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John Gribbin with Mary Gribbin, Stardust : Supernovae and Life—The Cosmic Connection. New Haven, Yale University Press, 2000. xviii + 238 pages; illustrated with photographs, line drawings, and graphs; includes "further reading" and index.
It's now been said so often that it's a cliché: "Humans are made of stardust." It's a poetic statement of humanity's connection to the cosmos. It's a call to arms for biblical literalists. It's also the literal thesis of this book. We can easily forget that the phrase does have a literal meaning—or very nearly literal—that was once an exciting new discovery.
In the mid twentieth century quantum theory still was young, our understanding of how the sun continued to shine through nuclear fusion, and the fact that the universe is expanding, therefore that the universe also had a beginning and a discoverable age, was all fresh, new knowledge. Isn't it surprising that all of these facts have been known for less than 100 years still.
The story really begins in the 1920s, when astronomers began to appreciate that a star like the Sun is indeed, even today, largely made of hydrogen and helium—before that, they had assumed that stars were made of much the same sort of material as planets like the Earth, which is rich in iron, the most stable element.
Beginning in the 1920s, the story of how we are made of stardust, and are therefore the children of the stars, involves the understanding of how stars themselves work that was developed over the next few decades. It is no coincidence that this understanding developed when it did, because it involved both the special theory of relativity and quantum physics, ideas which were themselves new to science in the early twentieth century. In the nineteenth century, the fact that stars stayed hot at all was one of the greatest puzzles confronting not just astronomy, but physics. [pp. x—xi]
And so the central idea of the book is stellar nucleosynthesis: how the various elements come to be made in stars and supernovae through nuclear fusion in a process that is well understood now but was a novel idea in 1939 when Hans Bethe published his groundbreaking paper in Physical Review on "Energy Production in Stars". Unraveling the mystery of how stars continued to shine, where their energy came from, synthesized new knowledge of nuclear processes and astrophysics, and led to the realization that heavy elements were not made in the Big Bang but were created in stars and supernovae.
This book explains the relationship between life and the Universe, from the Big Bang to the arrival of the molecules of life on the surface of the Earth. It is a complete and self-consistent story, describing our cosmic origins from stardust. But it is not necessarily the whole story of life and the Universe, and before delving into the details, I want to describe briefly some of the more intriguing current ideas that may, if they are proved correct, take us beyond the story so far. The caveat is that "intriguing" doesn't necessarily mean "correct." But science progresses by making reasonable speculations, then testing those speculations to see how well they stand up. And in a book which claims to offer the best available scientific evidence for our own origins, it would be derelict not to make it clear just how science arrives at these profound conclusions. [p. 1]
In a manner that readers of Gribbin's other books will recognize, the author makes this a mostly self-contained book about how stellar nucleosynthesis works and how we came to understand how it works, providing plenty of groundwork information and history to complete the story. He does this and keeps his writing lively, precise, and filled with scienticity. Sometimes that means a short digression on how science itself works, digressions that I thought enhanced his story.
The history of science isn't always as neat and tidy as some of the accounts you read in books might suggest. Discoveries may come out of sequence, with notable insights that would have speeded the development of understanding sometimes not turning up for years, while on other occasions the relevance of a scientific discovery becomes clear only long after it is made. The parallel development, after about 1930, of the understanding of how stars work and of how the Universe came to be the way it is was particularly messy and confused, and although both developments depended on the new technology of improved telescopes and the new physics of quantum theory (which is why they proceeded in parallel), it took forty years for all the pieces to fit together into a self-consistent picture of how stars had evolved within an expanding Universe and where the elements that we are made of had come from. Remember that is was only at the end of the 1920s that astronomers even began to realize that stars are not made of the same stuff that the Earth is made of, and that their composition is dominated by hydrogen. At exactly the same time, Edwin Hubble and his colleague Milton Humason, working with the largest and best telescope then available on Earth, the 100-inch telescope on Mount Wilson, in California, discovered that the Universe is expanding. It was the discovery that would lead to the realization that the Universe had been born in a Big Bang, some 15 billion years ago, and that what had emerged from the Big Bang to form the first generation of stars was a mixture of roughly 75 percent hydrogen and 25 percent helium, with just a smattering of other light elements (including, crucially, deuterium). But that would not become clear until the end of the 1960s, after crucial developments in our understanding of how the heavier elements are made inside stars. [pp. 112—113]
Because he wants his exposition about nucleosynthesis to be complete, some of the discussion in the earlier part of the book covers some ground that Gribbin has written on in other books. This is not surprising since cosmology and quantum physics are topics that he visits frequently; I never felt like he was just repeating himself without something new to say. It seems to me that Gribbin approaches each new book project with a fresh outlook, so new insights appear even with familiar material. I found several provocative and profound thoughts in this volume to keep me satisfied.
In my view, one of the most profound discoveries made by science in the twentieth century is that the Milky Way Galaxy, which is, as far as we can tell, a typical representative of the myriad of galaxies that fill the Universe, is itself packed with the raw materials for life, and that these raw materials are the inevitable product of the process of star birth and star death. We have answered the biggest question of them all—where do we come from? [pp. 213—214]
I very much enjoyed the reading of this book and the thinking that it induced in me. Of the books by Gribbin that I've read so far I think I had a slight preference for The Birth of Time, but that may be as much a matter of personal taste as anything.
-- Notes by JNS