Stars elemental abundance classification

In the MK system, most J-type stars are classified as C4-5 4-5 stars which show very strong C and CN bands. Yamashita(l972,l975) classified many C7-9 stars most of which are CS or SC stars. His classification of C7-9J stars is mainly based on Cl2Cl3(0,l)band at 6168 A, C13N(4,0)band at 6260 A, and Lil 6708 A line. In most of C7-9 stars, lines of s-process elements are greatly enhanced. It is a question if in all J-type stars abundances of s-process elements are nearly normal or not. 

Figure 17.2 shows the relative abundance of the elements of the universe and of the earth. The abundances are approximate, as a consequence of die difficulties in their assessment and limitations of experimental techniques. The abundances in the universe (based on spectral measurements on stars and interstellar matter) are used as a refinement of data obtained for the solar system. Stellar light is divided in spectral classes depending on the surface temperature of the star, see Fig. 17.1. The various classes (Harvard Spectral Classification) show lines of the elements as listed below in approximately decreasing intensity ...

Spectroscopy is the key to unlocking the information in starlight. Stellar spectra show a variety of absorption lines which allow a rapid classification of stars in a spectral sequence. This sequence reflects the variations in physical conditions (density, temperature, pressure, size, luminosity) between different stars. The strength of stellar absorption lines relative to the continuum can also be used in a simple way to determine the abundances of the elements in the stellar photosphere and thereby to probe the chemical evolution of the galaxy. Further, the precise wavelength position of spectral lines is a measure of the dynamics of stars and this has been used in recent years to establish the presence of a massive black hole in the centre of our galaxy and the presence of planets around other stars than the Sun. 

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