Stellar nucleosynthesis is the process by which elements are created within stars by combining the protons and neutrons together from the nuclei of lighter elements. All of the atoms in the universe began as hydrogen. Fusion inside stars transforms hydrogen into helium, heat, and radiation. Heavier elements are created in different types of stars as they die or explode. The idea that stars fuse together the atoms of light elements was first proposed in the 1920s, by Einstein's strong supporter Arthur Eddington. However, the real credit for developing it into a coherent theory is given to Fred Hoyle's work in the aftermath of World War II. Hoyle's theory contained some significant differences from the current theory, most notably that he did not believe in the big bang theory but instead that hydrogen was continually being created within our universe. (This alternative theory was called a steady state theory and fell out of favor when the cosmic microwave background radiation was detected.) The simplest type of atom in the universe is a hydrogen atom, which contains a single proton in the nucleus (possibly with some neutrons hanging out, as well) with electrons circling that nucleus. These protons are now believed to have formed when the incredibly high energy quark-gluon plasma of the very early universe lost enough energy that quarks began bonding together to form protons (and other hadrons, like neutrons). Hydrogen formed pretty much instantly and even helium (with nuclei containing 2 protons) formed in relatively short order (part of a process referred to as Big Bang nucleosynthesis). As this hydrogen and helium began to form in the early universe, there were some areas where it was denser than in others. Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space. Once these clouds became large enough, they were drawn together by gravity with enough force to actually cause the atomic nuclei to fuse, in a process called nuclear fusion. The result of this fusion process is that the two one-proton atoms have now formed a single two-proton atom. In other words, two hydrogen atoms have begun one single helium atom. The energy released during this process is what causes the sun (or any other star, for that matter) to burn. It takes nearly 10 million years to burn through the hydrogen and then things heat up and the helium begins fusing. Stellar nucleosynthesis continues to create heavier and heavier elements until you end up with iron. The burning of helium to produce heavier elements then continues for about 1 million years. Largely, it is fused into carbon via the triple-alpha process in which three helium-4 nuclei (alpha particles) are transformed. The alpha process then combines helium with carbon to produce heavier elements, but only those with an even number of protons. The combinations go in this order:
Other fusion pathways create the elements with odd numbers of protons. Iron has such a tightly bound nucleus that there isn't further fusion once that point is reached. Without the heat of fusion, the star collapses and explodes in a shockwave. Physicist Lawrence Krauss notes that it takes 100,000 years for the carbon to burn into oxygen, 10,000 years for the oxygen to burn into silicon, and one day for the silicon to burn into iron and herald the collapse of the star. Astronomer Carl Sagan in the TV series "Cosmos" noted, "We are made of star-stuff." Krauss agreed, stating that "every atom in your body was once inside a star that exploded...The atoms in your left hand probably came from a different star than in your right hand, because 200 million stars have exploded to make up the atoms in your body." Our world is made of elements and combinations of elements called compounds. An element is a pure substance made of atoms that are all of the same type. At present, 116 elements are known, and only about 90 of these occur naturally. Elements and the ‘Big Bang’ theoryDuring the formation of the universe some 14 billion years ago in the so-called ‘Big Bang’, only the lightest elements were formed – hydrogen and helium along with trace amounts of lithium and beryllium. As the cloud of cosmic dust and gases from the Big Bang cooled, stars formed, and these then grouped together to form galaxies. The other 86 elements found in nature were created in nuclear reactions in these stars and in huge stellar explosions known as supernovae. Elements and our SunFor most of their lives, stars fuse elemental hydrogen into helium in their cores. Two atoms of hydrogen are combined in a series of steps to create helium-4. These reactions account for 85% of the Sun’s energy. The remaining 15% comes from reactions that produce the elements beryllium and lithium. The energy from these nuclear reactions is emitted in various forms of radiation such as ultraviolet light, X-rays, visible light, infrared rays, microwaves and radio waves. In addition, energised particles such as neutrinos and protons are released, and it is these that make up the solar wind. Earth is in the path of this energy stream, which warms the planet, drives weather and provides energy for life. The Earth’s atmosphere is able to screen out most of the harmful radiation, and the Earth’s magnetic field can deflect the harmful effects of the solar wind. Dying starsWhen a star’s core runs out of hydrogen, the star begins to die out. The dying star expands into a red giant, and this now begins to manufacture carbon atoms by fusing helium atoms. More massive stars begin a further series of nuclear burning or reaction stages. The elements formed in these stages range from oxygen through to iron. During a supernova, the star releases very large amounts of energy as well as neutrons, which allows elements heavier than iron, such as uranium and gold, to be produced. In the supernova explosion, all of these elements are expelled out into space. Our world is literally made up of elements formed deep within the cores of stars now long dead. As Britain’s Astronomer Royal Sir Martin Rees said, “We are literally the ashes of long dead stars.” When you buy a party balloon that floats in air, it is filled with helium gas – most of which was created when the universe was only 3 minutes old! Examples of element making (nucleogenesis) in helium burning reactions:
Man-made elementsOnly 90 of the 116 known elements occur naturally, so where have the other 26 come from? The answer is to be found in the development of nuclear power plants and machines known as particle accelerators:
One of the habits of scientists is open-mindedness. Scientists need to be receptive to new ideas and suggestions. As new evidence is discovered, new ways of interpreting and understanding it may have to be considered.
Find out more about more about what an element is on Biology Online.
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