Ancient stars

The futnre is likely to be no less rewarding. Astronomers have not let success go to their heads and their thirst for discovery and knowledge has remained intact. They are certainly not so naive as to believe that the stars have delivered up all their secrets. The most ancient stars, those born before the Galaxy had assumed its present form, have now become a subject of intense interest. The next goal on the distant horizon is a complete picture of chemical evolution in space. In this context, it is quite clear that the early stages of this evolution are the least well understood. The end of the road is not yet in sight. 

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 their shape that gives us a clue. All galaxies probably comprise a flat disk and a spherical halo. They then differ according to which of the two components is pre-eminent. The disk is the place where stars are currently forming, whilst the halo attests to past activity, being a gathering of ancient stars. Only the... 

Rather unexpectedly, a striking similarity is observed between proportions of r-process elements measured in a handful of ancient stars and the Sun. The hallmark of the r process thus appears very early on, indicating the operation of a rapid and efficient process in the very first stages of galactic evolution. 

The detection of thorium in stars of very low metallicity by Patrick Frangois and Monique and Frangois Spite has opened the way to a direct determination of the age of the most ancient stars, and hence also of the age of the Galaxy. The age of some of the oldest halo stars has been estimated at 15.6 billion years, with an error margin of 2 billion years. This agrees with the classic determination of the age of the oldest globular clusters at 14.9 1.5 billion years, and the age of the Universe by means of remote type la supernovas at 14.2 1.7 billion years (Arnould Takahashi 1999). 

Heavy nucleus abundances in ancient stars are determined by rapid neutron capture, very probably associated with type II supernovas. 

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). 

N H2CH2C02H. Notably, about half of these interstellar molecules are carbon-based organic molecules. As discussed in Chapter 4, the atoms originated from the nuclear fusion of ancient stars. How interesting it is that these atoms then join together to form molecules even in the deep vac-cum of outer space. Chemistry is truly everywhere. 

Figure 7.6 Ancient stars have a carbon core surrounded by a helium-fusing shell which is itself encased by a helium and a hydrogen-fusing shell. Small stars, less than 1.4 times the Sun s mass, die quietly. Very massive stars exploded, belching their carbon-heavy elements into space. These heavy elements, like carbon, eventually found their way into primitive life-forms on Earth.

Second, an element must be soluble in water to be available for life. The concentration of the chemical elements in sea water may give us the result of both effects, elemental synthesis in stars and release from the Earth s crust into water (Table 1.1). Table 1.1 also provides information (enrichment E) if a given element has been enriched (or to the contrary) on its way from ancient stars to our sea water and from the Earth s crust to sea water. The term is expressed as the decadic logarithm thus, for example, lg(E) = 2 means a 100-fold enrichment, and lg(E) = — 2 a 100-fold impoverishment. Since the abundance of an element in the universe is compared to that of hydrogen, lg(E) = 0 for H in Table 1.1. 

An ancient star journeying here So far, so near, bright seer This unseen dance A brilliant chance —Anonymous... 

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