What trend is observed in the spectral lines of A-type stars?

In A-type stars, a trend observed in their spectra is that the Balmer lines, which are associated with hydrogen, become progressively weaker as the temperature of the star increases. This phenomenon can be attributed to the physical conditions present within the stars.

A-type stars are characterized by their high surface temperatures, typically ranging from about 7,500 to 10,000 Kelvin. At these temperatures, the majority of hydrogen atoms in the stellar atmosphere are in a higher ionization state due to the thermal energy present. As a result, the population of hydrogen atoms in the ground state—those capable of producing Balmer lines—diminishes. Consequently, the intensity of the Balmer lines weakens because there are fewer hydrogen atoms available to absorb and subsequently re-emit light at the specific wavelengths corresponding to the Balmer series.

This trend reflects the dynamic balance between temperature, ionization, and line formation in the stellar atmosphere, illustrating how the physical characteristics of stars can influence observable spectral features. In contrast, other options relate to behaviors of elements not primarily associated with A-type stars or do not accurately describe the spectral characteristics at these temperatures.