Very cool summary on synthesis of heavier elements in the universe from Google+ SciFi Author: Lacerant Plainer: Where do the elements come from ? : The early Universe had only subatomic particles, which combined to form the very light elements, largely Hydrogen and Helium. It is theorized that the early Universe was a very dark place filled with largely Hydrogen gas. How do we know this? We can still see the echoes from these first atoms in the Cosmic Microwave Background. All the other elements came from the fusion reactions in stars. Article Extract: The atoms left over by the big bang were gravitationally attracted to one another and condensed into huge clouds . The gravitational pressure on the centers of these clouds heated them to temperatures of millions of degrees. This led to the fusion of hydrogen into helium. Stars were born. Stars fuse lighter elements to form heavier ones.... Inside stars, Hydrogen is converted into Helium, through nuclear fusion – and some matter is turned in to energy in the process. Why does this produce energy? Since some of the mass in creating the heavier element was converted into Energy (remember E=mc2). This is called Nucleosynthesis. These reactions continue to form Li, Be, B, C, N, O, F, Ne.... and so on. Upto Fe (Iron). Until the time the Star starts creating Iron, the star is still not dense enough for gravity to overcome the nuclear reactions in the star. Once Iron is formed, the density of the star is so great, it collapses on itself due to gravity. So where do the other heavier elements come from? : The heaviest elements are created in supernovae, the fantastic death of supergiant stars. As the core of the supergiant becomes saturated with iron, its pressure and temperature increase. Eventually, the blackbody radiation from the core produces gamma rays powerful enough to break apart the iron atoms in the core. This further increases the pressure to a point where electrons and protons are fused into neutrons. This releases lots of energy in the form of neutrinos. The core cools and contracts; the inner shells rush to fill the void. As the core reaches nuclear density it become rigid and even bounces back a little. When the onrushing material feels this bounce, it creates a wave. As the wave spreads to outer, less-dense regions, it speeds up. Soon it is a shock wave and combines with the wave of neutrinos. The star is doomed. This process blows the star apart releasing 10^46 joules of energy. This shock wave is the only place hot and dense enough to fuse elements heavier than iron, elements up to and including uranium. Main Source: haystack.mit.edu/edu/pcr/Astrochemistry/3%20-%20MATTER/nuclear%20synthesis.pdf How Stars make elements: foothill.edu/attach/938/Nucleosynthesis.pdf The Evolving Universe: mnh.si.edu/exhibits/evolving-universe/bigbang/ Making the elements in the Universe: scienceblogs/startswithabang/2009/06/25/making-the-elements-in-the-uni/ Wikipedia link: en.wikipedia.org/wiki/Stellar_nucleosynthesis Chemical composition of Stars: spiff.rit.edu/classes/phys240/lectures/elements/elements.html Supernova: en.wikipedia.org/wiki/Supernova Popsci Link: popsci/science/article/2012-12/our-solar-system-formed-ashes-countless-stars-not-one-supernova-study-says Pics courtesy: Wikipedia and Wikimedia. Youtube video gif created from NOVA (BBC Science).
Posted on: Sun, 03 Nov 2013 03:43:27 +0000
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