If you were to imagine a description of nature whose constituents are so bizarre that even its originator refuses to allow for their actual manifestation, you would not have to go past the theory of general relativity. Created almost a century ago, it was perhaps the most anticipatory advancement in the history of physics. Its development was so visionary that none of the four significant tests applied to it since – two of which were adjudged to be of Nobel quality – have truly exposed the core of this remarkable theory, where the most abstruse distortions to the fabric of space and time are imprinted.

Einstein suspended his belief at the thought of a universe that would permit singularities to form, in which matter collapses inexorably to a point and becomes forever entombed. Yet this was the boldest consequence of his new description of gravity.

Remarkably, the idea that a gravitational field ought to bend the path of light so severely that the heaviest stars should then be dark was actually forged much earlier, in the context of Newtonian mechanics, toward the end of the eighteenth century. The Reverend John Michell argued in a paper published by the Philosophical Transactions of the Royal Society that, if a star was sufficiently massive, its escape velocity would have a magnitude exceeding even the speed of light, which, being comprised of particles, would then slow down and fall back to the surface.

Clues to the chemical evolution of the Galaxy

The evolution of the composition of matter can be traced back through the various ages of the Galaxy by systematically examining surface abundances over a vary large population of stars by means of spectroscopic analysis (Table 8.1). One is particularly interested in elements observed in the spectra of ancient suns in the galactic halo. These little stars, the oldest we know of, are still shining valiantly today, boasting their exceptional longevity (Fig. 8.1).

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:

• the light nuclei Li, Be and B;

• the α nuclei, i.e. multiples of the helium nucleus, such as Mg, Si, S and Ca;

• nuclei around the iron peak, viz. Sc, Cr, Mn, Fe, Co, Ni, Cu and Zn;

• heavy s and r isotopes like Sr, Y, Ba and Eu.

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.

Does invisible matter exist?

Two arguments support the idea that some invisible substance exists in the Universe. The first is dynamic. It starts from observation of motions under the effect of gravity. The second is related to Big Bang nucleosynthesis, i.e. nuclear cosmology, which combines cosmology and nuclear physics.

Dynamical proof

The first observation is that, if we can go by what light is telling us, most of the matter in the Galaxy (and indeed any galaxy) is concentrated in the galactic bulge, a marked, reddened swelling at the centre of the star distribution. Therefore, if we assume that the mass distribution of luminous objects is representative of the total mass distribution in galaxies, every spiral galaxy should behave like a vast Solar System, with the stars and clouds playing the role of planets and the galactic bulge that of the Sun.

Nuclear statistical equilibrium

The most tightly bound nuclei, i.e. the most stable and robust, in the iron peak are not symmetric arrangements bringing together equal numbers of protons and neutrons (N = Z). Rather, they possess a neutron excess (N − Z) between 2 and 4. Close to iron, the most stable nucleus 56Fe has a number of neutrons which exceeds the number of protons by 4 units (N − Z = 4).

The isotopic and elemental abundance table shows that, in the Solar System, iron is more abundant than its neighbours. Analysis of stellar spectra confirms this result, giving it a universal character.

Theoretically, nuclear strength is enhanced by internal transmutations of protons into neutrons, under the mandate of the weak interaction, either by positron emission (p → n + e+ + ν) or by electron capture (p + e− → n + ν). However, the weak interaction is much slower than the strong interaction. The question remains as to whether it will happen inside the star, or outside, once the matter has been expelled, i.e. after the explosion. This is not just an academic question. The answer we give will determine whether or not we can corroborate explosive nucleosynthesis by observation.

Any attempt to understand the conditions in which iron and its kin were created, and identify the astrophysical site of their birth, must focus on the idea of nuclear statistical equilibrium. The situation is the exact nuclear analogy of the ionisation equilibrium occurring in hot gases.

Aims and aspirations of nuclear astrophysics

The aim of nuclear astrophysics is threefold. Firstly, it seeks to determine the mechanisms whereby the various nuclear species occurring in nature are built up, from deuterium with its two nucleons to uranium with 238 nucleons. Secondly, it seeks to identify the astrophysical site in which these species are produced. And thirdly, it attempts to unravel the temporal sequence of the nuclear phenomena that fashion baryonic matter, the stuff of stars and humans, making up the galaxies. Beyond this, it aims to explain the composition of the Solar System and the main trends of chemical evolution in the Galaxy, such as the gradual enrichment in metals and the relative abundances of the elements. It explores in detail everything from the first stages of chemical evolution in the Universe through to the most recent and violent events of nucleosynthesis related for the main part to supernovas and large stars with strong stellar winds (Wolf–Rayet stars). Indeed, gamma photons of precise energies are emitted by the freshly fashioned nuclei in high-wind stars and supernovas, radioactive nuclei in search of their ultimate form. Without a doubt, these high-energy photons constitute the purest clues as to the mechanisms producing atomic nuclei in the Universe. Hence, the spectroscopy and mapping of celestial gamma sources should provide us with fresh evidence of nucleosynthesis and locate its centres in our own galaxy and beyond.

Chemical evolution of galaxies

The key word in modern theory is ‘evolution’. The impressive consistency of the astronuclear view of the heavens has established the idea of an evolution of nuclear species which has the same significance for astrophysics as the evolution of living species for biology. It is itself preceded by an evolution of particle or corpuscular species, which would have been very short, lasting less than 1 second. This process was of a quite crucial nature in determining the components available to build up atoms, that is, those stable particles, protons and neutrons, that serve as the building-blocks, and the forces that bind them together.

Once the elementary particles are produced, nuclear evolution precedes and determines all others, including geological and biological evolution, and its main agent is the stars. There are four main arguments to support the idea of a stellar genealogy for atomic matter. These can be described as the poverty of the ancients, the evolutionary trail, the great galactic cycle, and stellar alchemy. They are not independent of one another. Quite the contrary, they are very deeply related through the dialectic between big and small, astronomical and nuclear.

Note that I do not say ‘infinitely small’, for there are things smaller than atomic nuclei, namely elementary particles. There are also things larger than the astronomical scale of stars and galaxies that concerns us here.

Modern cosmology is a physical and mathematical tale, telling of the creation of the Universe from nothing, or almost nothing, and describing its composition, structure and evolution. All the subtlety of this story lies in the word ‘almost’. A perfectly rational discourse on the origin of the Universe, taken in the absolute sense, would nevertheless appear to be impossible because in the beginning the terms ‘time’, ‘space’ and ‘energy’ are undefined. Zero time is an instant in time that does not yet exist. The quest for the origin, or rather some mirage of the origin, remains the principal driving force in cosmology. However, in order to escape from the self-contradictions of the initiating event and conceptual catastrophes it triggers, one might assign a less ambitious aim to astronomy and its related sciences. For example, one might begin simply by trying to give meaning to the words ‘Universe’, ‘matter’, ‘light’, ‘Big Bang’, ‘star’, and now ‘quintessence’. Spelling out this cosmic semantics, we arrive at the following (provisional) definitions.

The Universe is what extends, proving itself through an expansive motion, which seems to accelerate without respite and without hope of return, and through its evolution, an irreversible advance towards more complex atomic structures. Its components are matter and quintessence. Matter is what has weight, gravitating and curving space. In this sense, light is matter in a neutral material form. Quintessence is a latent state of nature, being invisible and impalpable.

Rising ideas in the sky of knowledge

The constellated sky has never ceased to foster the enthusiasm of men and women in search of illumination. In his Theogony, Hesiod tells us how Gaea, the wide-bosomed Earth, arose from the vast and dark Chaos, accompanied by Eros. Gaea first bore Uranus, the starry heaven, then the barren waters of the sea.

Almost three millennia separate this genealogy of the gods from the modern idea of the expanding cosmos and the stellar origins of human matter, summed up so concisely in the two statements:

• the Universe is expanding;

• we are made from star dust.

What brought human beings to invent cosmology and relegate the celestial genealogy of the gods to oblivion? Is there no more drama in the heavens? No genesis?

The stars are just the punctuation in the text of the heavenly narrative. And yet, in our recitation, we claim to know the whole history of the Universe. Formulating its plot on the stage provided by the space–time of matter, astrophysics expresses a cosmic vision as well as a body of scientific thought. The sky is a palimpsest. Under the first visible writing, the starry sky, modern astronomy has managed to bring out, at least in part, very ancient hieroglyphics and original engravings.

From Hesiod to Aristotle, from Aristotle to Galileo, from Galileo to Einstein, the Universe has undergone repeated mental reform. Visionaries of infinity have replaced poets and men of God.

The language of light: equivalence of colour, wavelength and temperature

Unplaiting the braids of light, we discover the colour blue. Weaving together the strands of blue, red and yellow, all are swamped in white. Light thus looks white if it has a similar spectrum to the Sun, and other colours can be described as a divergence from the latter. However, the word ‘colour’ is not precise enough to qualify this attribute of light. Of course, scientifically, each note of light, each elementary tone of green, yellow, crimson, mauve or any other hue, is designated a number called wavelength. The tints of light are thus quantified, each assigned its corresponding wavelength. Blue is just a length, expressed by a number (roughly 500 nm), and likewise yellow (around 600 nm) and red (650 nm). So yellow lies between blue and red, and that is all there is to it!

On extragalactic and cosmological scales, light is reddened by the receding motion of its source. The further the source, the faster it appears to move away and the more the source is reddened. However, distance across space also goes with remoteness in time and the past of the Universe is tinged with red. Still further back, it even slips into the infrared.

Light is a conscientious messenger, carrying information from one point of the Universe to another. Atoms in stars speak the language of light to atoms in eyes. Why should we move when light can bring this wealth to us?

Matter with and without light

To begin with, how do astronomers know what stars are made of? The answer is that they have learnt to decode the language of light, of all the different kinds of light, be they visible or invisible. Atoms in stars speak to atoms in eyes using the language of light. I say it now, and I will say it again until the message is clear. It is the identical nature of emitter and receiver that makes perception possible.

Light has become as a mother tongue for astronomers, a conscientious and light-footed messenger, speaking volubly in every region of space, transporting information from one point of the Universe to another. It is like an expressive covering for the atom. Carnal and material beings long believed that all matter was like itself composed of atoms. Materialism merged with atomism and was taught as a definitive doctrine. It is quite understandable that astronomy, science of light, would be tempted to confine the Universe to its visible aspect, imprisoning it somehow in its mere appearance.

Then suddenly, against her will but in all lucidity, Urania the ancient Greek muse of astronomy was compelled to admit that what is visible is only the froth of existence. She even came to give precedence to what cannot be seen, to what neither shines nor absorbs light. Astronomical observation and theoretical reasoning suggest that most of matter is actually invisible.

The Sun as reference

The motions of the Sun and Moon form the basis of our calendars. The measurement of mechanical time is largely based on the periodic reoccurrence of certain phenomena: the rhythm of day and night, the seasons, or the cyclic reappearance of the planets and stars in the sky. The flow of change is attested by the apparently irreversible global evolution of the Cosmos. Cosmic time, eternity's yardstick, is the measure of universal change, of the evolution of matter, and this evolution is essentially one of nuclear complexification, driven by stellar forces.

The material evolution we are speaking of here is at work in all galaxies. Every part of the Universe is evolving, and the driving force is the stars. Everywhere on Earth, there are men, women and children; everywhere in the sky, there are stars. The star seems to be the best-adapted form of the visible Universe.

The path that leads from the multitude of anonymous and abstract elementary particles generated in the original explosion to the grass in the meadows, to the rain and the wind, to the infinite variety of shapes and states, to the profusion of feelings, must necessarily pass through the stars. Stars are an essential link between the primordial raw material that came out of the Big Bang and complex material with the ability to think. Nuclear astrophysics is the bridge between elementary particle physics and life.

Sources of the elements

In the eighteenth and nineteenth centuries, chemists had so successfully isolated the elements that John Dalton was able to put together a genuine atomic theory. Dmitri Mendeleyev organised the elements into his periodic table, the culmination of scientific elegance.

Confirming the idea of atomic structure, J.J. Thomson discovered the electron and Ernest Rutherford the atomic nucleus. When the nucleus was broken, Rutherford succeeded in distinguishing the proton, whilst James Chadwick identified the neutron. J.J. Thomson and his son G.P. Thomson each received the Nobel prize, the first for showing that the electron is a particle and the second for showing that it sometimes behaves as a wave. The discovery in France of natural radioactivity opened the way to the prospect of spontaneous transmutation. In doing so, it also demonstrated that the elements of the periodic table are not eternal. Pauli hypothesised the existence of the neutrino and it was discovered twenty years later. In the meantime, quantum physics had taken a firm hold of the atom and its nucleus, which became its favoured plaything. Nothing relating to the microcosmos escaped its notice or evaded explanation by the new theory.

Today, physical chemistry has accomplished its great task of elucidating the microcosmos. The existence, properties and combinatory rules for atoms have been firmly established. The problem now is to work out where they came from.

History of the Sun

Nebulous birth

The sky is no empty arena and stars are not the only actors. The other player in the cosmic drama is the cloud.

The business of the perfect interstellar cloud is to confiscate or at least filter the light of stars lying behind or even within it. Certain clouds referred to as bright nebulas are lit up from within. They are in the process of giving birth to a generation of stars, for like rats, cats and fish, stars are born in broods. Hence, the large, dusty and icy interstellar clouds are not only repositories for the ashes of defunct stars, but also for the material that will give body to new stars. Those stars currently forming, still buried deep within this cloudy placenta, can be observed in the radio, millimetre and infrared regions. Indeed, absorption by gas and dust is minimal at these wavelengths.

Still curled up at the heart of the parent cloud, the stellar embryos attract more matter in order to embark upon the visible phase of an object of fixed mass in hydrostatic equilibrium. They then disperse any surrounding matter and begin their own lives as free and independent stars.

In truth, star formation from molecular clouds is no easy subject to study. This is because the processes involved change the density from 10−23 g cm−3 to about 1 g cm−3 within a space of only a few tens of millions of years.

Nuclear evolution

In the twentieth and twenty-first centuries, humankind has turned its attention not only to the energy of matter and attempts to master it, but also to the origin and evolution of the elements that make it up.

Nuclear evolution precedes and determines the evolution of life, and is itself preceded by the evolution of elementary particles. Such is the great scheme of material things. The idea of universal unity can only be strengthened by this knowledge. In this physical genesis, the star plays a crucial intermediate role between the Big Bang and life, and for this reason we owe it our closest attention.

The birth certificate of any member of the stellar society carries one of the three following comments:

• alone or accompanied;

• mass;

• metallicity.

Masses range between 0.1 and 100 M⊙ and metallicities from one-thousandth of the solar value up to the latter. Metallicity is a question of generation. It may be taken as our leitmotif that the most ancient of stars are also the poorest in metals.

This therefore defines the stellar condition. Binary stars behave differently to single stars, as is clear from the case of the type Ia supernovas. In order to simplify, let us leave them aside for the moment and consider the case of the only child.