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I understand that a scientific term need not be constrained by its etymology. But is there some significant reason why astronomers choose to call all elements heavier than helium "metals"?

Are metals (in the chemist's sense) commonly easier to detect by spectroscopy than non-metals? Or, were they easier to detect a hundred years ago? Do they typically have more diagnostic value or a more crucial role in stellar evolution? I know Iron specifically has a key role. They are very much less common by weight in the universe, unless you include Carbon as a metalloid, but of course they are much more numerous in the periodic table than non-metals.

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    $\begingroup$ Because H and He dominate, and everything else is a rounding error to astronomers (partly tongue in cheek). More tongue in cheek - One could argue that our sun has only 4 planets (Jupiter, Saturn, Uranus, Neptune) and the rest is a bit of residual rocky rubble - we are merely biased by our particular viewpoint from one chunk of rubble. $\endgroup$ – Jon Custer Dec 2 '15 at 19:18
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I have a feeling this has been answered before, but basically it is because H and He dominate the elemental abundances in the universe. When we look at what else there is we are guided by the elements we can ascertain are present in the photospheres of stars. It just so happens that the most prominent sgnatures are those due to atomic and ionic absorption features due to calcium, iron, sodium, magnesium, nickel and aluminium - i.e. "metals".

This is ironic, because actually, after H and He, the most abundant elements in the universe are oxygen, nitrogen and carbon (i.e. non-metals); but their signatures in the optical spectrum of stars are comparatively weak. As a result astronomers lazily refer to anything heavier than helium as "metals" and there is little motivation to change this nomenclature since there is an orders of magnitude gap between the abundance of helium and the abundance of the C,N,O etc. in the universe.

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  • $\begingroup$ Those most prominent signatures being something like the Fraunhofer lines. $\endgroup$ – user10851 Dec 2 '15 at 22:57
  • $\begingroup$ IRONic, hehe. :-) $\endgroup$ – LocalFluff Nov 9 '18 at 13:24

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