Q:  Why are the lines weak in O stars?

          The temperature is so high that most
          hydrogen atoms are ionized; they cannot
          absorb any photons.


  Q:  Why are the lines strong in A stars?

          The temperature is just right to 
          put the largest fraction of hydrogen
          atoms into the n=2 state, from which
          they can absorb Balmer photons.


  Q:  Why are the lines weak in K stars?

          The temperature is so low that almost
          all hydrogen atoms are in the ground
          state n=1, and very few hydrogen atoms 
          are in the n=2 state.  Thus, the
          atoms cannot absorb Balmer photons.