● White dwarfs are the remaining cores of low-mass stars.
● Electron degeneracy pressure supports then against the crush of gravity.
● White dwarfs cool off and grow dimmer with time.
Size of a White Dwarf
● White dwarfs with the same mass as Sun are about same size as Earth.
● Higher-mass white dwarfs are smaller.
The White Dwarf Limit
● Quantum mechanics says that electrons must move faster as they are squeezed into a very
● As a white dwarf’s mass approaches 1.4 Msun, its electrons must move at nearly the
speed of light.
● Because nothing can move faster than the speed of light, a white dwarf cannot be more
massive than 1.4 the white dwarf limit. (....)
● Astar that started with less mass gains mass from its companion.
● Eventually the mass-losing star will become a white dwarf…
● Mass falling toward a white dwarf from its close binary companion has some angular
● The matter therefore obits the white dwarf…..
● Kepler’s law: matter in the inner part of the disk moves faster than that in the outer disk.
● Friction between orbiting rings of matter in the disk transfers angular…….
● The temperature of accreted matter eventually becomes hot enough for hydrogen fusion.
● Fusion begins suddenly and explosively, causing a nova.
● The nova star system temporarily appears much brighter.
● The explosion drives accreted matter out into space.
Two Types of Supernova
● Massive Star Supernova:
○ Iron core of a massive star reaches white dwarf limit and collapses into a neutron
star, causing a total explosion.
● White Dwarf Supernova:
○ Carbon fusion suddenly begins as a white dwarf in a close binary system reaches
the white dwarf limit, causing total explosion.
● One way to tell supernova types apart is with a light curve showing how luminosity
changes with time. Supernova Type: Massive Star or White Dwarf?
● Light curves differ
● Spectra differ (exploding white dwarfs don’t have hydrogen absorption lines)
Nova or Supernova?
● Supernovae are much more luminous (about 100 thousand times)
● Nova: H to He fusion of a layer of accreted matter white dwarf left intact.
● Aneutron star is the ball of neutrons left behind by a massive star supernova.
● Degeneracy pressure of neutrons supports a neutron star against gravity.
● Electron degeneracy pressure goes away because electrons combine with proteins,
making neutrons and neutrinos.
● Neutrons collapse……
● Aneutron star is about the same size as the distance fromAmherst to Northampton, but is
more massive than the sun.
Discovery of Neutron Stars
● Using radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio
● Pulsar at the center of the Crab Nebula pulses 30 times per second.
● Apulsar is a neutron star that beams radiation along a magnetic axis that is not aligned
with the rotation disk
● Pulsars spin fast because a stellar core’s spin speeds up as it collapses into a neutron star
due to Conservation ofAngular Momentum.
● The radiation beams sweep through space like lighthouse beams as the netron st