Hydrogen lines are weak in the spectra of hot stars because many of the hydrogen atoms have their electrons in levels above the second level.

Question

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Hydrogen lines in the spectra of hot stars are weak because the high temperature causes many of the hydrogen atoms to have their electrons excited to levels above the second level, resulting in fewer hydrogen atoms in the ground state available to emit photons in the visible range.

Hydrogen Lines Are Weak in the Spectra of Hot Stars