What happens to the Balmer lines as we move from B to A-type stars?

As we move from B to A-type stars, the Balmer lines, which are spectral lines corresponding to transitions of electrons in hydrogen atoms, become weaker. The primary reason for this trend is related to the temperature and the ionization of hydrogen in the stellar atmospheres.

B-type stars are generally very hot, with surface temperatures around 10,000 to 30,000 K. At these high temperatures, the presence of hydrogen in its ionized state is significant. Consequently, the Balmer lines, which arise from transitions of electrons in neutral hydrogen, can be very strong under specific conditions.

As stars cool down to A-type (with surface temperatures around 7,500 to 10,000 K), although they still have prominent hydrogen lines, the effective temperatures lead to changes in the population of hydrogen atoms and their ionization states. The balance shifts, and in A-type stars, the overall strength of the Balmer absorption lines starts to diminish because there are relatively fewer neutral hydrogen atoms available to absorb the light at the Balmer wavelengths.

Thus, as we progress from B to A-type stars, the Balmer lines become weaker due to the decrease in the number of neutral hydrogen atoms and the effects of temperature on the excitation and ionization