The Stuff of Which We Are Made

There are some 92 natural chemical elements, including the oxygen that we breathe, the carbon that forms the skeleton of organic molecules, the silicon that makes up rocks, and the iron that forms the basis of the machinery we use. Yet when the universe originally formed, some 15 billion years ago, it was endowed only with hydrogen and a very minor amount of helium. We know that there has been very little change in the abundances of the various elements since the earth formed 4.8 billion years ago, so the synthesis of the elements which comprise us and our environment must have occurred between 15 billion and 5 billion years ago. How?

We are all familiar with ordinary burning, in which atoms recombine to make different molecules. Oil, for instance, is made up of carbon and hydrogen, which combine with the oxygen of the air to make water vapor (hydrogen oxide) and carbon dioxide. In stars, such as our sun, a different kind of burning occurs, in which atoms are combined to make new kinds of atoms. This process, called nuclear fusion, releases enormous amounts of energy, but can take place only at very high temperatures.

In the center of the sun nuclear fires burn at temperatures of 100 million degrees. There elemental hydrogen is fused into helium, and helium is further fused into carbon, nitrogen and oxygen. Thus in ordinary stars the elements that go into living systems are formed. Several billion years from now, the sun will consume its hydrogen fuel. When that happens it will go through its death throes by growing into a red giant large enough that its surface will enclose the orbit of the earth. It will then collapse into a dense white dwarf, the size of the earth. Very little of its mass will be ejected into interstellar space where it would be available for forming other planets, nor would any great amount of the iron and silicon that make up the earth be available if it did.

The cycle of a more massive star, one of about 20 solar masses, is quite different. At higher temperatures and pressures, hydrogen is burned quickly to helium and helium is burned to more massive elements such as carbon and oxygen. However, because of the higher temperatures and pressures, oxygen is further burned to form sulfur, and so the chain of nuclear synthesis continues. When one type of nuclear fuel is consumed, gravity forces further compress and heat the stellar matter, so that another, hotter type of nuclear fire can ignite. At each stage of making heavier elements energy is released until the element iron is synthesized. Elements heavier than iron, however, must have energy put into the process to make it work, rather than releasing energy. Therefore nuclear burning cannot explain the formation of heavy metals such as lead and silver.

When a massive star has burned all its fuel, it undergoes a very violent death. Without the continued production of energy, there is nothing to maintain the pressure at the stellar center. The powerful gravitational forces cause a collapse of the stellar shell, with an enormous release of gravitational energy in a supernova explosion. Some of the energy goes into forcing together atoms that would not normally combine, allowing iron to be converted into heavier elements. The rest goes into an enormous explosion. The supernova shines with a luminosity of ten billion suns, and almost the entire mass of the star is blown away. Residents of the southern hemisphere were recently treated to the sight of one of these explosions, which occurred in a companion to our Galaxy, the Greater Magellenic Cloud. The explosion spreads the elements produced both by normal nuclear burning and by the supernova itself through nearby space, where they are available for formation of stellar systems such as our own. The remnant of the star becomes a very dense neutron star or a black hole.

Some theories hold that the sun and solar system were formed in the spiral arm of our Milky Way galaxy as a result of a supernova explosion that occurred 5 billion years ago. The condensation of the sun and planets are thought to have been triggered by the shock wave of that cataclysmic event. It is sobering to realize that the materials upon which our existence depends were formed in a crucible of almost unbelievable violence.