Very Brief History of the Universe
Origin & Evolution of Everything from the Big Bang to Life on Earth
Jul 30, 2007
The Big Bang
The universe formed about 14 billion years ago in the big bang. It was initially an unimaginably hot dense state. The matter was initially in the form of elementary particles such as protons and electrons. Hydrogen nuclei are essentially protons, so the initial composition was entirely hydrogen.
As the universe expanded it cooled. It went through a stage when the temperature was similar to temperatures in the cores of stars like the sun. During this stage some of the hydrogen fused to form helium, just as it does in the core of the stars. The composition of the universe became about 25% helium. The remainder was still hydrogen.
Fusion reactions that fuse helium into heavier elements require higher temperatures than the reactions that fuse hydrogen into helium. Hence, by the time there was enough helium in the universe for the helium to fuse into heavier elements, the universe had cooled below the temperatures required for these reactions. A few minutes after the initial big bang the composition of the universe was fixed at about 70 to 75% hydrogen and 25 to 30% helium. There were trace amounts of lithium and beryllium. All the other 88 naturally occurring elements formed later in stars.
Galaxy Formation
Within a million years or so after the big bang, galaxies began to form. We don't really understand the process in detail, but it is most likely from density fluctuations that occurred during the big bang. The largest clumps of matter ultimately became clusters of galaxies and galaxies within these clusters. Within galaxies, individual stars formed. They did not however all form at once. Some of the primordial matter remained in the interstellar medium to be tapped for future generations of stars.
First Stars and the Origin of the Elements
This first generation of stars to form was composed of entirely hydrogen and helium. The less massive of these stars are still around and still consist of just hydrogen and helium. They are called Population II stars to distinguish them from later generations of stars, Population I.
It is however the more massive stars in this first generation that are more important to us. Fortunately for us, they deplete their hydrogen fuel very rapidly, in only a few million years as opposed to ten billion years for stars with about the Sun's mass. When these massive stars run out of hydrogen they begin burning helium and then successively heavier elements until they have an iron core.
These stars then explode as a type II supernova. The supernova simultaneously manufactures the heaviest elements, via the r process, and blasts material containing all the elements that the star has made in its lifetime back into the interstellar medium.
Gradually after a few generations of massive stars and supernovae, the recycled stellar material, containing all the naturally occurring elements, mixes with the primordial hydrogen and helium in the interstellar medium. Later generations of stars form out of interstellar gas that is enriched in elements heavier than hydrogen and helium by the previous generations of massive stars and supernovae.
The Sun and Solar System
The sun, interstellar medium, and recently formed Population I stars contain only 98% hydrogen and helium and about 2% of all the other 90 naturally occurring elements. Our solar system formed from the interstellar medium not with the first generation of stars, but with the second to third generation stars. It therefore contains heavy elements, from the previous generations of massive stars, which serve as the raw material for life on Earth.
We are all recycled stardust!
Further Reading
Barrow, J.H. and Silk, J., The Left Hand of Creation, Oxford, 1983.
Silk, J., The Big Bang, Times Books, 2000.
Harrison, E.R., Cosmology The Science Of the Universe, Cambridge, 1981.
Friedman, H., The Astronomer's Universe, Norton, 1998.
Zeilik, M., Astronomy The Evolving Universe, Cambridge, 2002.
