Neutron Stars and Black Holes
PHYS390: Astrophysics
Professor Lee Carkner
Lecture 18
Question
1) Should there be a lower limit for the mass of observed white dwarfs? Why or why not?
Yes, size of white dwarf depends on initial mass of star, very low mass stars have not had time to evolve to white dwarf yet
Neutron Stars
None detected until the 1960s
Principally observed as pulsars rapidly rotating neutron star
producing beamed radio emission
Neutron Degeneracy
Density of neutron star ~ Star is like a big ball of ~ Acceleration of gravity at surface ~ trillion
meters per second2
Mass limit of ~3 Msun
Neutron Star Formation
Start with iron core At high densities electrons become relativistic
and combine with protons to produce neutrons (and neutrinos)
As density increases neutrons “drip” outside of nuclei Can form superconducting superfluid
Neutron Star Structure
inner crust of heavy nuclei and free neutrons
interior mostly neutrons maybe a core of sub-
nuclear particles?
Rotation
Ratio of initial and final periods:
Pf/Pi = (Rf/Ri)2
End up with neutron star rotation periods ~ 1 second
Flux Freezing
Magnetic fields get “frozen” into core material and concentrated as core shrinks
Bf/Bi = (Ri/Rf)2
Again, hard to know initial core magnetic field Typical neutron star B ~108 T
Pulsars
P ~ 1 sec Only something very
small and compact could change that fast
Many pulsars have large space motions
Can be found in the center of SNR
Pulsar Model
Changing magnetic field produces magnetic dipole radiation
If the cone intersects the Earth, we see the radio pulse
Energy is drawn from rotation and the pulsar slows down over time
Black Hole Gravity is so strong that the escape velocity exceeds the
speed of light Point occurs at the Schwarzschild radius
RS = 2GM/c2
Marks the event horizon
At the center is the singularity
Even outside of the event horizon, tidal forces are very strong Material nearing a black hole is violently ripped apart Can heat up material causing it to emit
Rotation
Maximum angular momentum is:
Lmax = GM2/c
May cause frame dragging of local spacetime
Types of Black Holes
3-15 Msun, stellar remnant black holes
100-1000 Msun: intermediate mass black holes possible explanation for superbright X-ray sources
105-109 Msun, supermassive black holes
Create Active Galactic Nuclei (AGN) when active, hard to find if not active
NS and BH Binaries
Called an X-ray binary If the mass of the
compact object is greater than ~3 Msun, it is a black hole
More than anything else but annihilation
Types of Binaries X-ray pulsar
Matter falls onto pulsar, heating it up to X-ray temperatures (107 K)
X-ray hot spot may be eclipsed
Mass transfer may spin-up the pulsar, decreasing the period
X-ray burster If the magnetic field is too
weak the material will build up in a layer on the surface
Next Time
Test 3 Same format as 1 and 2 For Friday:
Read 24.2-24.4 Homework: 24.15, 24.32
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