Sciency Words: Metallicity (An A to Z Challenge Post)

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, M is for:


Astronomers. All the other scientists had a meeting, and they all agree: there’s something wrong with those astronomers. For some reason, astronomers do not understand what is or is not a metal.

According to astronomers, the only elements that aren’t metals are hydrogen and helium.

Now it does make sense for hydrogen and helium to be special in astronomers’ eyes. By mass, something like 75% of the observable universe is hydrogen. Helium makes up almost all of the remaining 25%. And the hundred-plus other elements on the periodic table? All combined, all that other stuff constitutes less than 1% of the observable universe.

So for astronomers, it’s convenient to have a word that lumps all this “other stuff” together. But why does that word have to be metal? I’ve never found a wholly satisfactory answer for this, but I do have a personal theory.

Turns out that in technical shorthand, the amount of “other stuff” in a star is represented as [Fe/H]. That’s the chemical symbols for iron (Fe) and hydrogen (H). In other words, the amount of “other stuff” is quantified as a ratio (sort of) of iron to hydrogen (the math is a little more complicated than a simple ratio, but I won’t to get into that here).

I’m guessing that out of all the non-hydrogen, non-helium atoms you might expect to find in a star, iron must be the easiest—or at least one of the easiest—to identify with a spectroscope, and thus iron serves as a convenient proxy for everything else.

The quantity represented by [Fe/H] is called metallicity. Everyone would agree that iron is a metal, so that makes sense. But since metallicity actually tells us more than just the iron content of a star—since it also gives us a sense of how much carbon and silicon and argon etc is in that star—suddenly the word metallicity is covering metals and non-metals alike, in a way that comes across as very odd to everyone who isn’t an astronomer.

Next time on Sciency Words: A to Z, an electron by any other name would still be negatively charged.

11 thoughts on “Sciency Words: Metallicity (An A to Z Challenge Post)

  1. It’s interesting that they focus on iron for this, since it’s too much iron in the star’s core that reportedly causes it to collapse, sometimes generating a supernova in the process. Maybe Fe/H is a measure of where the star is in its life cycle.

    Liked by 2 people

    1. I’m trying to keep these posts short, so I didn’t go into that, but that is totally a thing that metallicity helps determine. More iron generally means older stars. There’s also ongoing debate about the relationship between a star’s metallicity and planet formation. I’m hoping to explore that more at some point in the future.

      Liked by 2 people

    2. That’s probably the reason they picked the iron ratio as a reasonable estimate. I would expect it to be one of the more abundant, if not the most abundant, element aside from H and He. Because of the way our universe worked out, elements below iron release energy when they fuse into higher elements. Elements above iron absorb energy when they fuse, but will release it when they decay back down towards iron. So iron is the low point of energy that everything else tends to move towards.

      Liked by 3 people

      1. That’s probably a big part of it. I’m not sure if it would be the most abundant after hydrogen and helium (oxygen is the third most abundant element in the universe, and carbon is the fourth). But iron’s got to be a pretty big part of what’s in a star, for the reasons you mentioned.


    1. It’s definitely an astronomy only thing, and it is weird. I’ve seen it a ton in astrophysical papers, and I still can’t get used to it. The math isn’t too hard, in my opinion, but for the purposes of a blog post I figured it’s better to skip it.


    1. I try not to let myself get bogged down in math. I’m not that good at it, first of all, and ultimately, I’m researching this stuff to use in science fiction. I am happy when the math makes sense to me, but I don’t feel I need to spend a lot of time and effort on it. At least not for my purposes.

      Liked by 1 person

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