You're right! The hydrogen Balmer lines are strongest in spectra of medium-temperature stars (like our Sun), and weaker in both hot and cool stars. Here's why:

1. Excitation and Ionization:

* Hot Stars: Hot stars have very high temperatures (around 10,000 K and above). This means their hydrogen atoms are highly excited and often ionized. Ionized hydrogen (protons) doesn't produce the Balmer lines. Instead, hot stars show strong lines from highly ionized atoms like helium and oxygen.

* Medium-Temperature Stars: Stars with temperatures around 5,000-10,000 K (like the Sun) have the right temperature to excite hydrogen atoms to the energy levels responsible for the Balmer series. The balance between excitation and ionization is optimal for producing strong Balmer lines.

* Cool Stars: In cool stars (below 5,000 K), most of the hydrogen atoms are in the ground state (lowest energy level). While they can still be excited to the Balmer levels, the probability is lower, leading to weaker Balmer lines.

2. Absorption and Emission:

* Absorption Lines: The Balmer lines are typically observed as absorption lines in stellar spectra. This means that the hydrogen in the star's atmosphere absorbs light at specific wavelengths corresponding to the energy transitions between the Balmer levels.

* Emission Lines: In some cases, like in certain types of nebulae, hydrogen can emit light at the Balmer wavelengths. However, in the spectra of stars, absorption dominates.

3. Spectral Class:

* A Stars: The Balmer lines are particularly strong in A-type stars, which have surface temperatures around 7,500-10,000 K. This is why A stars are often used as a reference point for understanding the Balmer series.

In summary: The strength of the Balmer lines in stellar spectra is a direct consequence of the star's temperature and the balance between excitation and ionization of hydrogen atoms. Medium-temperature stars have the ideal conditions for producing strong Balmer lines, while hot stars are too hot and cool stars are too cold.