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How Stars Produce Energy The sun warms the earth and allows life to exist. It provides us with most of our energy and with light. But how does it keep giving off light? Will it ever stop shining? The answer to the 2nd question is that the sun has been burning for 5 billion years and should burn for another 5 billion years. What does it use for fuel? The fuel for the sun and other stars is hydrogen, the simplest of atoms. It is a single proton surrounded by one electron. The sun’s mass is about 75% hydrogen and because it has a mass of approximately a million times the earth it has enough fuel to continue to burn for another 5 billion years. The sun doesn’t burn its fuel in the same way hydrogen burns on the earth. On earth, hydrogen and oxygen burn to form water in a chemical reaction. In a star, hydrogen goes through a nuclear reaction, which gives off much more energy than a chemical reaction. Nuclear reactions bond 4 hydrogen atoms to form 1 helium atom in the sun’s core, as shown in the diagram. This process is called fusion because the 4 small atoms are fused (joined) to form a larger atom. (Fusion is the opposite process from fission, which is used to power nuclear reactors on earth. Fission breaks a larger atom apart and makes smaller ones). When the hydrogen fuses into helium, a small amount of mass is lost – about 0.07%. This confused scientists for years until Albert Einstein’s equation explained it: E = mc2 The missing mass is converted into energy and can be calculated. This energy is what makes the star shine. In order to have hydrogen fuse, positively charged protons must be brought together to form a larger atom. However, like charges repel, so the protons push each other apart. Fusion occurs when you get the protons close enough together that another force, called the strong nuclear force, overpowers the electric force. Only then will the helium atom form. But how can the positively charged protons get close enough to fuse? Fusion occurs in the center, or the core of the sun. In the core, the temperature is very hot, 10 million degrees. The pressure is also huge because of the weight of the rest of the sun is pushing on it. Since atoms move faster when they are hot, they move very fast at the high temperatures in the core. The electric force is not strong enough to stop them. They collide and join together. Fusion occurs only in the core of the sun. The core is at a high enough temperature and pressure for fusion to occur. The high pressure makes the hydrogen very dense, meaning that the atoms are very close together. In order for 2 protons to fuse, the positively charged protons must hit other protons head-on. If the gas is very dense, the chances of having a proton hit another proton head-on are much greater. Therefore, the core of the sun, with its high temperature and very dense gas, is the only place fusion can occur. The energy released by fusion results from the strength of the nuclear force that holds the helium nucleus together. It is very, very strong. Once it over-powers the electric force, it releases huge amounts of energy. This energy is millions of times more than is released in traditional chemical burning. Fusion occurs when the strong nuclear force can pull two nuclei together. In order to overcome the normal repulsive electric forces of the protons, the nuclei must have lots of kinetic energy (energy of motion). A very high temperature can give the nuclei the high kinetic energy needed to fuse together. Fusion on earth hasn’t been achieved because it must overcome the same obstacles. Hydrogen bombs use atomic bombs with plutonium or uranium to heat and compress the atoms that are to be fused. Attempts to get controlled fusion on earth have involved either using lasers to heat and compress or use of magnetic bottles. Neither of these methods has yet been successful in giving off more energy than has been put in the system to get it started. Nuclear reactions allow the sun to give off a great deal of energy because they are much more energetic than chemical reactions. An atomic bomb can be made out of 20 lbs. of plutonium and have the explosive power of more than 1000 tons of TNT. This difference between chemical and nuclear energy is hard for most people to understand. A star could not produce enough energy with just chemical reactions. The energy produced in the sun’s core actually takes thousands of years to be released. This is because the sun is so huge. In the inner part of the sun, the heat comes out by radiation, while in the outer part it comes out by convection. Because the sun is so dense, the transfer of heat by radiation is very slow. Photons keep hitting atoms on their way to the outside. Because the atoms are so closely packed, a photon bounces around for thousands of years before it reaches the area where convection occurs. Review Questions: 1. What is the main fuel that stars use to generate their energy? (same element most stars are made of) 2. In a star this substance (from #1) doesn’t really BURN as it does on the earth. Fully explain how it works. 3. Explain the main difference between the processes of FISSION and FUSION. 4. When helium forms in a star some mass from the initial products is “lost”. What happens to that missing mass? Who figured it out? List the equation that calculates it. 5. How long has the sun been “burning” for? How much longer will it continue to “burn” for? 6. Why can’t we get fusion to work efficiently on the earth? 7. Why does it take so long for energy produced in the core of the sun to be released? 8. Electrical force says that Protons (+) charges want to repel each other. How do they eventually join together? 9. What is the approximate temperature inside the core of the sun?