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Stellar Interiors Astronomy 315 Professor Lee Carkner Lecture 10 What is Inside? Why does a given star have a given T, R, M and L? How are T, R, M and L related? Must be due to what is going on in the star The Insider Can only see the very outer layers directly Our best information comes from the Energy However, the energy must somehow get to the surface and pass through the regions along the way We want to determine 2 things: How is energy transported? Equations of Stellar Structure Weight of each layer of a star is balanced by the pressure of the layers beneath it A star is a sphere and as you move out from the center you enclose more and more mass (no gaps) Relationship between pressure, temperature and density Hydrostatic Equilibrium Star wants to contract Star wants to expand Star must be in a state where gravity and thermal pressure balance Basic Hydrostatic Equilibrium Cross Section of H.E. Central T and P We can use the equations to stellar structure to find things like the pressure and temperature We know mass from binary stars PC = 3 X 109 atmospheres for Sun T = 1.5 X 107 K for Sun Stellar Models Requires large computing power Temperature, pressure and density are strongest near the core and fall off toward the surface Model of the Sun’s Interior Why Does the Sun Shine? What could power the Sun for this length of time? Chemical energy (burning) -Kelvin-Helmholtz contraction (gravitational energy) -Nuclear Fusion Reactions -- Fusion Hydrogen fusion requires the atoms to be moving fast (high T) and to be packed tightly together (high P) Each reaction converts 4 H to 1 He and some mass to energy (E=mc2) Rate of reactions depends on the temperature and pressure Hydrogen Fusion Scientific Notation The numbers is astronomy are often too large to represent with words or even by writing out all digits Example: number of seconds in a year Move decimal point seven places to the left, so our exponent is 7 To put into calculator use “EE” key in place of “X10” However, you should always write out the “X10”, don’t write “EE” in what you hand in “There are 3.1536 X 107 seconds in one year.” The Main Sequence Explained Why is there a main sequence? The stronger the pressure the higher the temperature and the more reactions High mass stars produce more energy in their cores and thus have a larger temperature and luminosity Energy Transport In general energy is transported in 3 ways: Conduction -Radiation -Convection -- Star have no conduction Radiation and convection both very important Solar Granulation Convection in Granules Opacity Opacity determines whether energy is transported by radiation or convection Opacity -- High opacity -- very little radiation can penetrate Low opacity -- radiation penetrates easily Ionization Atoms can only absorb photons if they have electrons High ionization means low opacity At high temperatures it is easy for photons to move freely through a gas M < 0.4 Msun Completely Convective Low temperatures mean atoms are not ionized and can absorb radiation better 0.4 Msun < M < 4 Msun Inner Radiative Zone Free electrons and protons don’t absorb light very well so the primary means of energy transport is radiative Outer Convective Zone The atoms absorb the radiation and heat up Interior of a 1 Msun Star Energy Transport M > 4Msun Inner convective zone Even though the opacity is low, there are so many photons that enough get absorbed to produce convection Outer radiative zone Energy Transport Down the Main Sequence What is a Star? A big sphere of gas (mostly hydrogen) Powered by fusion reactions in the core Energy gets out via radiation or convection depending on the opacity Next Time Read Chapter 16.4-16.5
