Birth, Evolution and Death of Stars
It has been known for a long time that stars are similar to our Sun. But it was only in 1810 that they were shown to be made of an incandescent gas. The chemical composition of this gas began to be determined in 1860. In 1940, it was demonstrated that the energy radiated by the stars is of thermonuclear origin. How stars form from interstellar matter and how they evolve and die was understood only recently, with our knowledge still incomplete. It was also realized recently that close double stars present a wide variety of extraordinary phenomena, which are far from being completely explored. This book explains all these aspects, and also discusses how the evolution of stars determine that of galaxies. The most interesting observations are illustrated by spectacular images, while the theory is explained as simply as possible, without however avoiding some mathematical or physical developments when they are necessary for a good understanding of what happens in stars. Without being a textbook for specialists, this book can be profitably read by students or amateurs possessing some basic scientific knowledge, who would like to be initiated in-depth to the fascinating world of stars. The author, an emeritus astronomer of the Paris Observatory, worked in various domains of astronomy connected with the subject of this book: interstellar matter and evolution of stars and galaxies. He directed the Marseilles observatory from 1983 to 1988 and served for fifteen years as Chief Editor of the professional European journal Astronomy & Astrophysics. He has written many articles and books about physics and astronomy at different levels. © 2013 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
The Solar System has the structure of a disk, in which the planets, their satellites, asteroids and comets evolve at great distances from each other. The mass of this disk is very low with respect to that of the Sun: it is essentially distributed in the planets. The observation of objects in the Solar System orbiting the Sun has made it possible to discover the laws governing their mutual motions. Today, large computers make it possible to study the dynamics of the Solar System and its evolution on long time scales using the equations defining the N-body problem. From a physical point of view, the main difference between the Sun and the objects of the Solar System is the fact that the latter have no radiation of their own, whereas the Sun, like all stars, produces intrinsic radiation due to the nuclear reactions responsible, within it, for the fusion of hydrogen into helium.
The Galaxy seen from the Earth appears as a light trail along a large circle of the celestial vault: it is the Milky Way, which appears more or less divided in two by a dark band, due to the extinction of starlight by interstellar dust. Our anthropocentrism has long led us to believe that our Solar System, of which the Earth was to be the center, was unique in the Universe. The Copernican revolution gradually placed the Sun, and not the Earth, at the center of the Universe, and some have even imagined that other planetary systems could well exist, around stars similar to the Sun. For the planetesimals to form before the dissipation of the disk, the grains need to stick together fast enough to form larger bodies, of decimetric size, which will be less coupled with the gas and will not fall on the star.
It now appears very unlikely that our astronauts will encounter any kinds of life in our solar system that are of the intelligent persuasion anywhere close to our Earth-bound multicellular animal or even human types. However, as I indicated in Chap. 1, many of our space scientists now think it is possible, and a few believe even likely, that early stage primitive forms of life may have been able to evolve in the earlier histories of some of our inner planets (e.g., Venus and Mars) or on some of the moons of the outer planets, and some of these creatures may still be alive today. Although single-cell or microbial forms of life might still be present today in some locations, no multicellular forms are likely, assuming, of course, that life elsewhere is based on something equivalent to our “cells”. If man wants to search for life forms that are anywhere close to being like us in terms of being intelligent and even capable of communicating with us in some manner, we will need to extend our search for ET to other worlds outside our own solar system. At least for the near future, rocket science will not help us very much with this task. Mankind will need to become incredibly patient and somehow learn to cope with the real possibility that it may be our great-great-great…grandchildren and not us that will first view the exciting pictures and other data/information that our incredibly fast unmanned inter-stellar rocket ships will beam back to us sometime in the far distant future. This would, of course, assume that the speed of light is our ultimate speed barrier, or that nothing like “worm holes” or other fancy future technologies could be developed that might make it possible to somehow cancel or alter what appears to be a insurmountable and universal time/space restraint.
In the present book, in addition to taking advantage of my professional background as a neuroscientist to take on the difficult task of discussing how I believe alien nervous systems might be able to develop on other worlds in the universe, I also wanted to tackle the complex issues related to how difficult it might be for any possible life forms to survive in what we now believe is a universe that has frequently been characterized in the popular and scientific literature as either “life friendly” or, at other times, “unfriendly or hostile” to the existence of any known forms of life. This is, without any doubt, quite problematic to the story I am trying to present in the present book. What I would like to do is focus on how different kinds of life in different parts of the universe are affected by their own local environmental conditions. However, since I have knowledge of only one kind of life on only one kind of planet, I will have to limit my discussion to what our scientists currently believe they know about how our world may have, in the past, or could, in the future, host physical events that could support or impede the development of life right here under our own noses.
1. Overview 1.1 Stellar populations and chemical compositions of galaxies 1.2 Galaxy formation and evolution 1.3 Plan of this review 2. The Formation and Evolution of Stars 2.1 Basic physical properties of stars 2.2 The initial mass function 2.2.1 The local IMF 2.2.2 The IMF at other times and places 2.3 Rates of star formation 2.3.1 The local SFR 2.3.2 The SFR elsewhere 2.3.3 Factors affecting the SFR 2.4 Stellar evolution beyond the main sequence 2.4.1 Stars near solar mass 2.4.2 Stars of 1-4 M? 2.4.3 Stars above 8 M? 2.4.4 Stars of intermediate mass 2.4.5 Effects of initial composition 3. Aims and Methods of Chemical Evolution 3.1 Basic assumptions and equations 3.2 Analytical approximations 3.2.1 A closed system, initially unenriched gas 3.2.2 A system with infall balanced by star formation 3.2.3 Generalities 3.3 Numerical models 4. Chemical Evolution in the Solar Neighborhood 4.1 Outline of relevant data 4.2 The "G-dwarf problem" 4.2.1 Infall 4.2.2 Pre-enrichment of the disk gas 4.2.3 Variable IMF 4.2.4 Metal-enhanced star formation 4.3 Effects of galaxy formation 4.3.1 Metals from the young halo 4.3.2 Later metal-poor infall 4.3.3 Time scales for chemical evolution 4.4 Relative abundances of the elements 4.4.1 Primary elements from different stars 4.4.2 Secondary elements 4.4.3 Radioactive elements 5. Chemical Evolution of Galaxies 5.1 Outline of relevant data 5.2 Abundance gradients in spheroidal systems 5.2.1 Dissipative collapse 5.2.2 A gradient in the IMF 5.2.3 Finite stellar lifetimes 5.3 The metallicity-mass relation for elliptical galaxies 5.3.1 Supernova-driven winds 5.3.2 Bursts of star formation in merging subsystems 5.3.3 Mergers of stellar systems 5.4 The intergalactic medium and gas loss from galaxies 5.4.1 Loss of metals from galaxies 5.4.2 Overall gas loss from galaxies 5.4.3 Ejection from evolving stars in elliptical galaxies 5.5 Abundance gradients in disks 5.5.1 Effects of infall 5.5.2 Effects of radial gas flows 6. Approaches to Photometric Evolution 6.1 Aims and methods 6.1.1 Population synthesis 6.1.2 Evolutionary models 6.1.3 Analytical approximations 6.2 Evolution of a single generation of stars 6.2.1 Content and luminosity 6.2.2 Remnants of dead stars 6.2.3 The ratio of giants to dwarfs in the light 6.2.4 The stellar mass loss rate relative to luminosity 6.2.5 The mass-to-luminosity ratio 6.2.6 Evolution of luminosity and the Hubble diagram 6.2.7 Evolution of colors 7. Colors and Star Formation Rates 7.1 UBV colors of normal galaxies 7.1.1 "Standard" models 7.1.2 Possible effects of errors 7.1.3 Variations in age 7.1.4 Variations in metallicity 7.1.5 Variations in the IMF 7.1.6 Relevance to the formation and structure of normal galaxies 7.2 Colors of peculiar galaxies 7.2.1 Bursts of star formation and blue colors 7.2.2 Highly reddened galaxies 8. Conclusion
Morphologically normal and peculiar galaxies show very different distributions in the (U-B, B-V) diagram. To interpret these differences, an extensive grid of galaxy models with decreasing star formation rates (SFRs) and with bursts on various time scales has been constructed. Normal galaxies have colors that are consistent with a monotonically decreasing SFR, and very few can have experienced large variations in SFR with time scales not exceeding 500 million years. In contrast, the peculiar galaxies have a large scatter in colors that is consistent with bursts as short as 20 million years involving up to about 5% of the total mass. Nearly all of this scatter is associated with galaxies showing evidence of tidal interaction; moreover, interacting systems that are at early stages of dynamical evolution, as inferred from the absence of long tidal tails, have colors consistent with the most recent bursts. These results provide evidence for a 'burst' mode of star formation associated with violent dynamical phenomena.