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Unformatted text preview: Chapter 12 Star Stuff 357 summary of key concepts 12.1 Star Birth How do stars form? Stars are born in cold, relatively dense molecular clouds . As a cloud fragment collapses under gravity, it becomes a rapidly rotating protostar surrounded by a spinning disk of gas in which planets may form. The protostar may also fire jets of matter outward along its poles. How massive are newborn stars? Newborn stars come in a range of masses but cannot be more massive than about or less massive than Below this mass, degeneracy pressure prevents gravity from making the core hot enough for efficient hydrogen fusion, and the object becomes a failed star known as a brown dwarf . 12.2 Life as a Low-Mass Star What are the life stages of a low-mass star? A low-mass star spends most of its life generating energy by fusing hydrogen in its core. When core hydrogen is ex- hausted, the core begins to shrink while the star as a whole expands to become a red giant , with hydrogen shell burning around an inert helium core.around an inert helium core....
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- Spring '08