The Origin of the Elements

The Origin of the Elementsedited by David L. Alles Western Washington University e-mail: [email protected] - pdf za darmo

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The Origin of the Elements edited by David L. Alles Western Washington University e-mail: [email protected] This web paper was last updated 9/10/04.

Introduction The ordinary matter in our universe (known as baryonic matter) is made up of 94* naturally occurring elements, the familiar beasts of the periodic table. And it is one of the stunning achievements of twentieth century science that the question of where these elements came from has now been answered. The story of the origin of the elements is intimately intertwined with the evolution of our universe. It is also a central part of the evolution of life on Earth. The elements that make up our bodies reflect the cosmic abundance of the elements, and their presents on the Earth is, itself, part of the evolutionary history of stars. As Neil de Grasse Tyson, an astrophysicist and the director of New York City’s Hayden Planetarium, has put it: “We are not simply in the universe; we are born from it.” (Tyson 1998). Web Reference for Periodic Table *Web Reference for 94 Naturally Occurring Elements

Allan Sandage on Stellar Evolution "Historians of science a hundred years hence will remember twentiethcentury astronomy for two main accomplishments. One is the development of a cosmology of the early universe, from creation through consequent expansion. The other is the understanding of stellar evolution. Although not as well known among nonscientists as the Big Bang, the notion of the evolution of stars provided the foundation upon which astronomers built the grand synthesis of cosmological origins. The idea that stars change as they age and that these changes in turn alter their local environment and the chemical makeup of their parent galaxy—an idea that has developed only within the past fifty years—stands in the same relation to astronomy as the Darwinian revolution does to biology. It is a conceptual breakthrough that makes possible the modern understanding of the origin, evolution, and fate of the universe. Because all elements heavier than helium have been nucleosynthesized by stars, all the heavier chemical elements that are the raw materials of life were one time part of a stellar life cycle. We are the product of the stars. This is one of the most profound insights to have arisen out of twentieth-century astronomy. Life is clearly a property of the evolving universe made possible by stellar evolution." (Sandage 2000) Web Reference for Allan Sandage

The Origin of the Light Elements The origin of all the naturally occurring elements fall into two phases: Big Bang or Primordial Nucleosynthesis—the origin of the “light” elements; and Stellar Nucleosynthesis—the origin and production of the “heavy” elements. When astronomers refer to the “light elements”, they refer mainly to hydrogen and helium and their isotopes, and for very important reasons. Hydrogen is the simplest possible atom by definition, one proton and one electron. Anything less and it is no longer an atom; it is a subatomic particle with very different properties from the energetically stable atom. With this in mind it is easier to understand that the most abundant atoms in our universe should be the ones that formed first from subatomic particles. Big Bang nucleosynthesis refers to the process of element production during the early phases of the universe, shortly after the Big Bang. It is thought to be responsible for the formation of hydrogen (H), its isotope deuterium 2H, helium (He) in its varieties 3He and 4He, and the isotope of lithium (Li) 7Li. Nuclei of hydrogen (protons) are believed to have formed as soon as the temperature had dropped enough to make the existence of free quarks impossible. For a while the number of protons and neutrons was almost the same, until the temperature dropped enough to make its slight mass difference favor the protons. Isolated neutrons are not stable, so the ones that survived are the ones that could bond with protons to form deuterium, helium, and lithium. Why didn't all the neutrons bond with protons and make all the elements up to iron? While the temperature was dropping, the universe was also expanding, and the chances of collision were getting smaller. Also very important is the fact that there is no stable nucleus with 8 nucleons. So there was a bottleneck in the nucleosynthesis that stopped the process there. In stars, this bottleneck is passed by triple collisions of 4He nuclei (the triple-alpha process), but in the expanding early universe, by the time there was enough 4He the density of the universe had dropped too much to make triple collisions possible. Using the Big Bang model, it is possible to make predictions about elemental abundances and to explain some observations which would otherwise be difficult to account for. One such observation is the existence of deuterium. Deuterium is easily destroyed by stars, and there is no known natural process other than the Big Bang which would produce significant amounts of deuterium. Web Reference

The observed abundance of baryonic matter in our universe shows hydrogen makes up ~75% and helium ~25% of ordinary matter. All the other elements are a small fraction of the total (~1%) and represent the material that has been subjected to high enough temperatures and densities in stars to burn helium and make the heavier elements. Observation therefore closely matches the theoretical predictions of the standard Big Bang model. Note the chart uses a log scale in order to show the rarer, heavier elements on the vertical axis.

The Origin of the Heavy Elements In recent decades, astronomers have gained a reasonably good understanding of how stars proceed through the various evolutionary stages from birth to death—how stars change their temperatures and densities while struggling to reestablish their burning cycles and how they create most of the heavy elements, without which rocky planets, life itself, and intelligent beings could not exist. Relative abundances of the elements in the universe reveal the processes that synthesized heavier elements out of the hydrogen and helium from the Big Bang. Fusion in stars created more helium, skipped over lithium, beryllium (Be) and boron (B) to carbon (C) and generated all the elements up to iron (Fe). Massive stars can synthesize elements heavier than oxygen (O); these stars eventually explode as supernovae. Elements heavier than iron are made in such explosions. The chart on the preceding page has a logarithmic scale, in which abundance increases by a factor of 10 for each unit of height. Elements heavier than cadmium (Cd) are too rare to be displayed. Web Reference

Stars, such as our Sun (above), are the only place in our universe where the elements heavier than hydrogen and helium are produced. Stars with a mass similar to our Sun can produce heavier elements up to oxygen. Web Reference

Planetary Nebulae 95% of all stars that we see in our own galaxy, the Milky Way, will ultimately become "planetary nebulae". This includes the Sun. Planetary nebulae are formed when a red giant star ejects its outer layers as clouds of luminescent gas, revealing the dense, hot, and tiny white dwarf star at its core. The other 5% of stars—that is, those born with masses more than eight times larger than our Sun—end their lives as supernovae. The name "planetary nebula" is a misnomer. The name arose over a century ago when early astronomers looking through small and poor-quality telescopes saw these objects as compact, round, green-colored objects that reminded them of the view of Uranus. However, "planetary nebulae" are not made of planets, and no planets are visible within them. Rather, they are the gaseous and dusty material expelled by a geriatric star just before death. A far better name for these objects would be "ejection nebulae". Think of ejection nebulae as a cloud of smoke which escapes from a burning log as it collapses and crumbles into embers. Web Reference

The Life of a Star like the Sun by Bruce Balick The Sun generates all of its heat in its core. This heat both warms the Earth and prevents gravity from forcing the Sun to undergo a catastrophic gravitational collapse. The fuel which supplies the heat is hydrogen. Hydrogen nuclei are converted to helium as heat is released. Five billion years from now the Sun's hydrogen fuel will be depleted. Gravity will then force the spent core, now almost pure helium, to shrink, compress, and become even hotter than at present. The high temperatures will eventually ignite the helium ashes. The result is carbon nuclei and even more heat. The "second wind" of heat release will be furious, increasing the light emitted from the future Sun's surface by a thousand fold. Meanwhile, the same heat will cause the outer layers of the present Sun to expand and form a huge "red giant". As stellar time goes, the helium won't last long—certainly less than a mere few hundred million years. With its helium transformed into unburnable carbon, the solar core shrinks suddenly (a few thousand years) until just over half the mass of the present Sun is packed into a hot (million degree), dense (a ton per teaspoon) ball the size of the Earth. This amazing stellar remnant is called a white dwarf. The remnant's fuel reserves are now finally gone. Its shrunken stellar core is now entering retirement. Even so, one large final fling lies ahead for this star. The story shifts from the dying core to the star's distended outer layers. The core, their underlying foundation, now has all but imploded. The outer layers of the Sun fall inward toward the core. But the base mat...

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