Chapter 13 Evolution of high mass stars high mass stars stars with masses greater than about 8 solar masses live shorter lives than low mass stars higher luminosities more mass means stronger gravity and therefore more force on the core more force means higher pressure faster reactions and greater luminosity 13 1 High Mass Stars Follow Their Own Path Carbon nitrogen oxygen CNO cycle hydrogen nuclei interact with carbon which forms a nitrogen nucleus first step in high mass stars carbon is a catalyst for the fusion of hydrogen to helium as the high mass star runs out of hydrogen in core the weight of outer star compresses it before it s electron degenerate the pressure and temp are high enough for helium burn ing and the star responds to temp increase but luminosity changes very little once hydrogen core is degenerate helium burns in core and hydrogen in shell like a low mass horizontal branch star size grows surface temp falls moves off to the right on HR leaves main sequence stars with more than 10 solar masses will be red supergiants during helium burning high mass star exhausts helium core collapses temp is high enough to burn carbon which can produce even more massive elements like sodium neon and magnesium now the star has a carbon burning core surrounded by a helium shell and a hydrogen shell from carbon neon begins then oxygen many concentric layers fill the star The H R high mass stars leave the main sequence then move horizontally back and forth across the diagram as they pass through the instability strip they become pulsating variable stars Cepheid variables at top RR Lyrae at bottom of strip pulsate growing larger and smaller star alternately traps and releases thermal energy causing it to expand and contract gravity and pressure compete the pulsations don t affect nuclear burning just the light that escapes the star is brightest and bluest when it expands faintest and reddest when it falls in Cepheid variables most luminous pulsating variable stars periods of 1 100 days the longer the period the more luminous this is the period luminosity relationship it s consistent can be used to find the distances to galaxies beyond our own RR Lyrae variables unstable low mass horizontal branch stars less luminous than Cepheid can we used as standard candles the pressure of the intense radiation on gas at the surface of a massive star overcomes the star s gravity and causes winds with speeds of 3000km s and huge mass loss 13 2 High Mass Stars Go Out with a Bang high mass stars burn up from hydrogen to iron like an onion nuclear reactions form all stable isotopes less massive than iron but chain of nuclear fusion stops there just like gasoline which is self sustaining once it starts as hydrogen burns to helium the energy released maintains the temp needed to keep the reaction going binding energy the energy bound up in an atomic nucleus a nuclear reaction that in creases it releases energy decreasing it absorbs energy moving up from helium to carbon binding energy increases so fusing helium to carbon releases energy once you pass iron and go to heavier elements fusing absorbs energy iron fusion is not self sustaining because it absorbs energy in the reaction star s balance is like a leaky inflatable ball the larger the leak the more rapidly you have to pump air into the ball a star burning hydrogen helium is like a ball with a slow leak energy escapes by radiation and convection which is inefficient because the outer layers insulate the star but in carbon burning energy is carried in easily escaping neutrinos thus the outer layers fall in the star is denser and hotter and fuses faster as neutrino cooling goes on each element s fusion goes faster than the one before even though silicon burning gives off 200 million times more energy than helium the star isn t much more luminous because of neutrino cooling once iron core is formed there s nothing to replenish energy escaping in neutrinos iron core is not burning so it collapses causing gravity density temperature to skyrocket Earth size core is degenerate high pressure of outer layers caused temp and density to rise at these high temperatures 1 photons can break iron nuclei photodisintegration reversing nuclear fusion 2 high density forces electrons to combine with protons to form neutrons these processes absorb rest of star s energy as well as neutrino release collapse accelerates in less than a second material in core is denser than an atomic nucleus the force becomes repulsive half of core slows in its inward fall other half slams into the innermost part of the star and bounces sending shockwaves out in a second 20 of core becomes neutrinos some don t even escape because of such high densities energy of these neutrinos drives up the pressure and temp inflating a bubble of extremely hot gas and intense radiation around the core in a minute the shock wave has pushed its way out through the helium shell in a few hours it reaches the surface blasting material outward in a Type II supernova RECAP Not even electron degeneracy pressure can stop the collapse of an iron ash core As the core collapses the temperature is so high that thermal gamma ray photons photodisinte grate iron and the core is so dense that electrons and protons form neutrons and release neutri nos Photodisintegration and electron absorption rob the core of pressure support The collapse accelerates until nuclear forces suddenly become repulsive The overcompressed core bounces driving its outer layers outward through the star The expanding shock is strengthened by the pressure of a hot bubble of trapped neutrinos from the core The shock continues through the outer layers of the star blasting forth in a Type II supernova and leaving behind the collapsed remains of the core a neutron star 13 3 The Spectacle and Legacy of Supernovae Type II supernovas are huge expanding bubbles of million kelvin gas that glow in X rays and drive visible shock waves into the surrounding interstellar medium bubbles remain after thousands of years explosions compress nearby clouds triggering the initial collapse that be gins star formation only the least massive elements were formed at the beginning of the universe hydrogen helium lithium beryllium boron all the rest through nuclear reactions low mass stars form elements as massive as carbon and oxygen high mass stars form ele ments as massive as iron usually electric repulsion keeps positively charged atomic nuclei far apart extreme temp is