<p>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. (<a href="http://www.chemteam.info/Chem-History/Kirchhoff-Bunsen-1860.html" rel="noreferrer">http://www.chemteam.info/Chem-History/Kirchhoff-Bunsen-1860.html</a>)</p>

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

<p>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.</p>

CC BY-SA 4.0
2024-07-02T12:29:36.813Z
Except that the most abundant "metals" are in fact non-metals.
ProfRob
CC BY-SA 3.0
2014-06-12T01:15:58.797Z
2014-06-12T01:15:58.797Z
Aaron
<p>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?</p>

<p>Because we don't care. Why, really?</p>

<p>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 <strong>metals.</strong></p>

<p>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.</p>

CC BY-SA 4.0
2022-11-02T16:36:55.847Z
Stellar spectroscopic physics dates back to the beginning of the 19th century.
ProfRob
CC BY-SA 3.0
2014-05-23T05:40:21.083Z
ya. I understand you. I just hoped that imagination that an astronomer didn't just think-let me call the rest elements as metals-that randomly. I hoped there might be a good story behind it. Something happens and it leads to another and that leads to another. It always is a chain reaction. My high hopes led to this question. Anyway, thank you for your inputs. :)
MycrofD
CC BY-SA 3.0
2014-05-24T10:34:32.880Z
Actually we do give a lot of 'crap' about those other elements and metals. Even in those days when solar physics was born, there was quite some interest in the chemical composition of stars. I'd still think the naming came for rather simplifying reasons.
AtmosphericPrisonEscape
CC BY-SA 3.0
2014-05-24T11:38:44.080Z
@AtmosphericPrisonEscape Yeah, that's what I said. The naming 'came' for simplifying reasons. 'Now' we don't give a crap.
Cheeku
CC BY-SA 3.0
2014-05-24T12:31:32.023Z
@Cheeku: I wasn't hitting on that, I was hitting on the myth that we don't care about metals in astrophysics&astronomy.
AtmosphericPrisonEscape
CC BY-SA 3.0
2014-05-23T00:32:43.217Z
2014-05-23T00:32:43.217Z
Cheeku
<p>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. </p>

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

CC BY-SA 4.0
2020-01-28T21:02:15.233Z
Apart from lithium and beryllium.
ProfRob
CC BY-SA 4.0
2020-01-28T15:32:13.427Z
2020-01-28T15:32:13.427Z
usernumber
<p>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 <em>not</em> true in many circumstances and elements can be grouped by synthesis process - for example we can talk about &quot;alpha elements&quot; - 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 &quot;metal-poor&quot; stars, but Ba/Fe gets smaller. So talking about a single &quot;metallicity&quot; parameter only gets you so far, and the truth is more complex (and interesting).</p>
<p>The next point is why they are referred to as &quot;metals&quot; rather than another term like &quot;heavies&quot; 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 <a href="https://en.m.wikipedia.org/wiki/Fraunhofer_lines" rel="nofollow noreferrer">Fraunhofer</a>). The most abundant elements heavier than He are in fact <em>not</em> metals; they are Oxygen, Carbon, Nitrogen and Neon. However, the signatures of <em>these</em> 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.</p>
<p>Thus, there <em>is</em> a reason and some history behind the name &quot;metals&quot;. 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.</p>

CC BY-SA 4.0
2022-11-02T19:59:20.437Z
Before 1925, it was generally believed  "that there were no significant elemental differences between the Sun and Earth". It was quite a surprise to learn that stars had so much hydrogen. https://en.wikipedia.org/wiki/Cecilia_Payne-Gaposchkin
PM 2Ring
CC BY-SA 4.0
2015-10-08T13:56:34.243Z
2022-11-02T16:36:15.300Z
2022-11-02T16:36:15.300Z
ProfRob
<p><code>Metallicity</code> of objects refers to the amount of chemical elements present in it other than Hydrogen and Helium.</p>

<blockquote>
  <p>Note: The other elements may or may not be actual metals in the true sense of their defintion.</p>
</blockquote>

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

<p>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?</p>

<star><galaxy><history><definition><stellar-structure>
Metallicity of Celestial Objects: Why "Metal = Non-metal"?
MycrofD
# Metallicity of Celestial Objects: Why "Metal = Non-metal"?

(Asked by: username_0 on 2014-05-22)

`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?

**Tags:** star, galaxy, history, definition, stellar-structure


---

## 4 answers

### Answer by username_2 on 2015-10-08. Score: 19 | Accepted answer

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](https://en.m.wikipedia.org/wiki/Fraunhofer_lines)). 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.

> **Comments:**
> **username_6**: Before 1925, it was generally believed  "that there were no significant elemental differences between the Sun and Earth". It was quite a surprise to learn that stars had so much hydrogen. https://en.wikipedia.org/wiki/Cecilia_Payne-Gaposchkin  

---

### Answer by username_1 on 2014-06-12. Score: 9

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.

> **Comments:**
> **username_2**: Except that the most abundant "metals" are in fact non-metals.  

---

### Answer by username_3 on 2014-05-23. Score: 7

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.

> **Comments:**
> **username_0**: ya. I understand you. I just hoped that imagination that an astronomer didn't just think-let me call the rest elements as metals-that randomly. I hoped there might be a good story behind it. Something happens and it leads to another and that leads to another. It always is a chain reaction. My high hopes led to this question. Anyway, thank you for your inputs. :)  
> **username_4**: Actually we do give a lot of 'crap' about those other elements and metals. Even in those days when solar physics was born, there was quite some interest in the chemical composition of stars. I'd still think the naming came for rather simplifying reasons.  
> **username_3**: @username_4 Yeah, that's what I said. The naming 'came' for simplifying reasons. 'Now' we don't give a crap.  
> **username_4**: @username_3: I wasn't hitting on that, I was hitting on the myth that we don't care about metals in astrophysics&astronomy.  
> **username_2**: Stellar spectroscopic physics dates back to the beginning of the 19th century.  

---

### Answer by username_5 on 2020-01-28. Score: 3

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.

> **Comments:**
> **username_2**: Apart from lithium and beryllium.  

---

