# In galaxy optical spectra, why are Hbeta and Hdelta sometimes absorption lines?

I've heard that $\mathrm{H\beta}$ and $\mathrm{H\delta}$ lines in galaxy spectra might be either emission lines or absorption lines, is that correct?

From Wikipedia, Balmer lines are usually emission lines in the spectra of spiral and irregular galaxies probably due to the presence of type A stars. If ellipticals lack hot (O, B and A type?) stars, and are gas poor(?), then is it the remaining stars doing the absorbing?

Also, are the CaII-H and -K doublet ever emission lines?

• For H lines, isn't that there is enough hydrogen out and around? Jan 15 '18 at 11:50