About five thousand million years ago, a new star was bom in our Milky Way galaxy. It was an average star, neither over-bright nor over-faint, and was formed like all other stars by condensation of the interstellar gas under the all-pervading force of gravity. In all such contractions some degree of rotation exists and this causes the material to form into a circulating disk within which condensations occur to form a number of gravitationally bound objects.
Whether these objects become a star or a planet depends entirely on the amount of material condensing in the gravitational contraction. If this is large enough, the interior will heat up to the ultra-high temperature needed to ignite a nuclear furnace which generates enormous energy by fusing the most abundant element, hydrogen, into helium. In our Solar System, only the Sun reached this critical mass and the other condensations resulted in the formation of the planets. In more than half of other star formations, more than one body exceeded the critical mass to become a star. This is demonstrated by the fact that more than half of the stars in the sky are multiple, with two or even three stars in orbit about themselves, possibly with some planets.
The interstellar gas from which the Solar System was formed was composed mainly of hydrogen (74 per cent by mass) with 24 per cent helium and all the other heavier elements from carbon to uranium contributing only 2 per cent by mass. These elemental abundances are reflected in the composition of the Sun and the giant planets, Jupiter, Saturn, Uranus and Neptune, but the Earth and the other terrestrial planets, Mercury, Venus and Mars, are rocky in nature, indicating a chemically selective process in their formation which favoured the heavier elements and allowed most of the hydrogen and helium to escape.
Yet the heavier elements did not exist at the start of the Universe when the primeval matter produced in the Big Bang was composed entirely of hydrogen and helium, with minute traces of lithium, beryllium and boron; but there was no carbon, nitrogen, oxygen or any of the other 87 elements found on Earth. Hence, the very first stars that were formed in the rapid star-burst era that marked the beginning of our galaxy contained no heavy elements, and any planets formed at that time could not have been even remotely like the Earth.
But many of those early stars were more massive than the Sun and, consequently, evolved more rapidly to the extent that they had gone through their whole life-cycles by the time the Solar System was formed. As will be related later in this book, they had successively fused hydrogen into helium, helium into carbon, nitrogen, oxygen, silicon and all the other elements in the periodic table up to iron; then, in an immensely explosive event called a supernova, all the elements heavier than iron, from cobalt to uranium, were formed, and these, together with the lighter elements, were hurled into space in a high-velocity expanding shell.
Far from being a contamination of the primeval interstellar gas, this was, as far as we are concerned, a crucial enrichment of it, because it provided those elements essential to life. Indeed, apart from the hydrogen present in the water of our bodies, all the other elements that constitute more than 90 per cent of what we are made of are the result of nuclear processing in the interiors of massive stars and the cataclysmic explosion that heralds the end of their life. We are children of the stars.
These astronomical processes, the nuclear synthesis of the heavier elements in stellar interiors, their ejection into the interstellar medium, and the selective condensation of those elements in the formation of the four terrestrial planets in our Solar System, set the scene for the great miracle—the development of life on one of them. Earth, whose size and distance from the Sun were just right. But the development of life and its evolution was slow…very, very slow.
The Earth was formed four and a half thousand million years ago but almost a full thousand million years were to pass before the first micro-organisms appeared and a further thousand million years before marine algae and primitive plants started to generate pure oxygen, not tied up in carbon dioxide, into the atmosphere until, when the Earth was three thousand million years old, it reached a critically important stage for life and the environment when it had an oxygen-bearing atmosphere similar to that of today (except for artificial pollutants).
At this point, evolution accelerated greatly and, over the next thousand million years, fish, insects, toothed birds, large reptiles and primitive mammals appeared. Then, 65 million years ago, a catastrophic event, whose cause has just recently been established as a giant meteor or asteroid, led to the extinction of the dinosaurs. It was then that the mammals proliferated and, some 3-4 million years ago, the first human types emerged. But evolution of our own species, Homo sapiens, did not occur until a hundred thousand years ago and the earliest civilizations did not develop until after the most recent ice age had ended about 12000 years ago.
To give some idea of the timescale of the development of life on Earth, the important milestones are listed in the table which also scales real times to one year, that is, as if the Earth were formed on 1 January and its present age is midnight on New Year’s eve. This demonstrates vividly the very slow initial development, and then the very rapid later evolution of life, together with the relatively brief presence of Homo sapiens.
Since this story relates the attempts of the human race to study and understand the Universe it lives in, it is confined, in human terms, to the very brief period of civilization which, on scaling to one year, covers only the last two minutes; but in astronomical terms, it goes back to the very beginning when, most astronomers now believe, the Universe started in a single, immense, explosive event—the Big Bang.
List of Illustrations
PART I THE EARLY DEVELOPMENTS IN ASTRONOMY
1 The Beginning
2 Ancient Astronomy
3 The Greeks
4 The Interlude
5 The Renaissance
PART II THE ERA OF THE TELESCOPE
6 The Classical Post-Newtonian Period
7 The New Natural Philosophy
8 Astronomy in the Early Twentieth Century
PART III MODERN ASTRONOMY
9 The New Astronomies
10 Probing the Solar System
11 The Stars—their Birth, Life and Death
12 The Great Post-war Astronomical Discoveries
13 The Nature, Origin and Evolution of the Universe
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