AST 101 Lecture Notes - Lecture 13: Planetary Nebula, Cosmic Distance Ladder, Event Horizon
21 Jul 2016
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Off main sequence -> planetary nebula and white dwarf. Double-shell burning ends with a pulse that ejects the h and he into space as a planetary nebula. The core left behind becomes a white dwarf. White dwarfs are the remaining cores of dead stars. No fusion in white dwarf, but radiation at low level. White dwarfs cool off and grow dimmer with time. Degeneracy pressure is not strong enough to support a white dwarf with >1. 4msun against its gravity. A white dwarf cannot be more massive than 1. 4msun, the white dwarf limit (or chandrasekhar limit) If nothing happens , white dwarfs cool off and grow dimmer with time, and fade away forever. Mass falling toward a white dwarf from its close binary companion has some angular momentum. The matter therefore orbits the white dwarf in an accretion disk. Friction between orbiting rings of matter in the disk transfers angular momentum outward and causes the disk to heat up and glow.
