Fundamental Spectroscopy & Photometry

Analyzing Star Light

Star light is the total sum of energy of many wavelengths. When this light is spread out to display spectrum, it supplies us various and important astrophysical information depending upon the presence or absence of certain wavelengths. Spectral classification started in the second half of the nineteenth century and from that time, various and important developments have been bring on it. The two most important systems in spectral classification, are the Harvard and the Yerkes. Classification of stars is based on patterns gradation of spectral lines and it is independent from theoretical considerations. The Harvard classification sequence is characterized by the rise or decline in the strength of the hydrogen lines throughout its extension. Lines of other chemical elements could become prominent or less at different stages of the sequence [ Ionized helium lines appear toward hotter end sequence - O type stars - while bands of chemical compounds appear toward cooler end sequence - K, M, R, S stars -]. First Harvard classification scheme, divided spectra into classes defined by capital letters arranged in alphabetical order and successive revisions rearranged the order of the groups to produce this final order sequence: O,B,A,F,G,K,M and sub groups W,R,N. Subsequently the system was complemented with subdivision denoted by range number 0 to 9 providing thus, a kind of decimal class intervals. While Harvard system is based on objective prism spectra the Yerkes system is based on slit spectrograms obtained with a prismatic spectrograph. The advantage of Yerkes solution - slit spectrograms over objective prism spectrogram.
+s - is due to the fact that the definition of spectra is independent by the quality of images. Yerkes solution therefore gives much higher quality material in resolution and contrast. Yerkes system is a development of Harvard classification and its main difference from Harvard is represented by the introduction of a second parameter related to the luminosity of the stars (distinction from dwarfs, giants and supergiants). More on spectroscopy classification and related arguments under the following links.

© 2006 - Valter Arnò.