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University of Toronto Mississauga
J B Lester

AST 201S - Stars and Galaxies University of Toronto Mississauga Chapter 13 Summary th Essential Cosmic Evolution, 5 Edition 1. White dwarf stars • Average density (M/Vol) = 1 billion kg/m , more than lead or any other normal substance • Electrons are crushed to “degenerate” state - packed as closely as allowed by laws of physics: no two electrons can be at the same location with the same energy • Unusual property of degenerate matter: larger mass → smaller diameter • White dwarf stars have a maximum mass, 1.4 M Sun, where the diameter goes to 0 km = Chandrasekhar limit 2. White dwarfs in double stars • More massive star finishes its life as a white dwarf • Less massive star ages more slowly, becomes a red giant • Matter is transferred from the red giant onto the white dwarf • Transferred matter is from the surface of the red giant, 90% H • Fresh H on the surface of the white dwarf begins fusion in full view → “nova” explosion, luminosity ≈ 100,000 LSun • Nova explosion does not alter the system, transfer of fresh matter continues • Nova explosions repeat, from hours to centuries between explosions • Nova explosion does not blow off all the transferred matter → white dwarf gradually gains mass over time 3. White Dwarf Supernova • White dwarf in a double star can gain enough mass to exceed its maximum mass of 1.4 M Sun • Weight > degenerate pressure → sudden collapse • Collapse → sudden release of stored gravitational energy • Sudden heating → explosive fusion of the whole white dwarf → supernova, luminosity = 10 billion Sun continued on page 2 AST201H5S – Chapter 13 Summary page 2 of 2 4. Massive-Star Supernova - again • Explosion blows the stellar envelope out, but implodes the core • Crushed core releases stored gravitational energy • Energy tears apart nuclei (C,N,O, ... Iron) → p , n , e − • C
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