Caption: Spectrum (in black) with continuum scaled to 1 from Caffau's star (AKA SDSS J102915+172927) (Caffau et al. 2012, A primordial star in the heart of the Lion, p. 7).

Features:

1. Caffau's star is very metal-poor star (subdwarf star ??? with stellar classification ???) whose low metalliticity (i.e., mass fraction of metals) was discovered in 2011 (see Caffau et al. 2011, Nature, 477, 67; Caffau et al. 2012, A&A, 542, A51; Bonifacio et al. 2018, arXiv:1804.10419). Its metalliticity was a record low in 2012. For the current record low, see Current Record Low Metallicity Star below.

2. Its metalliticity Z ≅ 6.9*10**(-7) (see Wikipedia: SDSS J102915+172927: Side table). The solar metalliticity Z_☉ = 0.02 (fiducial solar metallicity) = 0.0134 Asplund 2009 value: see Asplund et al. 2009) = 0.0153 (Caffau et al. 2012, A&A, 542, A51 value). We have Z/Z_☉ ≅ 5*10**(-5) using the Z_☉ = 0.0153 value.
   Note the significant figures of the values 7.40*10**(-7) and 0.0153 show their precision (i.e., reproducibility measured in significant figures) given the modeling assumptions and techniques used to generate them and NOT their accuracy (i.e., closeness to truth) which as a guess is probably of order 50 %.

   The upshot of the metalliticity determination is that Caffau's star is an EXTREME Population II star.

   It seems that metalliticity Z ∼ < 0.001 = 0.1 % is a common definition of Population II star.

3. Caffau's star is in constellation Leo (symbol ♌) and is in the Milky Way halo at a distance of 1.27(15) kpc (see Caffau et al. 2011, Nature, 477, 67).

   Its age is ∼ 13 Gyr and its mass ≤ ∼ 0.8 M_☉.

   According to the Λ-CDM model (which fits all cosmological observations very well), the age of the universe = 13.799(21) Gyr.

4. The spectrum is from the Fraunhofer G' line region (i.e., Hγ 434.046 nm = 0.43406 μm region). The Hγ line is seen as absorption line near 434 nm.

   The continuum is scaled to 1 and the spectrum of another very metal-poor HE1327-2326 (with continuum scaled to 2 also in black) is shown for comparison.

   Caffau's star and HE1327-2326 are distinct in that HE1327-2326 is richer in carbon and this gives it stronger carbon absorption lines as we see by examination of the region ∼ 429.5--432 nm region which is expanded in the insert zoom-in for Caffau's star (AKA SDSS J102915+172927).

   The zoom-in shows model spectra with carbon abundance -3.81 dex below solar composition in red and abundance -2.48 dex below solar composition in green.

   The model spectrum with carbon abundance -3.81 fits the observed spectrum better.

5. Current Record Low Metallicity Star:

   As the current record low metallicity star Caffau's star ([Fe/H] = -4.89) has been beaten. Circa 2022, the certain record lowest metallicity star is SMSS J0313-6708 (age ∼ 13.6 Gyr, [Fe/H] ≤ -7.1). From the chemical abunance ratio

   [Fe/H] = log[ (f_Fe/f_H)/(f_Fe/f_H)_☉] ,

   yours truly estimates Z_SMSS J0313-6708 ≅ 7*10**(-9).

Images:

1. Credit/Permission: © David Jeffery, 2012 / Own work.
   Image link: Placeholder image alien_click_to_see_image.html.
   Image link direct: Itself.
   
2. Credit/Permission: © Caffau et al. 2012, A&A, 542, A51, 2012 / No known permission.
   Image link: A&A site.