<p>Stars are mostly hydrogen and helium. The presence of small amounts of other elements does change their luminosity, and their spectra. The heavy elements were formed in other stars.</p>

<p>After the big bang there was a short period in which the conditions were right for hydrogen to fuse into helium. The conditions were not suitable for any heavier elements to form, the state of the universe right after this time was about 3/4 hydrogen, 1/4 helium.</p>

<p>After some time clouds of hydrogen and helium began to form (around clumps of dark matter) and inside some of these clumps, smaller clouds collapsed to form the first generation of stars. These were large and they had no elements other than hydrogen and helium. These stars produced in their cores heavier elements like Carbon and Oxygen, and when they had reached the end of their lives, being big stars, they exploded in supernovae, and these elements were returned to the interstellar space, along with other heavier elements that were formed by the supernova.</p>

<p>The next generation of stars formed from a mixture of hydrogen, helium and the heavier elements formed in the first generation. These stars still had less heavy elements than modern stars, but were more like normal stars. Some of these stars are still around, they are known as Population 2 stars. They are found mostly in globular clusters <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html" rel="noreferrer">http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html</a>.</p>

<p>After the population 2 stars had lived their lives and died, either in a supernova or in a planetary nebula, elements that had been made in their cores was returned to the intersellar space, futher enriching it with heavy elements. It was from one of these clouds, seeded with elements from stars that had gone before it, that the sun and the solar system formed.</p>

<p>This is why Carl Sagan says <a href="https://www.google.co.uk/url?sa=t&amp;rct=j&amp;q=&amp;esrc=s&amp;source=web&amp;cd=1&amp;cad=rja&amp;uact=8&amp;ved=0ahUKEwiJwM2QicXSAhWlI8AKHSmTDI8QyCkIHDAA&amp;url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DtLPkpBN6bEI&amp;usg=AFQjCNHOIJ3CRxlExzJHhEBtoCq5XKPX1g&amp;sig2=fHb_SQYFjaGznPVf_28k4w&amp;bvm=bv.148747831,d.d24" rel="noreferrer">We are star stuff</a>.</p>

CC BY-SA 3.0
2017-03-08T16:31:40.800Z
Worth mentioning that Fe is the largest element which can be formed via "normal" fusion & all larger come from supernovas.
Carl Witthoft
CC BY-SA 3.0
2017-03-10T16:46:24.647Z
@CarlWitthoft (and whomever gave it an uptick) this is partially incorrect. Approximately half of the elements heavier than iron (up to lead) are NOT from supernovae and are made by slow neutron capture in intermediate mass stars.
ProfRob
CC BY-SA 3.0
2017-03-10T18:07:48.153Z
@RobJeffries you are correct - but only the s-process.  TIL rather a lot about the different processes for burning/forming heavy elements, in particular that almost all those processes other than s-process occur during or just barely prior to the supernova shock wave.   I should have said Fe was the limiting element produced via fusion rather than neutron (or putatively, proton) capture.
Carl Witthoft
CC BY-SA 3.0
2017-11-04T00:53:39.067Z
@CarlWitthoft Furthermore it is now becoming apparent that supernovae may not even by responsible for creating most of the r-process elements.
ProfRob
CC BY-SA 3.0
2017-03-07T18:59:39.647Z
2017-03-07T18:59:39.647Z
James K
<p><a href="https://astronomy.stackexchange.com/a/20318/1577">James' answer</a> already covers the fact that since helium was formed in the Big Bang (see <a href="https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis" rel="nofollow noreferrer">Big Bang Nucleosynthesis</a>), stars always formed with some helium (about 1/4 by mass). Those first stars fused the hydrogen and helium into heavier elements, and some of them eventually spewed these products into their neighbourhoods, so the next generation of stars was born with those "metals" (all elements heavier than helium) from the start. Each subsequent "generation" of stars was born with more and more of the detritus of the previous generations, so the Sun was born with a wide range of metals. All that said, the metal content in your average stars is still low as a fraction of mass. e.g. around 2% in the Sun.</p>

<p>To answer the title question, however, I suggest starting with a <a href="http://adsabs.harvard.edu/abs/2009ARA%26A..47..481A" rel="nofollow noreferrer">widely-cited review article</a> from 2009.  Their Table 1 gives the logarithm of the number densities relative to hydrogen, plus 12. i.e.
$$\log_{10}\epsilon_X=\log_{10}(n_X/n_\mathrm{H})+12$$
The values for carbon, sodium, silicon, and magnesium are 8.43 ± 0.05, 6.24 ± 0.04, 7.51 ± 0.03 and 7.60 ± 0.04, respectively. If you follow the paper trail, you'll find other estimates too.</p>

<p>Incidentally, the metal abundance of the Sun is actually a contemporary problem. Though this oversimplifies the details substantially, we generally measure the abundances by comparing spectral lines with models of the Sun's outer layers atmosphere. In the late 1990's and early 2000's, those models leaped forward from generally one-dimensional, hydrostatic models, to three-dimensional hydrodynamic models.  Although the 3D models fit the spectral data better, they do so with lower abundances than we inferred with the 1D models.  This revision isn't itself a problem but it means that our models of the Sun are quite a lot worse than we used to think (see Fig. 8 in the review), so there's a fair amount of effort going into trying to find what's missing from the models to explain the remaining discrepancy.</p>

CC BY-SA 3.0
2017-03-08T08:55:56.200Z
2017-03-08T08:55:56.200Z
2017-04-13T12:59:07.017Z
Warrick
<p>I've just begun learning of Astronomy and I can't figure out why any stars would begin their life with such elements if nuclear fusion hasn't created them.</p>

<p>Don't all stars begin life as Hydrogen?</p>

<p>I understand that the presence of these elements can change the luminosity of the given star. Is this also correct?</p>

CC BY-SA 3.0
2017-03-07T17:07:33.287Z
It would be true that all stars being life with just Hydrogen if only hydrogen gas clouds existed. However, we live in a universe that is 14 billion years old and many old and now dead stars have "polluted" the hydrogen clouds, filling them with trace amounts of heavier elements that then find their way into more recently formed stars.
zephyr
CC BY-SA 3.0
2017-03-07T19:14:58.203Z
See http://astronomy.stackexchange.com/questions/16311/how-can-there-be-1-000-stellar-ancestors-before-our-sun?rq=1
ProfRob
CC BY-SA 3.0
2017-03-07T19:18:20.263Z
And see table 2 of http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf
ProfRob
<star><stellar-evolution>
How much of carbon, sodium, silicon, and magnesium does the Sun have?
Jason Procka
# How much of carbon, sodium, silicon, and magnesium does the Sun have?

(Asked by: username_0 on 2017-03-07)

I've just begun learning of Astronomy and I can't figure out why any stars would begin their life with such elements if nuclear fusion hasn't created them.

Don't all stars begin life as Hydrogen?

I understand that the presence of these elements can change the luminosity of the given star. Is this also correct?

**Tags:** star, stellar-evolution

> **Comments:**
> **username_1**: It would be true that all stars being life with just Hydrogen if only hydrogen gas clouds existed. However, we live in a universe that is 14 billion years old and many old and now dead stars have "polluted" the hydrogen clouds, filling them with trace amounts of heavier elements that then find their way into more recently formed stars.  
> **username_2**: See http://astronomy.stackexchange.com/questions/16311/how-can-there-be-1-000-stellar-ancestors-before-our-sun?rq=1  
> **username_2**: And see table 2 of http://weft.astro.washington.edu/courses/astro557/LODDERS.pdf  

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## 2 answers

### Answer by username_3 on 2017-03-07. Score: 6

Stars are mostly hydrogen and helium. The presence of small amounts of other elements does change their luminosity, and their spectra. The heavy elements were formed in other stars.

After the big bang there was a short period in which the conditions were right for hydrogen to fuse into helium. The conditions were not suitable for any heavier elements to form, the state of the universe right after this time was about 3/4 hydrogen, 1/4 helium.

After some time clouds of hydrogen and helium began to form (around clumps of dark matter) and inside some of these clumps, smaller clouds collapsed to form the first generation of stars. These were large and they had no elements other than hydrogen and helium. These stars produced in their cores heavier elements like Carbon and Oxygen, and when they had reached the end of their lives, being big stars, they exploded in supernovae, and these elements were returned to the interstellar space, along with other heavier elements that were formed by the supernova.

The next generation of stars formed from a mixture of hydrogen, helium and the heavier elements formed in the first generation. These stars still had less heavy elements than modern stars, but were more like normal stars. Some of these stars are still around, they are known as Population 2 stars. They are found mostly in globular clusters <http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html>.

After the population 2 stars had lived their lives and died, either in a supernova or in a planetary nebula, elements that had been made in their cores was returned to the intersellar space, futher enriching it with heavy elements. It was from one of these clouds, seeded with elements from stars that had gone before it, that the sun and the solar system formed.

This is why Carl Sagan says [We are star stuff](https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiJwM2QicXSAhWlI8AKHSmTDI8QyCkIHDAA&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DtLPkpBN6bEI&usg=AFQjCNHOIJ3CRxlExzJHhEBtoCq5XKPX1g&sig2=fHb_SQYFjaGznPVf_28k4w&bvm=bv.148747831,d.d24).

> **Comments:**
> **username_4**: Worth mentioning that Fe is the largest element which can be formed via "normal" fusion & all larger come from supernovas.  
> **username_2**: @CarlWitthoft (and whomever gave it an uptick) this is partially incorrect. Approximately half of the elements heavier than iron (up to lead) are NOT from supernovae and are made by slow neutron capture in intermediate mass stars.  
> **username_4**: @RobJeffries you are correct - but only the s-process.  TIL rather a lot about the different processes for burning/forming heavy elements, in particular that almost all those processes other than s-process occur during or just barely prior to the supernova shock wave.   I should have said Fe was the limiting element produced via fusion rather than neutron (or putatively, proton) capture.  
> **username_2**: @CarlWitthoft Furthermore it is now becoming apparent that supernovae may not even by responsible for creating most of the r-process elements.  

---

### Answer by username_5 on 2017-03-08. Score: 4

[James' answer](https://astronomy.stackexchange.com/a/20318/1577) already covers the fact that since helium was formed in the Big Bang (see [Big Bang Nucleosynthesis](https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis)), stars always formed with some helium (about 1/4 by mass). Those first stars fused the hydrogen and helium into heavier elements, and some of them eventually spewed these products into their neighbourhoods, so the next generation of stars was born with those "metals" (all elements heavier than helium) from the start. Each subsequent "generation" of stars was born with more and more of the detritus of the previous generations, so the Sun was born with a wide range of metals. All that said, the metal content in your average stars is still low as a fraction of mass. e.g. around 2% in the Sun.

To answer the title question, however, I suggest starting with a [widely-cited review article](http://adsabs.harvard.edu/abs/2009ARA%26A..47..481A) from 2009. Their Table 1 gives the logarithm of the number densities relative to hydrogen, plus 12. i.e.
$$\log\_{10}\epsilon\_X=\log\_{10}(n\_X/n\_\mathrm{H})+12$$
The values for carbon, sodium, silicon, and magnesium are 8.43 ± 0.05, 6.24 ± 0.04, 7.51 ± 0.03 and 7.60 ± 0.04, respectively. If you follow the paper trail, you'll find other estimates too.

Incidentally, the metal abundance of the Sun is actually a contemporary problem. Though this oversimplifies the details substantially, we generally measure the abundances by comparing spectral lines with models of the Sun's outer layers atmosphere. In the late 1990's and early 2000's, those models leaped forward from generally one-dimensional, hydrostatic models, to three-dimensional hydrodynamic models. Although the 3D models fit the spectral data better, they do so with lower abundances than we inferred with the 1D models. This revision isn't itself a problem but it means that our models of the Sun are quite a lot worse than we used to think (see Fig. 8 in the review), so there's a fair amount of effort going into trying to find what's missing from the models to explain the remaining discrepancy.


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