© CAMBRIDGE UNIVERSITY PRESS 1997

1. INTRODUCTION AND OVERVIEW

The existence and distribution of the chemical elements and their isotopes is a consequence of nuclear processes that have taken place in the past in the Big Bang and subsequently in stars and in the interstellar medium (ISM) where they are still ongoing. These processes are studied theoretically, experimentally and observationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmochemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium.

Fig. 1.1 shows a general scheme or ``creation myth'' which summarizes our general ideas of how the different nuclear species (loosely referred to hereinafter as ``elements'') came to be created and distributed in the observable universe. Initially - in the first few minutes after the Big Bang - universal cosmological nucleosynthesis at a temperature of the order of 10⁹ K created all the hydrogen and deuterium, some ³He, the major part of ⁴He and some ⁷Li, leading to primordial mass fractions X 0.76 for hydrogen, Y 0.24 for helium and Z = 0.00 for all heavier elements (sometimes loosely referred to by astronomers as ``metals''!). The existence of the latter in our present-day world is the result of nuclear reactions in stars followed by more or less violent expulsion of the products when the stars die, as first set out in plausible detail by E.M. & G.R. Burbidge, W.A. Fowler and F. Hoyle (usually abbreviated to B²FH) in a classic article in Rev. Mod. Phys. in 1957 and independently by A.G.W. Cameron in an Atomic Energy of Canada report in the same year.

Estimates of primordial abundances from the Big Bang, based on astrophysical and cosmochemical observations and arguments. lead to a number of interesting deductions which will be described in Chapter 4. In particular, there are reasons to believe that, besides the luminous matter that we observe in the form of stars, gas and dust, there is dark matter detectable only from its gravitational effects (including gravitational lensing). This dark matter, in turn, has two quite distinct components: one is ordinary baryonic matter, consisting of protons, neutrons and electrons, which happens not to shine at any detectable wavelength (and may have primordial chemical composition); there is evidence for such baryonic dark matter (BDM) in the form of substellar sized objects from gravitational microlensing events in the halo of our Galaxy, and other forms may also exist as cold molecular gas and as rarefied ionized intergalactic gas. The other kind of dark matter could consist of some combination of massive neutrinos (if any of these have mass, which is not known at present) with more exotic non-baryonic dark matter (NDM), consisting of some kind of particles envisaged in extensions of the ``Standard Model'' of particle physics. The most popular candidates are so-called cold dark matter (CDM) particles that decoupled from radiation very early on in the history of the universe and now have an exceedingly low kinetic temperature which favours the formation of structures on the sort of scales that are observed in galaxy red-shift surveys. The seeds of those structures would have been quantum fluctuations imprinted very early (maybe ~ 10^-35 s) after the Big Bang during a so-called inflationary era of super-rapid expansion preceding the kind of expanding universe that we experience now, which later enabled pockets of mainly dark matter slightly denser than the average to separate out from the general expansion by their own gravity.

Fig. 1.1. A scenario for cosmic chemical evolution, adapted from Pagel (1981).

The first few minutes are followed by ``dark ages'' lasting of the order of 10⁵ years during which the universe was radiation-dominated and the baryonic gas, consisting almost entirely of hydrogen and helium, was ionized and consequently opaque to radiation. But expansion was accompanied by cooling, and when the temperature was down to a few thousand K (at a red-shift of a few thousand), matter began to dominate and first helium and then hydrogen became neutral by recombination. ⁽¹⁾ The universe became transparent, background radiation was scattered for the last time (and is now received as black-body radiation with a temperature of 2.7 K) and eventually gas began to settle in the interiors of the pre-existing dark-matter halos, resulting in the formation of galaxies, stars and groups and clusters of galaxies. The epoch and mode of galaxy formation are not well known, but the compact ultra-luminous objects known as quasars or quasi-stellar objects (QSOs), and intergalactic gas clouds with spectral lines of heavy elements seen in absorption on the sight-lines to quasars, are known with red-shifts up to about 5, corresponding to an era when the expanding universe was only 1/6 of its present size. The emission-line spectra of quasars indicate a large heavy-element abundance (solar or more), suggesting prior stellar activity, and numerical simulations of structure formation suggest that galaxies might form at red-shifts up to 10 or so, corresponding to an age of the universe between about 500 Myr and 1.5 Gyr according to what kind of cosmological model applies to our actual universe. The first number refers to the so-called Einstein-de Sitter model, preferred by many scientists, in which there is just enough gravitating matter to eventually slow down the expansion to zero velocity, while the second refers to a so-called open universe that goes on expanding for ever. In the most popular models, galaxies form by cooling and collapse of baryonic gas contained in non-gaseous (and consequently non-dissipative) dark-matter halos, with complications caused by mergers that may take place both in early stages and much later; these mergers (or tidal interactions) may play a significant role in triggering star formation at the corresponding times.

Galaxies thus have a mixture of stars and diffuse interstellar medium. (The diffuse ISM generally consists of gas and dust, but will often be loosely referred to hereinafter as ``gas''.) Most observed galaxies belong to a sequence first established in the 1920s and 30s by Edwin Hubble. So-called early-type galaxies according to this classification are the ellipticals, which are spheroidal systems consisting of old stars and relatively little gas or dust detectable at optical, infra-red or radio wavelengths; they do, however, contain substantial amounts of very hot X-ray emitting gas. Later-type galaxies have a rotating disk-like component surrounding an elliptical-like central bulge, with the relative brightness of the disk increasing along the sequence. The disks display a spiral structure, typical of spiral galaxies such as our own Milky Way system, and the proportion of cool gas relative to stars increases along the sequence, together with the relative rate of formation of new stars from the gas. At the end of the Hubble sequence are irregular galaxies, such as the Magellanic Clouds (the nearest galaxies outside our own), and outside the sequence there is a variety of small systems known as dwarf spheroidals, dwarf irregulars and blue compact and H II (i.e. ionized hydrogen) galaxies. The last three classes actually overlap (being partly classified by the method or discovery) and they are dominated by the light of young stars and (in the case of H II and some blue compact galaxies) the gas ionized by those young stars (see Chapter 3). There are also radio galaxies and so-called Seyfert galaxies with bright nuclei that have spectra resembling those of quasars; these are collectively known as active galactic nuclei (AGNs), and are associated with large ellipticals and early-type spirals. The increase in the proportion of cool gas along the Hubble sequence could be due to differences in age, the most gas-poor galaxies being the oldest, or to differences in the rates at which gas has been converted into stars in the past or added or removed by interaction with other galaxies and the intergalactic medium (IGM), probably some combination of all of these.

The figure illustrates very schematically some possible interactions between galaxies and the intergalactic medium. The diffuse IGM is a somewhat ghostly entity which may recently have been detected in the form of absorption of light from distant quasars by the He⁺ Lyman- line at 304 Å. This has been spread out by red-shift into a continuum shortward of 304 Å in the quasar's rest frame, and then also red-shifted to the middle ultra-violet range above 1200 Å accessible to the Hubble Space Telescope (HST); this so-called Gunn-Peterson effect (Gunn & Peterson 1965) was first predicted for neutral hydrogen, but has not yet been detected in that case, presumably because the hydrogen is too highly ionized. More definite evidence for intergalactic gas is found in the form of distributed X-ray emission from clusters of galaxies, where heavy elements are also present with an abundance of the order of 1/3 solar. All other evidence for intergalactic gas comes from some form of clouds, e.g. those producing the absorption-line systems in spectra of quasars, which exhibit a wide range of column densities and chemical compositions, usually with low or very low heavy-element abundances. These may represent primitive galactic halos (tenuous ellipsoidal outer extensions of disk galaxies), disks or building blocks thereof. Neutral gas is detected in disk and blue compact galaxies from the hyperfine 21 cm transition of neutral atomic hydrogen H I, but very few isolated intergalactic H I clouds have ever been detected. The interactions between the ISM and IGM include expulsion of diffuse material from galaxies in the form of galactic winds, stripping of (especially the outer) layers of the gas content of galaxies by ram pressure in an intra-cluster medium when the galaxy is a member of a cluster and/or inflow of intergalactic gas into galaxies, which latter may be indicated by observations of high-velocity H I clouds at high galactic latitudes in our own Milky Way system.

The figure also gives a schematic illustration of the complex interactions between the ISM and stars. Stars inject energy, recycled gas and nuclear reaction products (``ashes of nuclear burning'') enriching the ISM from which other generations of stars form later. This leads to an increase in the heavy-element content of both the ISM and newly formed stars; the subject of ``galactic chemical evolution'' (GCE) is really all about these processes. On the other hand, nuclear products may be lost from the ISM by galactic winds or diluted by inflow of relatively unprocessed material. The heavy-element content of the intra-cluster X-ray gas in rich clusters like Coma (the nearest rich cluster of galaxies, in the constellation Coma Berenices) is thought to result from winds from the constituent galaxies (see Chapter II), or possibly from the destruction of dwarf galaxies in the cluster.

The effects of different sorts of stars on the ISM depend on their (initial) mass and on whether they are effectively single stars or interacting binaries; some of the latter are believed to be the progenitors of Type La supernovae (SN Ia) which are important contributors to iron-group elements in the Galaxy. Big stars, with initial mass above about 10M (M is the mass of the Sun), have short lives (~ 10 Myr), they emit partially burned material in the form of stellar winds and those that are not too massive eventually explode as Type II (or related Types Ib and Ic) supernovae ejecting elements up to the iron group with a sprinkling of heavier elements. (Supernovae are classed as Type I or II according to whether they respectively lack, or have, lines of hydrogen in their spectra, but all except Type Ia seem to be associated with massive stars of short lifetime which undergo core collapse leading to a neutron star remnant.) The most massive stars of all are expected to collapse into black holes, with or without a prior supernova explosion; the upper limit for core collapse supernovae is uncertain, but it could be somewhere in the region of SCMR.

Middle-sized stars, between about 1 and 10M, undergo complicated mixing processes and mass loss in advanced stages of evolution, culminating in the ejection of a planetary nebula while the core becomes a white dwarf Such stars are important sources of fresh carbon, nitrogen and heavy elements formed by the slow neutron capture (s-) process (see Chapter 6). Finally, small stars below 1M have lifetimes comparable to the age of the universe and contribute little to chemical enrichment or gas recycling and increasingly merely serve to lock up material.

The result of all these processes is that the Sun was born 4.7 Gyr ago with mass fractions X 0.70, Y 0.28, Z 0.02. These abundances (with perhaps a slightly lower value of Z) are also characteristic of the local ISM and young stars. The material in the solar neighbourhood is about 15 per cent ``gas'' (including dust which is about 1 per cent by mass of the gas) and about 85 per cent stars or compact remnants thereof; these are white dwarfs (mainly), neutron stars and black holes.