Ancient stars in the galactic halo

Let us now describe the method used. The most accessible elements are those possessing clear lines in the optical spectra of these fossilised objects. In contrast, certain elements like neon and argon are not determined in these stars, whether they be dwarfs or giants. In their normal state, the noble gases produce no optical emission. 

Families that lend themselves best to this evolutionary analysis are  

Among these, iron is relatively easy to measure and serves as a reference, as a metallicity index, and thus as an indicator of the degree of evolution. Indeed, it is common practice in astronomy to treat the terms iron content (Fe/H) and metallicity (Z) as synonymous. Solar metallicity is denoted Zq. 

Source Courtesy of Andre Maeder and colleagues in Geneva. 

Stellar compositions are traditionally discussed in terms of abundances relative to iron, X/Fe (where X is the relevant element), as a function of the ratio of iron to hydrogen, Fe/H, this being the most convenient arrangement for observational purposes. 

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The VLT, centrepiece of the ESO, promises an exceptional harvest of astronomical data. Understanding the chemical evolution of the Universe requires a coordinated study of the most remote objects, ancient stars in the galactic halo and absorbent clouds in the line of sight of quasars. To this end, the high-resolution spectrograph UVES (Ultraviolet Echelle Spectrograph) was set at one focus of Kueyen, one of the four components of the VLT, perched at the top... 

The first major observation is thus that the Sun is a rich star compared with the ancient stars in the galactic halo, placed like a crown around the Milky Way, but that its composition is close to that of the stars in the disk where it itself resides. Our daytime star therefore belongs to the wealthy fellowship of the disk. Whereas the halo is almost completely devoid of gases, the disk abounds in them. 

We thus arrive at the following composition for the ancestral cloud that spawned the Solar System in 1 gram of matter, we find 0.72 g of hydrogen, 0.26 g of helium and 0.02 g of heavier elements. Despite the superb efforts of past generations of stars, the Sun, like its nebulous father, is singularly poor in metals, since these make up a mere 2% mass fraction of its matter. This, however, is a small fortune compared with the ancient stars in the galactic halo. 

It is worth noting in passing that the ratios O/Fe, Mg/Fe, Si/Fe, Ca/Fe and Ti/Fe in the ejected matter are roughly three times greater than their solar counterparts. These excesses of a nuclei are observed in ancient stars of the galactic halo, suggesting that the explosion of massive stars (type II supernovas) may have produced them (see Chapter 8). 

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