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Chapter 18

Chapter 18 Summary Cosmic Perspective

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Western University
Astronomy 1021
Chris Racknor

CHAPTER EIGHTEEN The Bizarre Stellar Graveyard Despite the strange nature of stellar corpses clear evidence exists for white dwarfs and neutron stars and the case for black holes is very strong White dwarfs neutron stars and black holes can all have close stellar companions from which they accrete matter these binary systems produce some of the most spectacular events in the universe including novae white dwarf supernovae and xray bursters Black holes are holes in the observable universe that strongly warp space and time around them the nature of black hole singularities remains beyond the frontier of current scientific understanding White Dwarfs A white dwarf is the core left over from a low mass star supported against the crush of gravity by electron degeneracy pressure a white dwarf typically has the mass of the sun compressed into a size no larger than the earth no white dwarf can have a mass greater than 14M Sun White Dwarfs in Close Binary SystemsA white dwarf in a close binary system can acquire hydrogen from its companion through an accretion disk in which matter swirls toward the white dwarfs surface as hydrogen builds up on the surface it may ignite nuclear fusion and produce a nova that for a few weeks may shine as brightly as 100 000 suns in extreme cases accretion may continue until the mass exceeds the white dwarf limit of 14M at which point it will explode as a white Sundwarf supernovaNeutron Stars A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova it resembles a giant atomic nucleus 10km in radius but more massive than the sun How Neutron Stars were Discovered Neutron stars spin rapidly when they are born and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron stars spin we see such neutron stars as pulsars and these pulsars provided the first direct evidence for the existence of neutron stars Neutron Stars in a Close Binary System Neutron stars in close binary systems can accrete hydrogen rich material from their companions formin
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