A. What is a star's spectrum? Which spectrum is most common in stars? How can it differ from star to star?
What does the spectrum tell us about the star?

Basic empirical data related to the physical properties of stars are obtained from the measurement and analysis of the radiation they emit. Studying the spectral characteristics of stars gives us perhaps the most significant information about stars. According to the type and characteristics of their spectra, the stars are divided into spectral types (classes). The basic criterion for classification is the intensity and type of spectral lines (emission and absorption) and the appearance of different molecular bands in the spectra of each of them. As the chemical composition of the surface layers of the stars is practically the same, it is clear that it is the temperature that determines the degree of excitation of atoms and molecules, and hence the spectral type. Over 90% of all stars can be classified as one of the seven major spectral class types. The types are indicated by letters taken from the older Harvard system.

Explanation:

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