How can you tell a star is mainly comprised of hydrogen from emission spectra? I thought emission spectra were related to the discrete energy levels obtainable by the electron in the atom. But in a star, the temperatures mean that the matter is plasma right? Then aren't all the electrons stripped from the matter? How can spectra exist then? Specifcally the hydrogen ones.

The hydrogen lines from the sun are very weak, these are due to heating of gas around the sun, and nothing to do with the plasma. The Lyman lines are due to recombination and ionisation of hydrogen within the sparser areas of the plasma, along with heating as you suggest. The sun is not totally plasma, but a dynamic equilibrium of ionisation and recombination. In the outer layers where the plasma is not as dense, there is more opportunity for combined hydrogen to be excited.

Most of the spectra you see from the sun is due to metals like iron and lithium, I assume you have no problem in visualising how you can get emission lines from ionised heavier elements.

Ah I see thanks. Yeah I realised there was spectra when you heated gas i.e. hydrogen but I though the temperatures were so extreme everywhere in the sun that theere would be any opportunity to do so right? And if it only occurs on the outer layers, how is this method of looking at spectra accurate in determining a stars composition? Do they just make assumptions about the distribution predicted by theory + use temperature and size or star of something?