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  Table:  Cosmic Composition
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  Substance          Primordial      Cosmic Composition  Solar Composition     Solar Composition
                     Cosmic          Fiducial Values     Asplund et al. (2009) Rounded 
                     Composition                         (Approx. Modern
                     Fiducial Values                     Cosmic Composition)
                     (mass fraction) (mass fraction)     (mass fraction)       (mass fraction)
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  Hydrogen (H,X)     0.75            0.75-Z              0.7381                0.74 = 74 % 
  Helium (He,Y)      0.25            0.25                0.2485                0.25 = 25 % 
  Metalliticity (Z)  0.001           Z                   0.0134                0.01 =  1 % 
  Deuterium (D, H-2) 10**(-3)
  Helium-3 (He,Y)    10**(-4) 
  Lithium-7 (L-7)    10**(-9) 
  ________________________________________________________________________________________________ 

1. Keywords: age of the observable universe = 13.797(23) Gyr (Planck 2018) (see Planck 2018: Age of the observable universe = 13.797(23) Gyr), Big Bang nucleosynthesis, cosmic composition, cosmic time, deuterium (D,H-2), helium (He,Y), helium-3 (He-3), Helium-4 (He-4), hydrogen (H,X), lithium, lithium-7 (Li-7), mass fraction, metallicity (Z), metals, primordial cosmic composition, solar photosphere composition, Solar System composition, substance.
2. The solar photosphere composition is believed to be close to the modern cosmic composition, except for wide variation in metallicity.
3. Metallicity varies widely in the observable universe: from of order 10**(-6) (e.g., Caffau's star (AKA SDSS J102915+172927)) to of order 0.027 or higher (e.g., Mu Arae (AKA Cervantes): see Wikipedia: Mu Arae: Stellar characteristics). See also Table: Stellar Population Metallicity below.
4. References: Asplund et al. 2009, Wikipedia: Big Bang: Abundances of the primordial elements, Wikipedia: Metallicity: Definition.

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  Table:  Stellar Population Metallicity for the Milky Way
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                     Population I          Population II            Population III
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  Age                young disk <∼ 1 Gyr   ∼ 12--13 Gyr             >∼ 13 Gyr
  (lookback time     old disk ∼ 1--9 Gyr
  to formation)
  ________________________________________________________________________________________________
  Metallicity [Fe/H] ∼ -0.5--0.3           bulge ∼ -0.5--0.5        ∼ -∞
  (abundance                               halo ∼ -8--(-1)
  ratio)

  Metallicity        0.006--0.04           2*10**(-10)--0.002       0 for Fe, but 
  (mass fraction                                                    10**(-9) for Li-7.
  very approx.)                                                                 
  ________________________________________________________________________________________________ 

1. [Fe/H] = log(N_Fe/N_H)-log(N_Fe/N_H)_☉: which is the logarithmic ratio of the number density of iron (Fe) to the number density hydrogen (H) normalized to the same for the Sun. By definition [Fe/H]_☉ = 0. [Fe/H] is a proxy for metallicity Z: the abundance of metals by mass fraction. Note [Fe/H] ≅ log(Z/Z_☉) very approximately.
2. Keywords: abundance ratio (e.g., [Fe/H]), age of the universe 13.797(23) Gyr (Planck 2018) (see Planck 2018: Age of the observable universe = 13.797(23) Gyr), Big Bang, cosmic composition, cosmic time, galactic bulges, galactic disks, galactic halos, galaxies (spiral galaxies), hydrogen (H), iron (Fe), lookback time, metalliticity (Z, [Fe/H], etc.), metals, Milky Way, Milky Way disk, Milky Way halo, Milky Way bulge, Population I stars, Population II stars, Population III stars, primordial cosmic composition, spiral galaxies, star formation, stellar populations.
3. The ages (lookback times) and metalliticities (abundance ratios [Fe/H]) in Table: Stellar Population Metallicity for the Milky Way are just the ones yours truly could find online. There must be better ones somewhere.
4. The gap in the ages (lookback times) suggests that people may admit there are intermediate stellar populations between I and II.
5. The oldest stars observed are estimated to be ∼ 13.2 Gyr or from cosmic time t ∼ 0.6 Gyr (see HE 1523-0901, Wikipedia: Oldest star, Wikipedia: Timeline of the formation of the Universe: Renaissance). They are all Population II stars. It is thought that all Population III stars were massive and exploded as supernovae within a few megayears of the Big Bang.
6. References: Chris Flynn's Module 2: Stars and "population types" (scroll down ∼ 40 %), Wikipedia: Caffau's star (AKA SDSS J102915+172927): (Z ∼ 10**(-6), and so for the Milky Way halo [Fe/H]=log[(M_Z/50)/(M_H/1)] ∼ -8).

Credit/Permission: © David Jeffery, 2016 / Own work.
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