lecture16_ch12a - Luminosity The Life of a Low-Mass Star A...

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Unformatted text preview: Luminosity The Life of a Low-Mass Star A starʼs mass and age determine its other properties Temperature Gravitational equilibrium: Energy Balance: The outward push of pressure balances the inward pull of gravity The rate at which energy leaves the surface must equal the rate at which it it generated in the core How do stars form? Stars form in cold clouds where gravity can overcome thermal pressure Clouds continue to contract because they radiate away energy in the form of infrared light 1 X Stars form in cold clouds where gravity can overcome thermal pressure Clouds continue to contract because they radiate away energy in the form of infrared light The Orion nebula is one of the closest star-forming clouds X X Stars form in cold clouds where gravity can overcome thermal pressure Clouds continue to contract because they radiate away energy in the form of infrared light Disks around newborn stars in the Orion Nebula 2 Star Birth 1. Gravity causes gas cloud to shrink and fragment 2. Core of shrinking cloud heats up 3. When core gets hot enough, fusion begins and stops the shrinking 4. New star achieves longlasting state of balance A single star forming cloud can give birth to an entire star cluster How massive are newborn stars? Luminosity Very massive stars are rare Low-mass stars are common Gravitational equilibrium: The outward push of pressure balances the inward pull of gravity Why no stars with less than 0.08 MSun ? Temperature 3 Degeneracy pressure: Arises because laws of quantum mechanics prevent particles like electrons from occupying the same state in the same small region of space Gas pressure goes up as temperature rises What are the life stages of a low mass star? Brown dwarf Degeneracy pressure stops the contraction of a brown dwarf before it gets hot enough for fusion A star remains on the main sequence as long as it can fuse hydrogen into helium in its core Helium fusion requires higher temperatures than hydrogen fusion because the larger charge leads to greater repulsion Fusion of two helium nuclei doesnʼt work, so helium fusion must combine three He nuclei to make carbon MS seq lifetime and mass 4 X Luminosity rises because core thermostat is broken -- rising fusion rate in shell does not cause inert core to expand Star clusters help us test models of stellar evolution because they contain stars of same age but at different life stages. Onset of helium fusion expands the core, lowering the overall fusion rate Star clusters help us test models of stellar evolution because they contain stars of same age but at different life stages Hr diagr and age of cluster 5 How does a low mass star die? 6 Death Sequence of the Sun 7 Recall: • Why donʼt stars live forever? • What conditions trigger a shift from one phase of life to the next phase? • What is happening inside the star while it is increasing in size (radius)? • How will the Sun (a low-mass star) die? 8 ...
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This note was uploaded on 11/13/2011 for the course ISP 205 taught by Professor Donahue during the Fall '08 term at Michigan State University.

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