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Unformatted text preview: The Raw Material 1 Chapter Three The Raw Material: Synthesis of Elements in Stars During the explosive birth of our universe only two chemical elements, hydrogen and helium, were formed. The other 90 elements were produced over the course of universe history in the interiors of massive stars. These stars rapidly burn their nuclear fuel and then explode, casting forth into the neighboring regions of the galaxy a mixture of the 90 missing elements. The frequency of these awesome events is about one per galaxy per century. Evidence in support of this origin is imprinted in the relative abundances of the elements making up our solar system. For example, the high abundance of iron relative to its neighboring elements is consistent with the fact that iron is the ultimate product of the nuclear res at the centers of large stars. The radioactive decay of 78-day half-life 56 Co dominates the light given off after a supernova explosion. As 56 Co decays to 56 Fe, it heralds the production of iron in the massive star. That newly produced elements are present in supernova debris is also demonstrated by the existence of the spectral lines imposed by the element technetium on the light from the nebula created by these explosions. Since all its isotopes are radioactive, technetium can be present only in matter fresh from a nuclear furnace. Chinese astronomers observed a supernova in 1054. The debris cloud from this explosion is known as the Crab Nebula. Through the course of our galaxys history, the formation and demise of about 100 million red giants has converted about 2 percent of the galaxys hydrogen and helium into heavier elements. Contained in this 2 percent are the ingredients needed to build Earth like planets. Introduction By cosmic standards our Earth and its fellow terrestrial planets are chemical mavericks. They consist primarily of four elements: iron, magnesium, silicon, and oxygen. By contrast, we look out on stars which are made up almost entirely of two elements, hydrogen and helium. For the universe as a whole, all elements other than hydrogen and helium are small potatoes; taken together they account for only about 1 percent of all matter. Clearly, one of the prerequisites for habitability is that a planet have solid surface. Objects made primarily of hydrogen and helium gas offer no such base. Hence, high on our agenda must be an understanding of how elements heavier than hydrogen and helium were formed and how these elements were separated from the bulk gas and forged into rocky planets. In this chapter we deal with the rst of these problems, in the next chapter we deal with the second. The Chemical Composition of the Sun All stars form from the gravitational collapse of clouds of gas. Since the lions share of the matter in the collapsing clouds ends up in the star itself, the stars chemical composition must be representative of the parent cloud. Thus, if we could somehow determine the chemical composition of the Sun, we could de...
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This note was uploaded on 02/08/2011 for the course EAS 1601 taught by Professor Lynch during the Spring '08 term at Georgia Institute of Technology.
- Spring '08