Metallicity of Celestial Objects: Why "Metal = Non-metal"?

Question

Metallicity of objects refers to the amount of chemical elements present in it other than Hydrogen and Helium.

Note: The other elements may or may not be actual metals in the true sense of their defintion.

But why did astronomers use such a term as metallicity? What is the history and cause behind coining such a term that may be (or rather actually is being) confused with metal-content of that celestial object ?

Is there any scientific objective or explanation to this? I do not believe it is just a random acceptance, but perhaps there is a not-so-solid motive behind doing this. If there is, what is it?

Answer 1

To first order, the relative abundances of the heavier elements to iron (for instance) are constant. So the metal content of a star is shorthand for the content of any element heavier than He. We now know this is not true in many circumstances and elements can be grouped by synthesis process - for example we can talk about "alpha elements" - O, Mg, Si etc. produced by alpha particle capture; or s-process elements - Ba, Sr etc. produced by the s-process. We know that the ratio of say O/Fe gets larger in more "metal-poor" stars, but Ba/Fe gets smaller. So talking about a single "metallicity" parameter only gets you so far, and the truth is more complex (and interesting).

The next point is why they are referred to as "metals" rather than another term like "heavies" or something. I would guess this is down to a bit of history and the fact that initial abundance analyses in stars were (and still are on the whole) done in the visible part of the spectrum (e.g. in the early part of the 19th century by Hyde Wollaston and Fraunhofer). The most abundant elements heavier than He are in fact not metals; they are Oxygen, Carbon, Nitrogen and Neon. However, the signatures of these elements are not at all obvious in the visible spectra of (most) stars. Whereas the signatures (absorption lines) of elements like Fe, Na, Mg, Ni etc., which decidedly are metals, are often very prominent.

Thus, there is a reason and some history behind the name "metals". It is that apart from hydrogen and helium, the metallic elements have the most prominent features in the optical spectra of stars, whereas in most stars the signatures of the more abundant non-metals are hard to see.

Answer 2

In 1812 Fraunhofer measured a set of absorption lines in the sun. It wasn't until later that Kirchoff and Bunsen figured out that these absorption lines matched emission features from metals they were burning in the lab. (http://www.chemteam.info/Chem-History/Kirchhoff-Bunsen-1860.html)

In chemistry, most things in the periodic table are known as "metals"( http://en.wikipedia.org/wiki/Periodic_table_%28metals_and_nonmetals%29), so given the small fraction that "metals" make up of the overall star, it may not be that far off to call them metals.

Astronomy does tend to keep historical conventions around (see magnitudes), but using the name metallicity does have some use. In many cases objects, like stars, can be classified into "metal rich" or "metal poor" to describe differing behavior; the specific elemental abundances aren't necessarily important for some of these studies. Using a blanket name like "metals" makes categorization much simpler.

Answer 3

There is no 'history' behind it. Stellar physics is less than a 100 years old. Thus, it is not a terminology that came to be due to some anecdotal reason. This is the way it has been since the start. Why?

Because we don't care. Why, really?

Hydrogen and Helium are more important and abundant, so we need something to measure the others, which can be grouped under one name due to their low significance to us. One fine day, one fine astronomer was sitting in his balcony, drinking coffee, and he thought, "What do I call a group of elements most of which are metals?" Surprise surprise, He decided to call them metals.

I am not kidding with you, although it may have sounded like that. But that is how it is with astronomical terminologies. I am assuming you are new to the field, because this is not the only crazy thing we've done to express how much crap we give in for terms and notations and units.

Answer 4

Even though it arised for historical reasons outlined in the other answers, the distinction between metals and non-metals as defined by astronomers does continue to make sense today.

Metals are formed in stars and supernovas, whereas non-metals preexist stars. Therefore, the distinction is relevant when considering nucleosynthesis.