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PHYS 183 Lecture Notes - Electron Degeneracy Pressure, White Dwarf, Stellar Evolution

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PHYS 183
Tracy Webb

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Lecture 26: March 18th, 2013
midterm is non-cumulative covering everything from after the first midterm & up
until last Friday’s lecture
final exam will be cumulative
next Monday’s lecture is cancelled
assignment number 3 will be due April 3rd
the bizarre stellar graveyard is stellar remnants white dwarfs & binary systems,
neutron stars, black holes
recall: white dwarfs
o remnant of low-mass stars
o supported by electron degeneracy pressure
o upper limit for mass of 1.4 solar masses
o gravity wins which collapses the star, but clearly pressure wins again or else
it would continue to collapse
o as you make them more massive, they get smaller rearrange matter
within them to support the extra mass, which makes them smaller
white dwarfs in binary systems
o take low mass star & you let it evolve (burns H & He) & then it loses all outer
atmospheres in a planetary nebula then falls & cools passively essentially
forever (only happens if the white dwarf is on its own)
o an isolated white dwarf will simply cool & fade forever
o 50% of stars are born in binary systems
o but a white dwarf in orbit with a companion star suffers a very different fate
o important since white dwarfs are extremely small difficult to study them
unless they are in a companion system
binary system stars
o mass transfers between close stars
o happens often between 2 companion stars
o mass from high mass star (red giant) is stolen from the lower mass star
which then becomes more massive
o what happens as they evolve?
the evolution of the high mass star slows down
the evolution of the low mass star speeds up will now go through
moe massive stellar evolution
WD Novae (distinct from Supernovae)
o hydrogen builds up on the surface of the star
o the temperature & pressure increases as more mass is accreted
o eventually the whole thing will explode
o hydrogen ignites rapidly & there is a brief (week-long) burst of fusion
o distinct from supernovae since just have the surface of the star exploding
due to excess H so outer H shell erupts
o can reach 100 000 solar luminosities
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