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EPS 102 Lecture 6

EPS 102 Lecture 6 - EPS 102 Lecture 6 Thursday February 5th...

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EPS 102 Lecture 6 Thursday February 5 th , 2009 H, He, and Li comes from Big Bang, and the rest of the stuff comes from stars. Where does Carbon, Oxygen, Si, Fe come from? The sun will expand and the surface of the sun will expand beyond the orbit of the Earth. Our own sun is currently on the main sequence, or the HR diagram. It is burning away by fusing away the H and accumulating He in its core. As it accumulates He in the core, the density goes up as does the gravity and it gets compressed and hotter. The fusing reactions which are temperature dependent will go faster and the sun will get brighter. The radiation or the light radiating out from the sun provides support: gravity pulls it in, and the pressure associated with the plasma or the material that the sun is made of helps hold up the load, but the radiation provides pressure. If we increase gravity, the center collapses, but since it gets brighter, the radiation pressure increases and the sun will increase in radius a little bit. We get to the end: all of the H has been consumed. We have turned off a source of light and source of pressure that is helping to hold up the star against gravity. When these fires go up, the core contracts a little bit and the temperatures go up. You can reignite H that was slightly outside the core that previously wasn’t available and fusion occurs again: we’ve turned the star back on. When the reactions were happening in the core, we have a lot of mass there, but now we are burning H outside the core and we don’t have as much mass as before. The sun and solar pressure pushes the material out into space causing it to expand. Red giants are examples of this. It could extend out to the radius of Mars. On the HR diagram, the population of stars observed on the diagram are stars that have migrated off of the main sequence and gone through red giant stage as they run out of burning Hydrogen. Eventually, you can get temperatures in the interior to be high enough so that you can fuse Helium. What used to be ash in the center of the star can be used as fuel. Carbon and Oxygen are being produced as waste products of these reactions at the very center of the star. The region that’s available to burn, or fuse He is confined because it takes very large pressures to do this. It doesn’t take long for us to run out of He, and then the star will start to collapse. As a consequence, the star starts to expand but this time it doesn’t stop. You produce an object known as a planetary nebula. As this material drifts out into space, the shell of the star is lost and what gets left behind is very hot ash which was produced by the fusing of He, and the waste products= C and O. this is very dense material, and can form a white dwarf. Typical expected evolution of a solar mass star: HR diagram relates surface temperature and luminosity. The color of the star gets a lot more red, and the temperature implies that it is actually colder. You are taking a hot star
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