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Lecture

Chapter 13 The Bizarre Stellar Graveyard.docx

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Department
Natural Science
Course
NATS 1740
Professor
Randy Hoffman
Semester
Winter

Description
Chapter 13 The Bizarre Stellar Graveyard 13.1 White Dwarfs What is a White Dwarf?  White Dwarf: exposed core of a star that has died and shed its outer layers  quite hot when forms  slowly cools with time  have masses of stars but radii of earth (very dim)  support by electron degeneracy pressure (packed electrons push outward) White Dwarf Composition, Density and Size - A teaspoon of white dwarf matter would weigh several tons - White dwarf formed by stars like our son would leave behind carbon - Low mass stars form white dwarfs with helium - More massive white dwarfs are smaller in size The White Dwarf Limit (Chandrasekhar Limit) - No white dwarf can have mass greater than 1.4M sun What can happen to a white dwarf in a close binary system? - Can slowly gain mass - In a close binary system, gas from a companion star can spill toward a white dwarf forming an accretion disk (new energy source) around it Novae - a nova is caused by hydrogen fusion on the surface of a white dwarf in a binary star system - 100,000 times as bright as sun, shines for a few weeks, not as bright as a supernova White dwarf super novae - when it reaches 1.4M of the sun carbon is ignited and star explodes - massive star supernova occurs when massive star explodes - light of white dwarf supernova fade steadily - if a white dwarf gains enough matter to exceed the 1.4 solar mass white dwarf limit, it will explode in a super nova 13.2 Neutron Stars What is a neutron star? - neutron degeneracy pressure: same as electron pressure pushing outwards in white dwarfs but with neutrons instead - a neutron star is a ball of neutrons just a few km in radius but with a mass like the sun - a neutron star could fit in your hometown but its gravity would quickly destroy our planet - 300,000 times as massive as earth How were neutron stars discovered? - Neutron stars can spin rapidly and emit beams of radiation along their magnetic poles which we detect as pulses of radiation if the beams sweep by earth - Pulses come from the star which spins as a result of conservation of angular momentum - Pulsar: is a rotating neutron star that beams radiation along its magnetic axis - All pulsars are neutron stars but not all neutron stars are pulsars What can happen to a neutron star in a close binary system? - X-ray binaries: close binaries that contain a
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