continuous
bright line
dark-line
Relationship between spectral types
CLASSIFICATION OF STELLAR SPECTRA (references)
Spectroscopic analysis of starlight revealed a huge quantity of hydrogen in the universe, as well as much smaller amounts of the heavier elements.
While the work of Huggins and others had led to the ability to determine the chemical composition of stars, the large body of spectral data becoming available through the photographic technique led certain astronomers to attempt to find a pattern in the spectral types. Among these were Lewis M. Rutherfurd(1816,1892), an American amateur and Angelo Secchi. As detailed in Crowe's book Secchi divided the spectra into four categories:
1) whitish to blue
2) yellow
3) red
4) faint fiery red stars producing spectra different
from type 3)
In 1872 Henry Draper, a Harvard College physician and astronomer, made the first photograph of a star's spectrum.
When Henry Draper died his widow donated a large sum of money to Harvard College to undertake the photography, measurement, and cataloging of stellar spectra to be published as a memorial to her husband. This work began around 1886 under the direction of Harvard physicist Edward Charles Pickering(1846, 1919). Pickering came up with a new technique for obtaining many stellar spectra simultaneously by placing a low dispersion prism before the telescope's objective lens. Pickering hired a group of "Lady Computers" to help investigate and classify the stellar spectra.
The attempt to make sense of the large and diverse
body of stellar spectra led to a new classification scheme for them. He
was instrumentally assisted in this by Annie Jump Cannon who was hired
in 1895. She found a scheme whereby the spectra could be classified into
10 broader categories, which latter would be subdivided even more. The
sequence became: O, B, A, F, G, K, M, R, N, S.
This classification scheme is used today still. For
example, we speak of our sun as a type G2 star. Henry Norris Russell provided
a whimsical mnemonic for this: Oh Be A Fine Girl And Kiss Me Now, Sweetheart.
Draper catalogs came out between 1890 and 1924. Annie Jump Cannon had in
her life time classified 350,000 stars. This became a significant resource
for astronomical research. We will see its importance in connection
with the Hertzsprung-Russell diagram.
The letter scheme actually turns out to be connected to the star's surface temperature as seen in the diagram (13.12) from Kaler's book. The physical elucidation of the spectral classification was largely due to the work of Cecilia Payne-Gaposchkin (1900, 1979). See the discussion on black body radiation .
Harvard's spectral classification team : The Lady Computers
original link now defunct: http://physics.carleton.edu/Astro/pages/marga_michele/harvard.html
LOCAL LINK
Expanded Categories of Stellar Spectra
"Modern Theories of the Universe from Herschel to Hubble", Michael J. Crowe, Dover Publications, 1994
"The Norton History of Astronomy and Cosmology", John North, W.W. Norton & Co., 1995
"Astronomy: A Brief Edition", James B. Kaler, Addison-Wesley, 1997