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The Life Cycle of a Star Main Sequence Stars Once fusion begins, hydrogen is continually consumed and helium accumulates in the core of the star. Core temperature and pressure increase and the result is a stable state which we refer to as the equilibrium. The time a star stays in this state depends on its mass. Low Mass Stars Lowmass stars (red dwarfs) consume hydrogen at a very slow rate (~ 100 billion years). During this time, they lose significant mass, and eventually evaporate. The result is a very faint white dwarf. Intermediate Mass Stars Intermediatemass stars consume hydrogen at a faster rate (~ 10 billion years). As the core contracts from its own gravity, the outer layers expand. When the core reaches 100 000 000 oC, the helium fuses into carbon. Because of the expansion, the outer layers are much cooler than when the star was in the main sequence. It therefore appears as a red giant. Our Sun is considered an intermediate mass star and will evolve to this phase in about 5 billion years, its diameter expanding beyond what is now the size of the orbit of Mars. Stellar winds will peel away gases of the red giant and over time, its remnant will cool slowly and lose its brightness. It then becomes a white dwarf. The Sun red giant white dwarf 1 High Mass Stars Highmass stars consume hydrogen extremely fast (~ 7 million years). Their core gets so hot the helium fuses into heavier elements. This releases a vast amount of energy so the star swells into a “supergiant”. In high mass stars, the extremely high temperature of the core of the star allows fusion to create any of the elements that come before iron on the periodic table When a high mass star dies, its iron core __________________ and the outer portion of the star _________________ causing a supernova which is very bright Depending on how large the high mass star is, following supernova it can either become a ____________________ or a ________________________ Summary Chart Size Name/Example Lifetime supernova End of Life Lowmass star Red Dwarf 100 billion years Burns out into a white dwarf Intermediate Mass Star Ex.Our Sun 10 billion years Cools and expands into a Red Giant 7 million years Goes Supernova and becomes either a neutron star of a blackhole Massive Star Supergiant As the size of a star increases the length of its lifetime _______________ What size star can become a blackhole? ___________________ What type of star is our Sun? ___________________ 2