To determine the spectral class of a star, key pieces of spectral data that are necessary include:
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Spectral Lines: The presence and strength of absorption or emission lines in the star's spectrum are crucial. These lines correspond to specific elements and ionization states, which vary with temperature.
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Balmer Lines: The hydrogen Balmer series lines are particularly important in classifying stars. The strength and width of these lines can indicate the temperature of the star.
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Temperature and Color: The overall color of the star, which is related to its temperature, also aids in classifying it. Photometric data can help assess this when combined with spectral data.
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Continuum Spectrum: The shape of the continuum can provide information about the star's surface temperature and other properties.
Based on these spectral features, stars are classified into spectral classes (such as O, B, A, F, G, K, M), which correspond roughly to increasing temperature and decreasing color index. Additional subclasses (like 0-9) further refine the classification within each main class.
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