EPS 102 Lecture 5

EPS 102 Lecture 5 - EPS 102 Lecture 5 Tuesday February 2nd...

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EPS 102 Lecture 5 Tuesday February 2 nd , 2009 The Earth had begun in a Big Bang explosion and had expanded. Everything seems to be moving away from us, due to the Universe expanding. The further things are away, the faster they move away. Cosmic microwave: we start from an extremely hot explosion to a relatively low temperature world. At some point you form elements, and as they cool down, the neutrons become more stable. The production of elements: the universe is mostly hydrogen with about 25% He by mass and trace amounts of heavier elements. We are sitting here on a planet of Si and O and we ourselves are made of C. These are all from the products of stars. Star formation is the same thing as planet formation; planets form around stars. We have an enormous amount of heat and many nuclear reactions going on; protons and neutrons are combining, etc… Eventually, these protons and neutrons adjust to an equilibrium as the universe cools. At 1 second, we have a ratio of 1:6 proton: neutron ratios. Some of these reactions that are converting protons and neutrons to each other freezes out. A neutron outside the nucleus is unstable and will decay to a proton. Gradually, you lose neutrons over time. The expectation is that you can now have protons and such things colliding to make more complicated elements. What begins to happen at these temperatures is you can take a proton and neutron, ram them together and form Deuterium. It is still a Hydrogen but it has a little bit of extra mass in comparison to a regular H. You need high temperatures when you get Tritium (another type of H) and put another proton to make the Helium atom. These reactions become possible at these temperatures. Once this happens, most of the neutrons left will end up in H and He. This is where the abundance of He comes from. If we basically have 1 neutron for every 7 protons, then we can predict how much He we get at the end of the day. Equivilanetly, we have 2:14, and if all the neutrons go to Helium (it has 2 protons and 2 neutrons), then that leaves us with 12 protons we have available to make Hydrogens. We expect 12 Hydrogens to 1 Helium. Why doesn’t this process continue? Making other types of He and H are unstable. You have to collide these unstable elements with other particles. You are looking at a 3 ball collision. The He nucleus is hit with protons and neutrons to build up heavy elements. This doesn’t happen very often. At some point, the universe cools enough so that you can’t pull in anymore protons and make any more elements. That’s the end of the story in terms of Big Bang. Additional elements had to be added after the big bang process. What we see in the sun is what we expect from Big Bang. When you look into space, you see stars and blank dark. There is about as much matter in between stars as there is in stars. There are gigantic
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EPS 102 Lecture 5 - EPS 102 Lecture 5 Tuesday February 2nd...

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