Elements, Formation of
Elements, Formation of
Elements are identified by the number of protons in the nuclei of their atoms. For example, an atom having six protons in its nucleus is carbon, and one having 26 protons is iron. There are over 80 naturally occurring elements, with uranium (92 protons) being the heaviest (heavier nuclei have been produced in reactors). Nuclei also contain certain neutrons, usually in numbers greater than the number of protons. The number of neutrons in the atoms of a given element varies.
Heavy elements can be formed from light ones by nuclear fusion reactions; these are nuclear reactions in which atomic nuclei merge together. The simplest reactions involve hydrogen, whose nucleus consists only of a single proton, but other fusion reactions, involving mergers of heavier nuclei, are also possible. When the universe formed in an initial state of very high temperature and density, the big bang, the first elements to exist were the simplest, primarily hydrogen and helium (two protons). But we, and Earth, and all other objects including elements other than hydrogen and helium, are made of heavier elements, so a major question for scientists is how these heavier elements were created.
During the formation of the universe in the big bang, only the lightest elements were formed: hydrogen, helium, lithium, and beryllium. Hydrogen and helium dominated; lithium and beryllium were only made in trace quantities. The other 88 elements found in nature were created in nuclear reactions in the hearts of stars and in the huge stellar explosions known as supernovas. Stars like the sun and planets like Earth containing elements other than hydrogen and helium could only form after the first generation of massive stars exploded as supernovas, and scattered the atoms of heavy elements throughout the galaxy to be recycled.
The first indications that stars manufacture elements by nuclear reactions came in the late 1930s when Hans Bethe and C. F. von Weizsäcker independently deduced that the energy source for the sun and stars was nuclear fusion of hydrogen in a process that formed helium. They received the Nobel Prize in physics for this work.
George Gamow championed the big bang theory in the 1940s. Working with Ralph Alpher, he developed the theory that the elements formed during the big bang. With Robert Herman in the early 1950s, the pair used early computers to try to work out in detail how all the elements could have been formed during this cataclysmic period. The attempt was unsuccessful, but was one of the first large scientific problems to be tackled by computer.
Astronomers now realize that heavier elements could not have been formed in the big bang. The problem was that the universe cooled too rapidly as it expanded, and the extremely high temperatures required for nuclear reactions to occur did not last long enough for the creation of elements heavier than lithium or beryllium. By the time the universe had the raw materials to form the heavier elements, it was too cool.
In 1957, Margaret Burbidge, Geoffery Burbidge, William Fowler, and Fred Hoyle (referred to as B2FH) published a monumental paper in which they outlined the specific nuclear reactions that occur in stars and supernovas to form the heavy elements. Fowler received the 1983 Nobel Prize in physics for his role in understanding nuclear processes in stars.
During most of their lives, stars fuse hydrogen into helium in their cores, but the fusion process rarely stops at this point; most of the helium in the universe was made during the initial big bang. When the star’s core runs out of hydrogen, the star begins to die out. The processes that occur during this period form the heavier elements.
The dying star expands into a red giant star. A typical red giant at the sun’s location would extend to roughly Earth’s orbit. The star now begins to manufacture carbon atoms by fusing three helium atoms. Occasionally a fourth helium atom combines to produce oxygen. Stars of about the sun’s mass stop with this helium burning stage and collapse into white dwarfs about the size of Earth, expelling their outer layers in the process. Only the more massive stars play a significant role in manufacturing heavy elements.
Massive stars become much hotter internally than stars like the sun, and additional reactions occur after all the hydrogen in the core has been converted to helium. At this point, massive stars begin a series of nuclear burning, or reaction, stages: carbon burning, neon burning, oxygen burning, and silicon burning. In the carbon burning stage, carbon undergoes fusion reactions to produce oxygen, neon, sodium, and magnesium. During the neon burning stage, neon fuses into oxygen and magnesium. During the oxygen burning stage, oxygen forms silicon and other elements that lie between magnesium and sulfur in the periodic table. These elements, during the silicon burning stage, then produce elements near iron on the periodic table.
Massive stars produce iron and the lighter elements by the fusion reactions described above, as well as by the subsequent radioactive decay of unstable isotopes. Elements heavier than iron are more difficult to make, however. Unlike nuclear fusion of elements lighter than iron, in which energy is released, nuclear fusion of elements heavier than iron requires energy. Thus, the reactions in a star’ s core stop once the process reaches the formation of iron.
How then are elements heavier than iron made? There are two processes, both triggered by the addition of neutrons to atomic nuclei: the s (slow) process and the r (rapid) process. In both processes, a nucleus captures a neutron, which emits an electron and decays into a proton, a reaction called a beta decay. One proton at a time, these processes build up elements heavier than iron. Some elements can be made by either process, but the s process can only make elements up to bismuth (83 protons) on the periodic table. Elements heavier than bismuth require the r process.
The s process occurs while the star is still in the red giant stage. This is possible because the reactions create excess energy, which keeps the star stable. Once iron has formed in the star’s core, however, further reactions suck heat energy from the core, leading to catastrophic collapse, followed by rebound and explosion. The r process occurs rapidly when the star explodes.
During a supernova, the star releases as much energy as the sun does in 10 billion years and also releases the large number of neutrons needed for the r process, creating new elements during the outburst. The elements that were made during the red giant stage, and those that are made during the supernova
Beta decay— The splitting of a neutron into a proton and an electron.
Fusion— The conversion of nuclei of two or more lighter elements into one nucleus of a heavier element.
r process— Rapid process, the process by which some elements heavier than iron are made in a supernova.
Red giant— An extremely large star that is red because of its relatively cool surface.
s process— Slow process, the process by which some elements heavier than iron are made in a red giant.
explosion, are spewed out into space. The atoms are then available as raw materials for the next generation of stars, which can contain elements that were not made during the big bang. These elements are the basic materials for life as we know it. During their death throes, massive stars sow the seeds for life in the universe. We are made from the ashes of stellar explosions.
McWilliam, Andrew and Michael Rauch. Origin and Evolution of the Elements. Cambridge, UK: Cambridge University Press, 2004.
Hester, J. Jeff, et al. “The Cradle of the Solar System.” Science. 304 (2004): 1116-1117.
Kirshner, Robert. “The Earth’s Elements.” Scientific American (October 1994): 59.
Nittler, Larry R. “Nuclear Fossils in Stardust.” Science. 303 (2004): 636-637.
National Aeronautics and Space Administration (NASA). “Nucleosynthesis in the Early Universe.” 2005. <http://map.gsfc.nasa.gov/m_uni/uni_101bbtest2.html> (accessed October 30, 2006).
Science Express Reports. “Deep Mixing of 3He: Reconciling Big Bang and Stellar Nucleosynthesis.” October 26, 2006. <http://www.sciencemag.org/cgi/rapidpdf/1133065v1.pdf> (accessed October 30, 2006).
Paul A. Heckert