Which spectral classes do these mostly apply to and why, how can I find out

I'm not entirely sure of the question but the Balmer absorption lines refer to the light of the energy to raise electrons from the n=2 to higher orbitals in the Hydrogen atom.

I'm not sure what you mean by "spectral classes".

More info: Balmer series - Wikipedia, the free encyclopedia

Thanks,by spectralclassesi meant OBAFGKM it was A and B but I didn't know why

What on earth is OBAFGKM? :P

The spectral groups from blackbody radiation

I had actually never come across that clasification, I just would classify it by temperature, so our sun is a 5800K blackbody.

Interesting :)