The Universe by John Gribbin

Author:John Gribbin
Language: eng
Format: epub
Publisher: Penguin Books Ltd
Published: 2006-03-26T16:00:00+00:00

7

Where Did the Chemical Elements Come from?

Although Alfred Russel Wallace, writing more than a hundred years ago, had no idea of the true extent and complexity of the Universe, his remarks about the relationship between life on Earth and the Universe at large resonate today. In another example of anthropic reasoning, it seems entirely possible that a very large Universe, billions of years old, is an inevitable requirement as the ‘stage’ on which life forms like ourselves can perform. The fact that we exist means that when we look out into the night sky we must inevitably see a large, old Universe.

The reasoning runs like this, starting from the facts that the Universe is flat, expanding, with a small cosmological constant and contains irregularities caused by matter clumping together under the influence of gravity. How did some of those clumps of matter form into stars, planets and people? The order of that is important, because, as I have emphasized elsewhere,1 life begins with the process of star formation. We are made of a variety of baryonic material, not just hydrogen and helium – in fact, not out of helium at all. Every element in our bodies, apart from the atoms of hydrogen, has been manufactured inside a star, and that takes time – time during which the Universe kept on expanding. So the fact that we exist requires that the Universe is large and old.

The modern understanding of the way in which the chemical elements were manufactured inside stars is another archetypal example of the power of combining what we know of physics on the large scale – in this case, the scale of stars – with what we know of physics on the small scale – in this case, the scale of atomic nuclei. This time, the study of the physics of stars – astrophysics – pointed the way to one of the key features of quantum physics, the uncertainty associated with wave–particle duality.

To a physicist, a star viewed from outside is a simple thing. It is a ball of stuff held together by gravity, and prevented from collapsing further by the heat generated in its core, which sets up an outward pressure that balances gravity. If you know how bright a star is and how massive it is, it is a trivial calculation (really – secondary school stuff) to work out how hot it must be in its heart to prevent collapse. It doesn’t matter what the star is made of, or where it draws energy from, it has to have a certain internal temperature to provide the pressure to resist the pull of gravity and to shine as brightly as it does. The Sun is a fairly ordinary star, and close enough to be studied in some detail, so it was the first star investigated in this way; but, thanks to spectroscopy, astronomers can measure the temperatures of other stars, and, thanks to the way stars in binary pairs orbit around one another, in many cases they can also measure their masses.

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