Hello, friends! Welcome to another episode of Sciency Words, a special series here on Planet Pailly that’s all about those weird words scientists use. Today on Sciency Words, we’re talking about:
METAL
Yes, scientists use some very strange words. You know the kind of words I mean. Words that are hard to pronounce. Words with definitions that only make sense if you understand differential calculus. But you know what’s even weirder? When scientists take words you already know and redefine them. That’s what astronomers and astrophysicists have done to the word “metal.”
Approximately 75% of the matter in the universe is hydrogen. 24% of it is helium. And the remaining 1%? Ask an astrophysicist, and they’ll tell you the remaining 1% is all “metal.” If that seems weird to you, don’t worry. All the other scientists think it’s weird too.
For years now, I’ve been trying to figure out how this started. Who gets credit (or blame) for first messing up the definition of metal?
I don’t know, but I do have a pet theory. Perhaps certain chemical elements (like nickel or iron) are easier to detect in outer space than others. And if you’re trying to study that 1% of the material universe that isn’t hydrogen or helium, perhaps those easier-to-detect elements (which happen to be metals) serve as a convenient proxy for everything else—including nonmetals like nitrogen, carbon, and oxygen.
According to the Oxford English Dictionary Online, the earliest documented usage of either “metal” or “metallicity” (in the astronomy sense of those words) is this 1969 paper on the molecular composition of stars. Now I won’t pretend to have read the whole paper (it’s over 60 pages long), but based on what I did read, I can say this much: this cannot be the true first usage of the word metal (in the astronomy sense).
At one point, the authors, two astronomers from U.C. Berkley, categorize nitrogen as a metal. No explanation is offered. Clearly the authors expect their readers (i.e. other astronomers) to understand why nitrogen would be considered a metal, which suggests to me that most astronomers in 1969 already understood “metal” to mean “matter that isn’t hydrogen or helium.”
However, I can also say this: I think this paper supports my pet theory. The paper describes a new technique for determining the molecular composition of stars. In explaining this new technique, the authors focus on the spectroscopic signatures of three specific elements: sodium, magnesium, and calcium. Those three elements are then used as a proxy for all the other non-hydrogen and non-helium elements that might be found inside a star.
Sodium, magnesium, and calcium are all—wait, let me double check the periodic table—yes, all three of those elements lie on the metal side of the metalloid line. And thus through a process linguists call semantic generalization, the word metal is generalized to mean something more than it originally meant.
Next time on Planet Pailly, someone really wanted to pick a fight with me about life on Mars.
Planet Pailly 
Wow. This is why jargon can be such a barrier, both to interdisciplinary work and to us poor layfolk. Nice detective work.
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Thanks, friend! This has been a tough thing to research, and I don’t think I’ll ever really find out how this started. But that 1969 paper does at least give a hint about what astronomers were thinking.
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To make matters worse hydrogen can exist as a metal under certain extreme conditions. I first became aware of it as an ideal rocket fuel because of its density. It also may exit in the depths of Jupiter, where the pressures force the hydrogen to take on properties of a metal.
https://www.space.com/39370-what-is-bizarre-metallic-hydrogen.html
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Good point! And under similar conditions there can be metallic helium as well, so I guess everything can be metal.
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Ahh… excellent. Yeah, I always wondered about this one, but never made the jump to it being a proxy. I’d bet you’re pretty close if not right on. Another great one.
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Thanks! I’ve been trying to figure this one out for such a long time now. When I finally tracked down that 1969 paper it lined up well with what I already suspected. It’s still only my own pet theory, though. I’d love to find the true first usage (or misusage) of this word.
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Heh. I like that, “usage (or misusage).”
I know the more common use comes from Greek, through, Latin and it really just means… metal, which doesn’t really help…
…but!
it also means, kind of poetically, to mine and, more poetically, I guess, to search (as in “he mined the internet to find out what metal means”). Maybe the first astronomers to use “metal” knew how much they were about to find out they didn’t know, and how much searching for answers they’d be doing.
Yeah, it’s a stretch. 🙂
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Hah! A stretch, maybe, but I like the way you’re thinking!
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Probably the normal definition of metal does not make much sense to astronomers. Metals are materials with a solid state or liquid phase in which electrons can move freely. Inside a star, everything tends to be a plasma, so what makes metals metals only appears at conditions that normally do appear in the atmospheres of stars. No wonder they started using the term differently.
Try using Google’s Ngram viewer. If you enter “metallicity” and then search through the 1950s, you get for example a “Zeitschrift für Astrophysik” from 1958 using the term in the context of star spectroscopy- I have not tried to trace this further back but or look more systematically, but it might go quite some time back. The term first appears in the 19th century but might have had a different meaning back then. In any case, the ngram viewer is a great tool if you research history of concepts and terminology.
The corresponding German term “Metallhäufigkeit” (for Metallicity, the term “Metallizität” seems to be a recent retranslation from English) seems to appear in 1946, but the whole idea might be older. Perhaps this terminology was first developed in Germany before English became the dominant language of science after WW II.
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I use Google Ngrams a lot. It’s led me straight to the first published usage of many terms. But of course the conventional meaning of metal is so common that it’s impossible to tell when the astronomy sense of the word first appears. That’s why I turned to the O.E.D. for help.
I hadn’t thought about the possibility that the Germans might have started this, but that would make sense to me. So much early 20th Century science was written in German.
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This terminology of distinguishing hydrogen and helium on the one hand from other elements on the other seems to make sense only in the context of cosmological theories that single out hydrogen and helium as special. Nuclear fusion as the source of star’s energy was hypothesized by Eddington in 1920, but that is not enough. He wrote about fusion of Hydrogen into Helium. The work of Hubble brings us to the late 1920s and early 1930s, so I guess that the term Metal in the Astrophysical sense was coined after that. On should search in the 1930s and 1940s. If it was in Germany, I don’t know. The Nazis started driving out lots of scientists, especially Jewish scientists, starting from the early 1930s and after that, English replaced German as the dominant language of Science. It is certainly possible to find it out, but it would require more time than I currently have, although it is an interesting question.
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Since all elements apart from hydrogen and helium were created by stellar nucleosynthesis, it’s convenient for astrophysicists to lump them together in a single category. Shame they chose a word that was already in wide use.
Anyway, since 98% of all atoms in the universe are hydrogen or helium, chemistry is really just a rounding error. To a good first-order approximation, we can say that it doesn’t exist. And since 95% of all mass-energy in the universe is dark matter or dark energy, I think we can say that physics (apart from general relativity) is statistically insignificant too. From this cosmic perspective, metals are a blip on some noise, and who cares what we call them?
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I certainly can’t argue with that! I seem to remember something like that in one of the Hitchhiker’s Guide books… that the total population of the universe rounds to zero, or something like that.
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