Classification Scheme, her methodology was adopted by the International Astronomical Union in 1922 as the official system for stellar spectra classification.

Cannon discovered about 300 stars and classified the photographic spectra of about 325,000 more, for which Time profiled her as "Census-taker of the sky". She was the first woman officer of the American Astronomical Society, holding the post of Treasurer for seven years. She was nominated for membership in the National Academy of Sciences, but not elected, after noted biologist Raymond Pearl made an issue of her deafness. She was given an official appointment by Harvard as Astronomer in 1938, the first woman so honored. An outspoken suffragette, she said that her interest in the heavens was stoked by her mother, who also had an intense interest in stargazing.