I've looked up why before, tldr it's just because historically astronomers have never had to care in any detail about chemical reactions (this is not strictly true of course, but somewhat close for at least a large subset). So they just need a term for "crap that came from stars".
Eh, that grosses over the defining difference that hydrogen, helium, and a little bit of lithium are everywhere everywhen all at once because of The Bog Bang, whereas everything else is concentrated due to being produced in supernovas and neutron stars.
So for astronomers, metallic means "behaving like everything else other than hydrogen and helium".
In fact, I'm guessing "metal" (noun) came from use of "metallic" (adjective). The mention of helium betrays it - helium is what hydrogen behaves like normally "due to the insane pressures" (and heat), so what it basically says is just "not hydrogen".
Point is, not in the astrophycisists’ use of the word (and not really in any other use either – metallic hydrogen is not a metal, just behaves in some aspects like metal). To astrophysicists, hydrogen is not a metal by definition, which is literally "everything except hydrogen and helium". No matter how exotic a phase.
Nb. The subset of astronomers who call everything after helium "metals" is mostly disjoint from the subset that’s interested in gas giant interiors.
Reminds me of that article[0] that says the trebuchet is a gravity weapon. That's because it's the gravity that does both the launching and the dropping.
With some interesting consequences that sort of justify calling it that.
I really can't see why they are called categories... Do we have anything that we categorize and get categories from it? Looks like another synonym to "set" and "group" that wasn't used so far.
