ASTRON 1101 Lecture Notes - Lecture 27: Yttrium, White Dwarf, Planetary Nebula
31 Oct 2016
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Document Summary
About 90 elements have been found in nature (others have been made in labs) Even scarce elements must have been made somewhere. The universe began in a hot, dense, opaque state about 13. 7 billion years ago. When the universe was less than 3 minutes old, photons were energetic enough to photodissociate atomic nuclei. There were free neutrons, free protons, and free electrons: no helium nuclei, no lithium nuclei, etc. From 3-15 minutes after the big bang, there was an episode of nucleosynthesis. First, neutrons and protons fuse to form deuterium (2h) Second, deuterium and neutrons fuse to form tritium (3h) Third, tritium and protons combine to form helium (4he) Once lots of helium was made in big bang nucleosynthesis, a bit of lithium was made. After about 15 minutes, the universe was too cool for more fusion to occur. Most of the helium and lithium in the universe was made in the first quarter-hour after the.
