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Between 6600 and 8600 A The names assigned to the different spectral types have a historical origin and no physical significance. However, the order in which they appear in Fig. 6 is one of temperature, with the hottest stars at the top and the coolest at the bottom. This is apparent at once by considering the colors of the stars. The O and B stars are clearly blue, with their spectra rising toward the shorter UV wavelengths of the peak of the blackbody spectrum. The A, F and G stars become progressively “whiter”.

Under the conditions in most normal stars, the equation of state of a classical, nonrelativistic, ideal gas, provides a good description. Consider, for example, such a gas, composed of three different kinds of particles, each with its own mass mi and density ni . The mean particle mass will be n1 m1 + n2 m2 + n3 m3 ρ m ¯ = = . 68) n1 + n2 + n3 n The gas pressure will then be ρ Pg = nkT = kT. 69) m ¯ The mean mass will depend on the chemical abundance and ionization state of the gas. 5. 71) mH 4mH AmH where ZA is the mass abundance of an element of atomic mass number A.

The Sun is a G2 star, and its spectrum is largely indistinguishable from that of any other normal star of this type. As we will see, all of the main physical properties (mass, radius, luminosity) of the stars sharing a common spectral classification are the same. For completeness, we note that the spectral sequence extends beyond M stars to two cooler classes, labeled L and T. Strictly speaking, members of these classes are not stars but “brown dwarfs”, objects intermediate between stars and giant planets in their properties.

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