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Unformatted text preview: Astronomy 101 - Test 3 Review THE SUN The Sun is a star, a hot glowing ball of gas powered by fusion. It is a fairly average star. The Sun is about halfway through its expected 10 billion year lifetime. The basic regions of the Sun we discussed are the core, radiative zone, convective zone, photosphere and corona (others I didnt emphasize). The core is the Suns (and any stars) engine. The gas there is completely ionized, and it is so hot that fusion reactions can occur. In a fusion reaction, the nuclei of two atoms collide to make the nucleus of a new atom. Temperature is a measure of the random speeds of the ionized particles. You need high temperatures because you need to collide two nuclei at very high speeds for them to stick. This is because they both have a positive charge and so they repel each other. But if the collision speed is high enough to bring them close enough together, an even stronger, attractive force called the strong nuclear force makes them stick together. Fusion reactions make energy, which is how stars hold themselves up against their own gravity, and why they shine. This balance of gravity with the pressure from fusion is called hydrostatic equilibrium. The main reaction going on in the Suns core is the fusion of four protons (or hydrogen nuclei) into a helium nucleus, by a chain of reactions called the proton-proton chain. By such reactions, new elements are made. Energy is produced because the mass of the helium nucleus is slightly less than the mass of the four protons. That tiny mass difference gets converted to energy according to Einsteins famous equation E=mc 2 . In the Sun, hydrogen fusion into helium takes about 10 billion years. Core hydrogen fusion is the main phase of any stars life and how fast it happens sets the lifetime of stars. To be stable, a star must get rid of energy at its surface as fast as it makes it in its core. The energy produced by fusion initially gets to the surface of the star by radiation, which bounces its way off of particles, slowly inching its way out. Eventually, it reaches a cooler zone where it has trouble making any progress (because atoms and ions with bound electrons are present and absorb and re-emit photons in random directions), and then the energy is carried to the surface by convection, which is just rising pockets of hot gas and falling cooler gas. We can see this bubbling happening at the Suns surface in images, which show a mottled appearance. The layer of the Sun we see is called the photosphere. When we take a spectrum of the Sun, we see the continuous blackbody spectrum as well as thousands of absorption lines, each of which comes from a different atom or ion in a different state....
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- Fall '09