[astroH]

Indeed, but we define their "metallicity" (mass fraction of elements heaver than helium) typically by the gas that they formed from. And the key point is that since they form from metal-free gas, you don't expect to see emission lines from metals which come from the star illuminating the surrounding gas with radiation.

[LarryMullins]

It makes sense to me that these stars would lack planets, and metallic gases around them and whatnot. But wouldn't you still get metal emission lines from the star itself? Or can those emissions not escape the star because the heavier elements are deep inside it?

[soiler]

I interpret

> the star illuminating the surrounding gas with radiation

to mean that we're looking at the spectra of the gas around the star, or at best the corona or maybe the surface of the star. I think it's very difficult for photons in the core of a star to reach the surface, so we probably don't see light from the heavier interior elements often or at all.

[martincmartin]

Inside the star, or even on the surface, there is a lot of energy, so you won't see the light that specifically comes from a single electron, in the first excited state with a well defined energy, that then decays to the ground state.

At least that's my guess, I'm not an expert.