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Lecture18_110f2007

Lecture18_110f2007 - Main-Sequence Star Summary High-Mass...

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1 Main-Sequence Star Summary High-Mass Star: High Luminosity Short-Lived Large Radius Blue Low-Mass Star: Low Luminosity Long-Lived Small Radius Red Recap How do we classify stars? We classify stars according to their spectral type and luminosity class . The spectral type tells us the star’s surface temperature The luminosity class how much light it puts out. Why is a star’s mass its most important property? A star’s mass at birth determines virtually everything that happens to it throughout its life. Recap (Cont.) What is a Hertzsprung- Russell diagram? • An H–R diagram plots stars according to their surface temperatures and luminosities. Star Birth How do stars form? • Stars are born in molecular clouds consisting mostly of hydrogen molecules. • They form in places where gravity can overcome thermal pressure in a cloud.
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2 Star-forming clouds emit infrared light because of the heat generated as stars form. Cloud heats up as gravity causes it to contract. Conservation of energy and angular momentum apply! Contraction can continue if thermal energy is radiated away. Infrared light from Orion Orion Nebula is one of the closest star- forming clouds As gravity forces a cloud to become smaller, it begins to spin faster and faster. As gravity forces a cloud to become smaller, it begins to spin faster and faster. This is because of the conservation of angular momentum. As gravity forces a cloud to become smaller, it begins to spin faster and faster. This is because of the conservation of angular momentum.
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