Seven Pillars of Science: The Incredible Lightness of Ice, and Other Scientific Surprises by John Gribbin

Author:John Gribbin [Gribbin, John]
Language: eng
Format: epub
ISBN: 9781785788581
Google: LMLvDwAAQBAJ
Amazon: B08CK6MWQY
Goodreads: 55506616
Publisher: Icon Books Ltd
Published: 2020-04-15T00:00:00+00:00

The Carbon Coincidence

A combination of spectroscopy and an understanding of the physics of stellar interiors tells us that a star like the Sun is composed almost entirely of hydrogen and helium, with just a smattering of heavier elements (Pillar Two). In the heart of a star, these elements are not in the form of gases, as they would be on Earth today. The electrons have been stripped from their nuclei, which are squeezed together at enormous densities, without the empty space that makes up ordinary atomic matter (Pillar One). Observations of clouds of gas in space tell us that they have a similar composition, although there the elements are in their familiar atomic state, with the dust that is so important to life as we know it a barely significant fraction of the total amount of stuff in a galaxy like our Milky Way. There may be other things that contribute to the overall mass of the Universe, things called Dark Matter and Dark Energy, but they are outside the scope of the present book. What matters here is the kind of stuff we are made of, the chemical elements we learned about in school, referred to by physicists as baryonic matter. Where does it come from?

There is a wealth of evidence that the Universe as we know it emerged from a very hot, very dense state, known as the Big Bang, about 13.8 billion years ago. The evidence comes partly from observations that the Universe is expanding today, so that it must have been more compact in the past, partly from studies of radio noise left over from the primordial fireball (the so-called Cosmic Microwave Background Radiation) and partly from our understanding of the laws of physics. Basic physics tells us that the first baryonic matter produced from the energy of the Big Bang, in line with Einstein’s famous equation, would have been hydrogen, the simplest and lightest element. The equations also tell us that as the Universe expanded and cooled about 25 per cent of that hydrogen would have been converted into helium by nuclear fusion reactions while the young Universe was still hot. But after about three minutes the fireball in which the Universe was born would have cooled to the point where no more nuclear reactions could take place, leaving great clouds made of a mixture of hydrogen and helium, the raw material of the first stars and galaxies, moving apart from one another in the expanding Universe. It doesn’t take a great intellectual leap to realise that the other elements must have been manufactured later on, inside stars. But how?

To put things in perspective, and see just how much (or how little!) stuff we are talking about, we can look at the composition of the Solar System, which is representative of what we might expect to find in planetary systems orbiting other stars. As we saw earlier, in terms of mass the Sun is 71 per cent hydrogen, 27 per cent helium, and less than 2 per cent of everything else put together.

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