We are all made of star stuff, Carl Sagan said. This isn’t so much a factoid as a plain fact, but it’s worth taking a look at exactly what this astonishing fact really means.
The modern theory of stellar nucleosynthesis, or the theory of how atomic nuclei are created inside stars, started with a very influential 1957 paper by Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle called Synthesis of the Elements in Stars. The paper attempts to explain the observed distribution of elements in the universe, and in so doing, explains the mechanisms of nucleosynthesis in stars. At the time, it was a prevalent belief that all the elements had been created in the Big Bang, but this hypothesis had the disadvantage of requiring that the universe have very particular initial conditions for which there was no evidence. The B2FH paper, however, built upon known fusion processes inside stars.
The current theory goes that the elements heavier than lithium (including carbon and oxygen, those necessities of life) were created in stars. After the Big Bang, the universe was mostly hydrogen, but inside stars, hydrogen fuses to make helium, and from helium, carbon and oxygen can be made. This happens in cycles: at first, hydrogen is fused to helium in the star’s hot core. When most of the hydrogen in the core has been transformed into helium, a conversion of gravitational to thermal energy increases the star’s temperature, allowing helium burning to occur. The process by which carbon is made is called the triple-alpha process. Two helium nuclei fuse to make beryllium, and then the beryllium core collides with another helium nucleus to form carbon. Carbon can then fuse with another hydrogen core to create oxygen. Once the helium in the core is gone, carbon burning occurs, and so on.
Because beryllium is unstable, the probability that this process would occur is very small, unless it happens that the energy of an excited state of carbon-12 equals the energy of 8Be + 4He. Fred Hoyle predicted that this resonance must exist (because otherwise there would be very little carbon). When this prediction was confirmed, it was strong evidence in favor of the theory of stellar nucleosynthesis.
All the elements up to iron can be made successively in this fashion. The heavier elements can either be made through a slow-moving neutron capture process, or during the massive energy release of a supernova explosion. A dying star has an onion-like structure of elements, and when it explodes, all those elements are blown out into space, where they may eventually form planets and, through innumerable generations of chemical reactions, eventually make up you and me. Most of the matter on Earth was made in this way. Exceptions include 14C, the rare carbon isotope used for carbon dating, which is created when cosmic rays hit nitrogen in our atmosphere—carbon-14 has a half-life of five thousand years, and so would have decayed long before star-stuff could turn into a planet.
Another fun fact: Fred Hoyle was also the man who coined the term “Big Bang”. Hoyle, a proponent of the competing steady state theory, had intended it to be derogatory. William Fowler got the Nobel Prize in 1983 for his work on stellar nucleosynthesis; some have suggested that Hoyle might have shared it, if not for his unconvential ideas on the beginning of the universe.
dailymeh posted this on November 15, 2011
