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Chapter 17

Chapter 17

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PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 17: Star Stuff 17.1 Lives in the Balance  stars with large masses have greater fusion rates since they attain high core temperatures easier  rate of fusion is very sensitive to temperature so massive stars achieve equilibrium with fusion higher than lower mass stars  mass of main sequence star determines luminosity & lifetime  star’s mass also determines what happens when star exhausts core supply of H  low mass star: stars born with lees than 2 solar masses (2M sunf material  intermediate mass star: star with birth weight between 2 and 8 solar masses  high mass star: stars born with masses greater than about 8 solar masses  intermediate mass stars are more similar to high mass stars 17.2 Life as a Low Mass Star  the sun is a low mass star  currently a main sequence star, in the middle of its 10 billion year life  sun shines steadily due to self regulating processes (solar thermostat)  energy in low mass stars moves through a combination of radiative diffusion & convention from core to surface  depth of convention zone depends on internal temperature & therefore mass  radiative diffusion occurs deep inside the star  convection occurs in sun’s outer layers only  cooler temperature so photons more difficult to transport  transition from radiative diffusion to convection occurs 70% of way from centre to surface  stars smaller than sun have cooler interiors so deeper convection zones  highest mass stars have no convection zones near surfaces, but have convective cores since energy produced at rate that radiative diffusion can’t transport in enough time  convection plays role in determining whether star has activity similar to sunspot cycle on sun  most active stars are very low mass  flare stars: stars that have field lines that suddenly snap & reconfigure themselves which releases energy from the magnetic field in the form of flares  as number of particles in star drops, core shrinks & heats up to keep pressure in balance with gravity  raises fusion rate & luminosity  core shrinks due to gravity when nuclear fusion ceases but outer layers expand  subgiant: star that has ceased nuclear fusion and moves to the right on the H-R diagram as it grows in size  red giant: star that grows in size and luminosity with expansion of outer layer  at the end of the red giant stage, it’s 100 times larger in radius & 1000+ times brighter in luminosity  gravity will then shrink the inert helium core & surrounding shell of H, so H shell will become hot enough for hydrogen shell burning  core & shell continues to shrink & heat up until temperature reaches 100 million K which is hot enough or He to fuse together  large amounts of mass will escape via solar wind since increasing radius means weaker pull of gravity PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013  in very low mass stars, degeneracy pressure halts collapse in inert He cores so not hot enough to fuse He  “dead” cores become white dwarfs  He has 2 protons so greater positive charge than single proton so they repel each other more than H  helium fusion: occurs when nuclei slam into one another at higher speeds that those needed for hydrogen fusion; also called triple alpha; 3 He  1 C + energy  thermal pressure in He too low to counteract gravity, so degeneracy pressure works (doesn’t depend on temperature)  helium flash: rapid rise in temperature of core due to helium fusion and degeneracy pressure; releases energy into core until thermal pressure high enough to combat gravity (no longer need degeneracy pressure) & core begins to expand which pushes hydrogen burning shell outward (lowers temperature & fusion rate)  size and luminosity declines as it becomes a helium burning star  outer layers of stars have different masses depending on how much mass expelled through stellar wind o expelled more mass = smaller radii + higher surface temperature  the longer a star undergoes hydrogen shell burning, the larger & more luminous it becomes  horizontal branch: horizontal line of H-R diagram that consists of helium burning stars since all have the same luminosity but differ in surface temperature  when He core is exhausted, fusion will cease again and star will be out of balance  core will begin to shrink again under crush of gravity but general size of star will increase  star will become a double shell burning giant since H sell will burn atop He layer  both
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