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29.3 Stellar Evolution Basic Structure of Stars The mass of a star governs its temperature, luminosity, and diameter. Mass effects The more massive a star is, the greater the gravity pressing inward and the hotter and more dense it has to be in order to balance its own gravity. Fusion The density and temp in a star increase toward the center, where energy is generated by nuclear fusion. Stellar Evolution As long as a star is changing hydrogen into helium, nuclear fusion, it will be a main sequence star. Star formation The formation of a star begins with a cloud of dust and gas called a nebula. The nebula will then collapse on itself because of its own gravity. As the cloud contracts, its rotation forces it into a disk shape with a hot condensed object at the center called a protostar. Friction from gravity will continue to increase the temp until it reaches the temp for nuclear reactions to occur and then it becomes a new star. Fusion begins When the temp inside a protostar becomes hot enough, nuclear fusion begins. The first reaction is the conversion of hydrogen to helium Once this happens, the star becomes stable because it has enough internal heat to produce the pressure to balance gravity. The object is now a true star and takes its place on the main sequence according to its mass. Life Cycle of Stars Like the Sun (low mass) What happens next depends on the stars mass. It takes about 10 billion years for a star with a mass of the Sun to convert all of the hydrogen to helium, therefore the life cycle is about 10 billion years. The next step is to become a red giant. Red Giant When the hydrogen in the stars core is gone, the star will now have a helium center with outer layers of hydrogen. The energy in a thin layer at the outer edge of the helium core will force the outer layers of the star to expand and cool. The star then becomes a red giant because its luminosity increases while its surface temp decreases due to the expansion White Dwarf The helium in the core of a red giant will become really hot and react to form carbon. Eventually the helium is all used up leaving a core of carbon. Energy production ends because it is not hot enough. The star is now a white dwarf. Black Dwarf Eventually the white dwarf will lose its luminosity and becomes an undetectable black dwarf. Life Cycle of Massive Star Massive stars convert hydrogen to helium the same way as less massive stars do. But because they are much higher on the main sequence, their lifetime is shorter because the star is very luminous and uses up its fuel quickly. Supergiant The core heats up to much higher temps. Star will expand to a supergiant. Iron forms in the core and becomes too massive to be supported. The core collapses in on itself in a violent explosion. This is called a supernova. A supernova can go in two directions. 1. Neutron star When the collapsed core of a supernova is 1.4-3 times as massive as the sun, it will shrink to about 20km(12mi) in diameter. Only neutrons can exist here, thus the name neutron star. Neutrons are very dense. 1 teaspoon can weigh 600 million metric tons 2. Black hole If the remaining core of a supernova is more than 3 times as massive as the Sun, the core will collapse. It is now called a black hole. Gravity is very strong now and nothing can escape its gravity. In order for something to get sucked into the black hole, it has to cross its event horizon.