The Death of Stars - I. Learning Objectives ! Be able to sketch the H-R diagram and include stars by size, spectral type, lifetime, color, mass, magnitude, temperature and luminosity, relative to our Sun ! Compare Red Dwarfs to our Sun in terms of their different masses and interiors ! Know the physics of the evolution of low mass stars from Main Sequence to white dwarf (fusion, gravity, collapse, expansion, temperature, composition, equilibrium) ! How do evolving stars move around the H-R diagram (where do they go when expanding, helium burning etc?) ! How does electron degeneracy pressure stabilize a dead star? What is a white dwarf? What is a planetary nebula? Important bad drawing So much information in the HR diagram... The Evolution of Stars ! A star’s evolution depends on its mass ! We will look at the evolution of three general types of stars ! Red dwarf stars (less than 0.4 M⊙ ) ! Low mass stars (0.4 to 8 M⊙ ) ! High mass stars (more than 8 M⊙ ) ! We can track the evolution of a star on the H-R diagram ! From Main Sequence to giant/supergiant and to its final demise Red Dwarf Stars ! 0.08 M⊙ < Mass < 0.4 M⊙ ! Fully convective interior (no “radiative zone”) ! Burn hydrogen slowly as convection constantly circulates hydrogen to the core ! Live 100s of billions to trillions of years ! The Universe is only 13.8 billion years old, so none of these stars have died yet Stellar Demise! Low mass star Helium-burning White dwarf and red giant planetary nebula High mass Helium-burning Other supergiant Core-collapse star red supergiant phases supernova Path of a Solar-Mass (1 M⊙ ) Star Planetary nebula Asymptotic giant branch Helium flash Horizontal branch Red giant Protostar White dwarf Main sequence Life of a Low Mass Star Red giant Shell hydrogen Main sequence burning Core hydrogen burning Tcore ~ 15 million K Asymptotic branch giant Shell helium burning Helium flash Horizontal branch Core helium burning Tcore ~ 100 million K Hydrostatic Equilibrium ! The battle between Gravity and Pressure ! Pressure from fusion pushes out and gravity pulls in – an equilibrium ! This is why a Main Sequence star isn’t shrinking even though it’s a big ball of gas ! A Main Sequence star’s life is all about this battle The Red Giant Phase ! When the hydrogen is gone in the core, fusion stops ! The core starts to contract under its own gravity ! This contracting heats areas of the star near the core, and hydrogen fusion starts in a shell around the core ! The push from fusion in this shell expands the star ! As the star expands, its surface moves farther from the core and it cools – so it becomes a red giant Path of a Solar-Mass (1 M⊙ ) Star Planetary nebula Asymptotic giant branch Helium flash Horizontal branch Red giant Protostar White dwarf Main sequence The Horizontal Branch ! When the core reaches 100 million K, it can start to fuse helium into carbon ! this stabilizes the core ! The star shrinks slightly and the surface heats up ! But helium doesn’t last very long as a fuel ! Horizontal branch lifetime is only about 10% that of a star’s Main Sequence lifetime ! Our Sun will burn helium for about a billion years Path of a Solar-Mass (1 M⊙ ) Star Planetary nebula Asymptotic giant branch Helium flash Horizontal branch Red giant Protostar White dwarf Main sequence When The Helium Runs Out… ! Fusion in the core stops – the helium has been converted to carbon and oxygen ! This is where carbon and oxygen in our universe come from! ! Every carbon atom in your body is the remnant of a dying star ! The star’s core collapses under its own gravity ! A shell near the core starts to fuse helium and the star expands and cools (again). The star is now called an asymptotic giant branch (AGB) star Path of a Solar-Mass (1 M⊙ ) Star Planetary nebula Asymptotic giant branch Helium flash Horizontal branch Red giant Protostar White dwarf Main sequence End Game ! The outer layers of the AGB star are cast off T > 200,000 K ! Up to 80% of the star’s original mass ! The core remains, made of carbon and oxygen from helium fusion ! The core is very hot, above 200,000 K NGC 2440 ! Ultraviolet radiation from the core ionizes the cast off outer layers ! The star becomes a planetary nebula Planetary Nebulae What About the Core? ! Nuclear fusion has stopped, and gravity begins to win the battle ! The core contracts to the size of the Earth ! But it’s about 60% of the Sun’s mass ! Material in the core is compressed to a density of about 1,000 kg/cm3 ! The star ends its life as a white dwarf ! It slowly cools off over billions of years Path of a Solar-Mass (1 M⊙ ) Star Planetary nebula Asymptotic giant branch Helium flash Horizontal branch Red giant Protostar White dwarf Main sequence What stops a white dwarf from contracting further? ! A quantum effect: electron degeneracy pressure ! The electrons are squashed together towards a quantum limit that is classically unphysical ! This creates pressure to counteract gravity ! Which stops the contraction e e e e e p p p p p e e e e p SQUEEZE p p p p e p e e e e e p p p p Matter in the core Electron-degenerate matter of a normal star 1 ton per cubic cm Relative Size of a White Dwarf 12,000 km An utterly insignificant little A white dwarf – weighs blue-green planet about 0.6 M⊙ White Dwarfs! Next Time The Death of Stars II.