Why do A-type stars have especially strong hydrogen spectral lines?

A-type stars exhibit especially strong hydrogen spectral lines because their surface temperatures—approximately 9,000 K—are optimal for populating the second energy level of hydrogen atoms.1

The visible hydrogen lines, known as the Balmer series, are produced by electron transitions starting from this second energy level. For these absorption lines to be prominent, a sufficient number of hydrogen atoms must have their electrons in this specific excited state.23

• In cooler stars (such as red dwarfs), the surface temperature is too low to excite many electrons to the second level, resulting in dim hydrogen lines.2
• In hotter stars (such as blue giants), the temperature is high enough that most hydrogen atoms become ionized, meaning the electrons are stripped away and cannot produce these specific absorption lines.21

Therefore, A-type stars provide the "just right" thermal conditions to maximize the population of the second energy level, leading to the most intense hydrogen absorption features observed in stellar spectra.21