Credit/Permission: © David Jeffery,
- This is a semi-log plot.
The horizontal axis is a normal linear axis in
The vertical axis is a logarthmic axis in elemental
are by number fraction.
- 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).
- 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.
- Why the zigzag pattern
in the plot?
Well even-Z nuclei
tend to be more stable than
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
tend to be more abundant than
The upshot is a zigzag pattern
(which is NOT perfect)
in a mass fraction
versus atomic number (Z)
- The plot
is NOT exactly the composition of the Sun
or any particular
astro-body, but rather the best-determined composition
primordial solar nebula
out of which the Solar System formed.
It is obtained from observations of the
and from primitive meteorites: i.e.,
meteorites that seem to have
undergone little chemical processing since the
solar system formation.
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.
at the present cosmic time is believed to be similar throughout the
observable universe, except for variations
in metallicity as explained below.
- As one can see, hydrogen
and helium are the dominant
elements by far.
- Once you get to atomic numbers greater than
30 (which is for zinc (Zn)), the abundances become relatively small.
- Now for
(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
are everything that is NOT H
Just accept it.
Metals are NOT
to be confused with ordinarily-defined
- The leading metals in
Solar system composition
in decreasing order
by NUMBER are oxygen (O),
iron (Fe), and
The leading metals in
Solar system composition
in decreasing order by MASS FRACTION are oxygen (O),
(see the plot).
- For the
primordial cosmic composition
by mass fractions,
see the table below
(local link /
general link: solar_composition_metallicity.html).
- The formulae for
arise because Big Bang nucleosynthesis
created ∼ 75 % hydrogen,
25 % helium,
∼ 10**(-10) % lithium,
and only minute traces of anything else.
The metals grew during
cosmic evolution by
nuclear burning of
The formulae are crude since it is NOT clear how
affected helium abundance since
helium is both created and destroyed
in stellar nucleosynthesis.
The metalliticity Z is left as a variable, since
it varies with astronomical object
depending on how much
affected the astronomical object.
At the present cosmic time,
Z is about 2 %, but Z varies wildly for
stars even in one
from a low of ≤ ∼ 10**(-6)
(see Wikipedia: SDSS J102915+172927 (Caffau's star))
to a high of ∼ 4 %???.
- In non-stellar astro-bodies
like rocky planets, Z can go to nearly 100 %, of course.
The deep interior (i.e., the core)
of the Sun and other
stars is richer in
He because of ongoing
White dwarf stars
can be nearly all helium
or metals in the interior
due to late stellar evolution
- The record low-metallicity
for stars as of
2013 was ≤ ∼ 10**(-6)
SDSS J102915+172927 (Caffau's star)
which is located in the Milky Way halo.
- The relative abundances among the metals
vary much less strongly than the overall metallicity
since the metals
are produced by the
fairly constant, astronomical-object-specific
processes in, mainly,
and kilonovae whose
and relative rates of occurrence are
very roughly constant over cosmic time.
It is the amount of such production that determines
the overall metallicity.