astron lec13 white dwarfs and neutron stars

limit to how much electron degeneracy pressure a star can have leads to...

maximum mass for white dwarf of 1.4 Msun

white dwarf

dead low-mass star that cools and dims forever
- around same size as earth
- electron degeneracy pressure supports them against crush of gravity

White dwarf limit (Chandrasekhar limit)

1.4 Msun

White dwarf limit (Chandrasekhar limit) explained

quantum mechanics say that as electrons are squeezed into smaller space, they must move faster
as white dwarf's mass reaches 1.4 Msun, electrons must move at speed of light, and nothing moves faster than that

white dwarf in close binary system

accretes matter in accretion disk and explodes as nova

accretion disk

mass from close binary companion falls toward white dwarf and orbits it, creating accretion disk of matter
friction between orbiting rings transfers angular momentum outward and causes disk to heat up and glow

nova

star that suddenly becomes super bright and then fades
temp of accreted matter gets hot enough for hydrogen fusion, fusion begins suddenly and explosively, causing nova

2 types of supernova

1. big star supernova
2. white dwarf supernova

white dwarf supernova

white dwarf accretes matter and exceeds the Chandrasekhar limit, exploding

how to tell what type of supernova it is

1. light curve - white dwarf supernova goes up more then linearly down
2. spectra - white dwarfs don't have hydrogen absorption lines

neutron star

ball of neutrons left behind by massive-star supernova, supported by neutron degeneracy pressure against gravity
about same size as Durham

pulsar

spinning neutron star that produces pulses of radio waves
beams radiation along magnetic axis not aligned with rotation axis

X-ray burst

accretion onto neutron star, becomes hot enough for helium fusion, sudden onset of fusion produces X-ray bursts

neutron star limit

neutron degeneracy pressure cannot support past 3 Msun, turns into a black hole