The Solar Composition and the Cosmic Composition

The solar composition is, however, much more important than the Solar System because it is also approximately the cosmic composition (meaning inside modern galaxies: fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals) (i.e., the composition of the observable universe) for reasons explained below.

Features:

1. This is a semi-log plot.

   The horizontal axis is a normal linear axis in atomic number.

   The vertical axis is a logarthmic axis in elemental mass fraction.

   Many solar composition plots are by number fraction.

2. The data are from Asplund et al. (2005) and are compiled for easy reference at Table: Solar Composition and, in a different format, at Table: Solar Composition 2 and Table: Solar Composition 2 (txt).

3. The symbol for atomic number is Z (see Wikipedia: Atomic number).

   But Z is also the symbol for metalliticity (which we describe below).

   Context must decide which meaning applies as usual.

4. Why the zigzag pattern in the plot? Well, even-Z nuclei tend to be more stable than odd-Z nuclei for darn good nuclear physics reasons. Because they are more stable (and therefore tend to be more easily created, and more resistant to destruction than odd-Z nuclei), even-Z nuclei tend to be more abundant than odd-Z nuclei. The upshot is a zigzag pattern (which is NOT perfect) in a plot of mass fraction versus atomic number (Z).

5. The plot is NOT exactly the composition of the Sun or any particular Solar System astro-body, but rather the best-determined composition for the primordial solar nebula out of which the Solar System formed.

   It is obtained from observations of the solar photosphere and from primitive meteorites: i.e., meteorites that seem to have undergone little chemical processing since the solar system formation.

   The primordial solar nebula composition is a key datum in modeling the formation and evolution of the Solar System since it is part of the initial conditions from which all else followed.

   Assuming it came from well mixed interstellar medium (ISM), the solar nebula had a homogeneous composition.

6. As one can see, hydrogen and helium are the dominant elements by far.

7. Once you get to atomic numbers greater than 30 (which is for zinc (Zn)), the abundances become relatively small.

8. Now for metallicity.

   Metallicity (with symbol Z) is net abundance of astro-jargon metals which are usually just called metals.

   Recall metallicity Z and atomic number Z are different things. We had to recycle Z. A lot of symbols have to get recycled or we'd need more symbols than can be found in fonts.

   Metals are everything that is NOT H and He. Just accept it.

   Metals are NOT to be confused with ordinarily-defined metals though some metals are metals.

9. The leading solar composition metals in in decreasing order by mass fraction are oxygen (O), carbon (C), iron (Fe), neon (Ne), silicon (Si), nitrogen (N), magnesium (Mg), and sulfur (S) (see the plot).

   The leading solar composition metals in decreasing order by number fraction are: oxygen (O), carbon (C), neon (Ne), nitrogen (N), magnesium (Mg), silicon (Si), iron (Fe), and sulfur (S) (Cox-28--29).

Why is this so?

To foray into cosmichemical evolution:

1. Big Bang nucleosynthesis created primordial cosmic composition (fiducial values by mass fraction: 0.75 H, 0.25 He-4, 0.001 D, 0.0001 He-3, 10**(-9) Li-7) and nothing else.

2. Note that the primordial hydrogen and helium still dominates the abundances of those elements since their abundances have been only slightly affected by creation and destruction by stellar nucleosynthesis and supernovae since the Big Bang.

   Lithium has also been affected by creation and destruction in stellar nucleosynthesis and supernovae since the Big Bang, but how much is uncertain which is the cosmological lithium problem.

   Note that in the deep interior (i.e., the core) of the Sun and other stars is richer in He than solar composition because of ongoing hydrogen burning in the core.

   Note also that white dwarf stars can be nearly all helium or metals in the interior due to post-main-sequence evolution.

3. All the other elements (i.e., all the metals, except some lithium) have been created since the Big Bang in processes (mainly nuclear burning of hydrogen and helium in stars and more extreme nuclear reactions in supernovae and kilonovae). These processess have gone on in roughly the same ratio since after the reionization era (AKA cosmic dawn: cosmic time ∼ 0.150--1 Gyr, z∈∼[6,20]), and so give approximately the same distribution of metals in overall average.

   Thus, the relative composition of the metals (including lithium) is approximately universal: i.e., the cosmic composition (meaning inside modern galaxies: fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals) of metals.

   Note kilonovae may produce most of the R-process elements which are about half of all elements heavier than iron (Fe, Z=26). Kilonovae are the ejecta from neutron star mergers that have inspiraled due to loss of kinetic energy by gravitational waves. Most of the material from the 2 neutron stars ends up in a newly formed black hole or, sometimes, a larger newly formed neutron star.

4. The absolute overall abundance of metals does vary widely, however.

   The first stars the Population III stars had zero metals (aside from a little primordial lithium which goes without saying hereafter), but they are believed to have all been very large stars (because of formation with zero metals), and so exploded as supernovae within a few megayears and polluted the interstellar medium (ISM) with metals.

   The next early generations of stars (which formed from the polluted ISM) had varying low metallicity: i.e., the Population II stars. So they have the cosmic composition (meaning inside modern galaxies: fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals), but with low metallicity.

   The long-lived low stellar mass Population II stars are still around and show very low metallicity going down to Z ∼ 10**(-6) (e.g., Caffau's star (AKA SDSS J102915+172927)) which is 10**4 times smaller than Solar System metallicity Z ∼ 2.

5. The pollution of the interstellar medium (ISM) with metals increased with cosmic time for a few gigayears (Gyr), but eventually saturated causing all stars formed since saturation (the Population I stars) to have metallicity Z ∼ 2 to 3 at most???, but the exact amount varies widely.

   Why the saturation?

   Galaxies are NOT closed boxes. There are always outflows to and inflows from the intergalactic medium (IGM) The outflow remove ISM enriched in metals and the inflows inject IGM which has mostly just the primordial cosmic composition (fiducial values by mass fraction: 0.75 H, 0.25 He-4, 0.001 D, 0.0001 He-3, 10**(-9) Li-7)).

   For a discussion of the saturation process, see file metallicity_evolution.html

6. Note that in non-stellar astro-bodies like rocky planets and rocky-icy bodies metallicity Z can go to nearly 100 %. The hydrogen and helium of such astro-bodies is lost early after formation by atmospheric escape processes. The gravity of these astro-bodies is insufficient to hold the light gases of hydrogen and helium.

Credit/Permission: © David Jeffery, 2006 / Own work.
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