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The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.
S-type stars are intermediate between carbon stars and normal giants. They can be grouped into two classes: intrinsic S stars, which owe their spectra to convection of fusion products and s-process elements to the surface; and extrinsic S stars, which are formed through mass transfer in a binary system.
The MK classification assigned each star a spectral type—based on the Harvard classification—and a luminosity class. The Harvard classification had been developed by assigning a different letter to each star based on the strength of the hydrogen spectral line before the relationship between spectra and temperature was known.
As such, color-color diagrams can be used as a means of representing the stellar population, much like a Hertzsprung–Russell diagram, and stars of different spectral classes will inhabit different parts of the diagram. This feature leads to applications within various wavelength bands.
Blue giant is not a strictly defined term and it is applied to a wide variety of different types of stars. They have in common a moderate increase in size and luminosity compared to main-sequence stars of the same mass or temperature, and are hot enough to be called blue, meaning spectral class O, B, and sometimes early A.
The uses of spectroscopy in so many different fields and for so many different applications has caused specialty scientific subfields. Such examples include: Determining the atomic structure of a sample [32] Studying spectral emission lines of the sun and distant galaxies [33] Space exploration; Cure monitoring of composites using optical fibers.
The sharp series limit is the same as the diffuse series limit. In the late 1800s these two were termed supplementary series. In 1896 Arthur Schuster stated his law: "If we subtract the frequency of the fundamental vibration from the convergence frequency of the principal series, we obtain the convergence frequency of the supplementary series". [5]
The Xe, Xc, and Xk are transitional types between the plain X- and the corresponding E, C and K classes. Other spectral classes include the T-, D-, and V-types . The Ld-type is a new class and has more extreme spectral features than the L-type asteroid.