Correct, since any element heavier than hydrogen or helium is created in stellar cores or from supernovas; stars that are metal-poor are either very old or formed from virgin materials from the big bang.
That is, some 'metals' are present in the centre of stars, but not near the surface so it doesn't show up in the emitted light spectrum.
I would have expected the exact opposite.
They are very young, since they didn't have the time to form metal, yet.
They don't even mean elements, but spectral lines that correspond to elements if their spectrum was observed on Earth.
That's a weird distinction to try to make. Metal does mean the elements, not the spectral lines. The presence of those elements is usually inferred from the spectral lines, but if another method was used we would still say "metal poor/rich".
Other stars may as well be made of exotic particles that look like hydrogen to us.
Does anti-hydrogen have the same spectral signature as regular hydrogen?
I guess so, if its electron/positron shell is the same. But I dont mean just antihydrogen. I'm sure there are many nucleus configurations that have the +1 charge and thus look like hydrogen from the outside.
But a larger nucleus would be unlikely to produce the same spectral signature even with the same charge. The energy transitions would be different, especially with different orbital shells and subtle quantum effects.
> I'm sure there are many nucleus configurations that have the +1 charge and thus look like hydrogen from the outside.
Name three.
An anti-hydrogen star - or even an entire star system - surrounded by normal dust would show annihilation events in the spectrum. So it would be very distinctive and unusual.
And there may just as well be an invisible dragon in my garage. There's no theory nor evidence that such exotic hydrogen impostors exist, so by Occam's razor we must assume that if it looks like hydrogen and quacks like hydrogen, it is indeed hydrogen.