Nucleosynthesis is closely linked with cosmology. The present composition of the Galaxy has arisen because the history of the Universe took the course it did. Some early theories investigated single processes in the early Universe that might have resulted in the present abundances. One of these was the ALPHA-BETA-GAMMA THEORY, so-called because it was proposed by the three physicists Alpher, Bethe and Gamow. This theory is based on the idea of a very hot, dense, early stage to the Universe (the primaeval fireball) where protons and neutrons might combine rapidly, but of itself it fails to account for the amounts of heavier elements we have now. Another idea was the EQUILIBRIUM THEORY. If a high enough temperature can be reached (>5 x 109K) under suitable conditions, then the nuclear reactions that are occurring are just as frequently reversed, so that the composition remains fixed, depending only on the actual temperature. Again, this one process, which, it was supposed, might have taken place in the early Universe, cannot account for the abundances of all the elements at once, though such a process might have occurred at some point.

The observational and theoretical evidence all suggests that a sequence of events has changed the composition of the Universe, including our Galaxy, and that the heavier elements have been built up from light ones. As the outer layers of some of the oldest stars in the Galaxy are metal-poor, they are thought to represent the original composition of the Galaxy. These stars are still in existence because their masses are low and their consequent rates of evolution are slow. Massive stars that were born at the same time have long since evolved. Many of these will have exploded as supernovae, scattering some of the contents of their evolved interiors into the interstellar medium, though exactly how much mass is lost by stars in this way is uncertain. Recent research has shown that during the explosion itself, conditions are right for nucleosynthesis to occur. This may turn out to be a major source of heavier elements.

If the rate at which supernova explosions occur has always beenthe same as it is now, it could not explain the observed enrichment of stellar material. As only the oldest stars have the metal-poor composition, it looks as though a burst of element production took place in the Galaxy, after the first stars were formed. Perhaps early in the history of the Galaxy, there were many more massive stars than we observe today; these would rapidly have become super-novae. Of course, much of the material that is converted into heavier elements remains locked up in the remnants (white dwarfs or neutron stars) of highly-evolved stars and is not available for new stars. The tendency for the stars poorest in metals to be found away from the galactic centre suggests that this may have been the site of the earliest and most intense element production. However, there remain many problems in piecing together a unified theory, and this is still very much an area of active research.

We have been much concerned with the production of heavy elements, but one of the worst headaches is the so-called HELIUM PROBLEM. The question is, essentially: was all or some of the helium present today part of the original composition of the Universe ? One of the troubles is that helium is notoriously illusive to detect. Its spectral lines are measurable only in the hottest stars (0- and B-types), so we cannot be sure of the exact abundance of helium m the Galaxy, though it seems to be around 25 per cent by mass in younger stars. Only about 2 per cent of the matter is in the form of heavier elements, the remainder being hydrogen.

Astrophysicists are reasonably agreed that the- hydrogen is primaeval (i.e. it dates from the earliest phase of the, Universe, or shortly after); but although helium is a major product of hydrogen burning in stars, it in turn is consumed in successive processes. If only 10 per cent of the mass of the Galaxy has been changed into the helium that is observable now, (i.e. not counting that which has subsequently been turned into heavier elements, or- will be used up in time in low-mass stars), the amount of on orgy released would have been so great that the total luminosity of the Galaxy would sometime have had to be at least 50 times greater than it is now. Furthermore, if all the helium was produced in stars, it is difficult to account for the proportions of helium and heavier elements. One theory that tries to explain this suggests that hypothetical super-massive stars ( > 100 M0) might have exploded after they had converted a substantial proportion of their hydrogen to helium, but before the helium had been changed into other elements.

The idea favoured by most astronomers is that most of the helium found in our Galaxy has not been assembled inside stars, since the problems of explaining its ubiquity in the observed high proportion (one-quarter of all cosmic matter) seem insurmountable. It is thought that helium could have been made in the primaeval Universe by the following process. In the laboratory, the free neutron decays in about 10 minutes to produce a proton and an electron. It has been shown theoretically that all the helium could have been formed in the first ten minutes of the Universe’s existence, before half the neutrons would have naturally decayed. In the primordial neutron-proton soup, neutron-proton collisions Produce deuterons. Deuteron-pair collisions result in the production of helium-3 and helium-4 (3He and 4He). The exact details depend on the model of the Universe that one adopts, but it does seem entirely plausible that the observed helium was made in the primordial big bang

The light elements, lithium, beryllium and boron have also presented some special difficulties. These are rapidly destroyed in stellar interiors. Although their abundances are low compared with elements of neighbouring mass, the fact that they exist at all needs explanation. A plausible theory is that they are produced when cosmic rays collide with heavy nuclei and break them up.

Out of the tangled web of data from nuclear physics (information on energy levels in nuclei and nuclear reaction rates) and spectroscopy (abundance and distribution of the chemical elements), astronomers have drawn a coherent picture of the history of matter. Elements are assembled inside the stars and then broadcast through space by supernova explosions. From the enriched interstellar medium, new stars and possibly planets form, incorporating the ashes of a previous generation. All common materials of our world were made in stellar furnaces before our Sun and planets formed; every atom of our bodies was fused together in past aeons of an almost fantastic galactic history. In truth we are the children of the Universe.

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