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Astronomy & Astrophysics
Michael Reid

Chapter 17Star StuffHow a Stars Mass Affect Nuclear FusionAll mainsequence stars are in gravitational equilibriumoutward push of gas pressure balancing the inward pull of gravityBecause the force of gravity is greater within more massive stars the pressure inside must also be greaterTo provide this greater pressure the cores must be hotter and denser leading to a greater fusion rateWith higher core temperatures and higher fusion rates massive stars burn through their hydrogen gas much faster explaining why they live shorterLowmass stars are stars born with less than about the mass of our SunIntermediatemass stars have birth weights between about 2 and 8 solar massesHighmass stars are those stars born with masses greater than about 8 solar massesLowMass StarDIAGRAM 172Red Giant StageWhen a stars core hydrogen is finally depleted nuclear fusion ceases in the stars core The star becomes out of balance gravity shrinking its core sizeThe stars outer layers however will expand outward becoming a subgiantThe stars luminosity will then increase and further expand in size becoming a red giantDIAGRAM 174why stars expand in sizeAs the hydrogen in the core burns out helium is left behind However the cores vicinity still holds some hydrogenGravity then shrinks both the nonburning helium core and the surrounding shell of hydrogen allowing for hydrogen to sustainHydrogen burning shell will proceed at a greater fusion rate creating a greater luminosity Size increase because energy transport within its interior cannot keep pace with thislarger energygeneration rate so thermal pressure will build up and push the pressure outwardHelium BurningHelium fusion occurs only when nuclei slam into one another at much higher speeds than those needed for hydrogen fusion meaning that a higher temperature is neededThe helium fusion process converts three helium nuclei into one carbon nucleusDegeneracy pressure takes part in counteracting gravity so the core does not inflateThe rising temperature causes the helium fusion to soar upward called helium flash releasing an enormous amount of energy into the core allowing thermal pressure to dominate As a heliumburning star size is smaller and is hotter dropping its life trackDIAGRAM 175DIAGRAM 176
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