8B The Main Sequence A star is a delicate balance between the force of gravity pulling in, and pressure from the heat of fusion pushing out. Stars on the main sequence burn hydrogen in their core to produce heat. Longest phase of a star’s life.
8B What then? When the hydrogen in the core is almost consumed the balance between gravity pulling in and pressure pushing out is disturbed. The structure and appearance of the star changes dramatically. What happens then, depends on the star’s mass. Two cases: –Low-mass (< 8 x mass of Sun) –High-mass (> 8 x mass of Sun)
8B Low-Mass Stars Recall: low- mass stars are the lower right side of the H-R diagram. They are small so they take longer to exhaust their core hydrogen fuel.
8B Helium Ash After 10 billion years the stellar core is “choked” with helium “ash”. Hydrogen fusion continues in a shell around non-burning core.
8B The Red Giant Branch Without fusion energy pressure in core: –Helium core collapses (no counter to gravity) –Gravitational energy heats up core Also heats up hydrogen-burning shell. Fuses hydrogen faster in shell. Result: More energy is generated inside the star than before! More outward pressure The star gets bigger while its outside gets cooler.
8B From main sequence to the red giant stage: 100 million years.
8B Helium Fusion Outside radius = Mercury’s orbit Core radius = Diameter of Earth! Pressure and temperature great enough for the Helium Flash! 3He C + Energy Red Giant Branch: –hydrogen fusing in a shell –and helium fusing in core. Time: Lasts for 10-20 million years
8B The Onion Sun Red Giant Stars Layers of: –Non-fusing H –Fusing H –Fusing He –Non-fusing C “ash”
8B Red Supergiant What happens when the Sun runs out of helium in its core? Same as before. Core shrinks, surface expands.
8B Death Core is contracting and heating. –Surface is cooling and expanding. Will it finally become hot enough in core for Carbon to fuse? For the Sun: No. Gravity keeps contracting the core: 1000 kg/cm 3 ! What stops it? Electron degeneracy pressure!
8B Electron Degeneracy Pressure from motion of atoms
8B Electron Degeneracy Pressure from electron shells
8B Where are we know? Core dead – nothing happening. Shells – burning H and He, but soon stop too. Outside atmosphere of star still cooling and expanding. …and expanding Force of radiation from burning shells blows the atmosphere away.
8B White Dwarfs Q: What’s left once the atmosphere blows away? A: The exposed electron degenerate carbon core. Size of Earth. No more fusion. Glow by their heat alone. Eventually cool and fade away black dwarf.
8B High-Mass Stars Think back to the first carbon core. How they get from main sequence to the carbon core stage is a little different. Now however, there is enough mass that it becomes hot enough to fuse carbon? Hot enough to eventually fuse lots of elements.
8B The Iron Core 4H He + Energy 3He C + Energy 2C O + Energy Fusion takes place in the core when the end result also yields energy. This energy causes pressure which counters gravity. But Iron doesn’t fuse.
8B Core-Collapse Iron core – no outward pressure. Gravity wins! Star collapses rapidly! Electron degeneracy can’t stop it. Atomic structure can’t stop it. Electrons and protons crushed together to produce neutrons. Neutrons pushed together by force of gravity.
8B Supernovae The result of the catastrophic collapse is the rebound and explosion of the core. From start of collapse to now: 1 second! Matter thrown back into the interstellar medium. Matter rushing outwards, fuses with matter rushing inwards. Every element after Fe is made in the instant of a supernova!