Lecture_15

Lecture_15 - White Dwarfs are the remaining cores of dead...

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1 White Dwarfs, Neutron Stars and Black Holes Our Goals for Learning • What is a white dwarf? • What can happen to a white dwarf in a close binary system? The Bizarre Stellar Graveyard The Bizarre Stellar Graveyard White Dwarfs are the remaining cores of dead stars like the Sun. The outer layers of the original star have been blown into a planetary nebula during red giant phase. The carbon core (for a 1 M Sun star) is very dense. Electron degeneracy pressure balances gravity. The brightest star in the sky – Sirius – is actually a binary containing a normal star and a white dwarf. In X-ray light, the white dwarf if much brighter! White Dwarf - Sirius B With no internal nuclear energy source, White Dwarfs (WD) cool off, and grow dimmer with time until they become black. For a 1 M Sun star the core ends up as carbon, leading to a carbon WD; for a smaller mass star the core never fuses He to C and so the WD is made of helium. A white dwarf is about the same size as Earth! WD density = 333,000 x 5.5g/cc = 1.8 tons/cc A sugar cube of WD material would weight 1.8 tons on Earth! How is this possible? M Sun = 333,000 M Earth
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2 White dwarfs shrink when you add mass to them because their gravity gets stronger! How is this possible? Shrinkage of White Dwarfs Shrinkage of White Dwarfs Quantum Mechanics says that electrons in the same place cannot be in the same “state” Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space In order to avoid being in the same state some of the electrons need to move faster Is there a limit to how much you can shrink a white dwarf? Yes. The White Dwarf Limit Einstein’s theory of relativity says that nothing can move faster than light – so that must be the limit. When electron speeds in a White Dwarf approach speed of light, electron degeneracy pressure can no longer support the weight. Chandrasekhar found (at age 20!) that this happens when a white dwarf’s mass reaches 1.4 M Sun This is the famous Chandrasekhar Limit. S. Chandrasekhar Nobel Laureate; Chandra X-ray telescope named after him. In a binary star system with one star more massive than the other, the more massive star is first to become a red giant, eject its outer layers and end as a white dwarf. Eventually the second star (of lower initial mass) also swells up to become a red giant. What happens next? If the binary is close enough, the 2nd star swells up to fill is ROCHE LOBE, an imaginary tear-drop shaped shell of equal gravity, and dumps matter onto the WD.
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3 Friction in the disk makes it hot, causing it to glow. Friction also removes angular momentum from inner regions of disk, allowing them to sink onto white dwarf. View from above
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Lecture_15 - White Dwarfs are the remaining cores of dead...

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