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PASS Content Standard 5.2 All stars have a life cycle including birth, development, and death. Fusion reactions in the stars release great amounts of energy and matter over millions of years. A star's life cycle is determined by its mass. The larger its mass, the shorter its life cycle. The star's mass is determined by the amount of matter available in the giant cloud of gas and dust, nebula, from which it forms. QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture. Nebulae - 5 min Stage 1 Dust and gas are pulled together by gravity to form a protostar inside the nebula. The contraction causes heating to begin. Stage 2 When the star is hot enough, about o 15,000,000 C, nuclear fusion begins in the core of the star, and it begins to emit light and heat. Stage 2 At this point, the star has become a Main Sequence star. Stage 3 After millions of years, the star runs out of hydrogen in its core. The core becomes unstable and contracts, while the outer shell starts to expand. Stage 3 The star has now become a red giant. When our Sun becomes a red giant, it will expand as far as the orbit of the Earth. Stage 4 Stage 4 has two possibilities, depending on the mass of the star. Stage 4 (low mass) A star like our Sun will contract, becoming a white dwarf, which slowly cools, changing color as it does, until it can no longer be seen a black dwarf. Stage 4 (high mass) More massive stars may glow brightly again as they undergo further fusion reactions, expanding and contracting several times... Stage 4 (high mass) and forming the nuclei of heavier elements before becoming a supernova. Stage 4 (high mass) A supernova is an exploding star that throws its outer layer of dust and gas into space, leaving behind a very dense core called a neutron star. Stage 4 (high mass) If the star is very massive, roughly 3 or more times the mass of our Sun, the final object produced may be a black hole. Stage 4 (high mass) The dust and gas left behind by a supernova, supernova remnant, can be the raw material used to produce new stars. Stage 4 (high mass) Stars formed from the remains of supernovae are called Second Generation stars. Our Sun is a second generation star. Unlike chemical reactions, nuclear reactions involve the nucleus of the atom. Nuclear reactions release many times the energy of chemical reactions. There are two types of nuclear reactions. Fission is a nuclear reaction in which a heavy nucleus is split into two fragments. This picture is of a ground explosion of 20 pounds of plutonium, releasing the energy equal to 70 million pounds of TNT. A fusion reaction is one in which two or more small nuclei are forced together to form one larger nucleus. This is a picture of a "fusion" reaction equal to 2,180 million pounds of TNT. Fusion reactions power the stars, converting hydrogen into helium. Temperatures in the core o of our Sun (over 15 million C) are high enough to produce nuclei up to the size of iron, (atomic mass 56). To produce nuclei heavier than iron requires temperatures only found in supernova explosions, o (over 100 Billion C).