Why are the Balmer lines weak in the spectra of hot and cool stars, but strong in the spectra of intermediate-temperature stars?
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If a star is cool, there are few violent collisions between atoms to excite the electrons, so the electrons of most atoms are in the ground state, not at the second level. Electrons in the ground state can't absorb photons in the Balmer series. As a result, we find weak Balmer absorption lines in the spectra of cool stars.
In the surface layers of hot stars, there are many violent collisions between atoms. These collisions can excite electrons to high energy levels or even ionize some atoms by knocking electrons completely out of them. In that situation, there are few hydrogen atoms with their electrons in the second orbit to form Balmer absorption lines, and these stars have weak Balmer absorption lines.
In stars of an intermediate temperature, the collisions are just right to excite large numbers of electrons into the second energy level. Hydrogen gas at the temperature of roughly 10,000 K absorbs photons very well at the wavelengths of the Balmer lines and produces strong (dark) spectral lines.
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