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.

Properties of supernovas

At first glance, the spectral properties, absolute magnitudes (intrinsic luminosities) and shapes of the light curves of the majority of type Ia supernovas (SNIa) are remarkably similar. Only a few rather subtle photometric and spectrometric differences can be discerned from one object to another.

Hydrogen shines by its absence and the optical spectra of SNIa events feature spectral lines of neutral and once ionised elements (Ca+, Mg+, S+ and O+) at the minimum of the light curve. This indicates that the outer layers are composed of intermediate mass elements. SNIa events reach their maximum luminosity after about 20 days. This luminous peak is followed by a sharp drop amounting to three magnitudes per month. Later the light curve falls exponentially at the rate of one magnitude per month.

The exploding stars discussed here of a much more modest character than gravitational collapse supernovas, which arise when the core of a gigantic star gives way. However, they eject matter at very high speeds, up to 10 000 km s−1, and release a comparable amount of energy, some 1051 erg. They are genuine nuclear bombs, unlike the gravitational-collapse supernovas (SNII), which draw their energy from gravity.

In studying the Solar System, we find an important exception to our concept of astronomical objects being so remote that we cannot hope to visit them in the foreseeable future. People have already visited our nearest neighbor in the Solar System, the Moon, and brought back pieces to study in normal Earth-bound laboratories. Unmanned probes have landed on Venus and Mars and have visited all the other planets. Clearly, the opportunity for even limited close-up viewing has had a major impact on our understanding of the Solar System.

However, the study of the Solar System is not simply devoted to sending probes when we feel like it. The spacecraft have followed literally centuries of study by more traditional astronomical methods. By the time the first probe was launched to any planet, astronomers had already developed a picture of what they expected to find. Many of these pictures did not survive the planetary encounters, but they did provide a framework for asking questions, and for deciding what instruments were important to place on the various probes.

We have also had the advantage of having the Earth as an example of a planet to study. It has been possible to develop ideas about planetary surfaces, interiors, atmospheres and magnetospheres by studying the Earth. For that reason, we have devoted one whole chapter of this Part to the Earth, viewed not as our home base, but as just one planet.

In Chapter 10 we saw how stars evolve to the red giant or red supergiant stages, and how low mass stars (less than 5M⊙) lose enough mass to leave behind a white dwarf as the final stellar remnant. We also saw that electron degeneracy pressure can only support a 1.44 M⊙remnant. In this chapter we will see what happens to higher mass stars.

It is important to remember that stars lose mass as they evolve. This mass loss can be through winds, or the ejection of planetary nebulae. (In the next chapter, we will see that stars in close binary systems can transfer mass to a companion.) Though we only have estimates for the total amount of mass loss, it seems likely that massive stars can lose more than half of their mass by the time they pass through the red supergiant phase. A star's evolution will depend on how much mass it starts with, and how much mass it loses along the way.

Supernovae

Core evolution of high mass stars

In the core of a high mass star the buildup of heavier elements continues. If we look at nuclear binding energies (Fig. 9.3) we see that the isotope of iron 56Fe has the highest binding energy per nucleon. This makes it the most stable nucleus. This means that any reaction involving 56Fe, be it fission or fusion, requires an input of energy.

Distribution of galaxies

If we look at the distribution of galaxies, such as that shown in Fig. 18.1, we see that the galaxies are not randomly arranged on the sky. Among the patterns we see distinct groupings, called clusters of galaxies.

Clusters are interesting for a number of reasons. They may provide us with clues on the formation of galaxies themselves. This is especially true if, as many think, cluster-sized objects formed first and then broke into galaxy-sized objects. (The alternative view is that galaxies formed first and then gathered into clusters.) Clusters also pose us with interesting dynamical problems, including a dark matter problem of their own. Finally, when we reach the scale of clusters of galaxies, we are beginning to reach a scale which has some significance in the overall structure of the universe.

The cluster of galaxies to which the Milky Way belongs is called the Local Group. As clusters go, it is not a very rich one. Besides the Milky Way, it contains several irregulars, including our companions, the Large and Small Magellanic Clouds, the spiral galaxies M31 and M33, and a number of dwarf ellipticals. Other nearby clusters are named by the constellation in which they are centered. For example the Virgo, Coma, Hercules and Centaurus clusters are shown in Fig. 18.2.

Cluster dynamics

Just as with clusters of stars, clusters of galaxies may be isolated collections of masses interacting gravitationally. As such, they are interesting systems to understand.