Neutron Stars and Black Holes
Vela pulsar
Chapter 22 cover photo
Properties of Neutron Stars
High density, small radius
Neutron degeneracy
Large magnetic field, as core collapsed, magnetic field lines pulled in closer, thus amplifying the magnetic field
Rotation- fast, remember the smaller the object becomes, the faster it rotates
Calculate the "weight" of a 75 lb person on the surface of a neutron star
Problem 22-3
What is the surface gravitational acceleration of 1.4 solar mass neutron star with radius of 10 km?
What is the escape speed from a neutron star of same mass and radius 4 km?
Pulsars
Lighthouse model of emission
All pulsars are neutron stars
Not all neutron stars are pulsars
Depends on age and viewing angle
Fig. 22.3
Pulsars
Pulsed emission vs. time
Pulse intensities can be different
Regular intervals between pulses
First observed by Jocelyn Bell, 1967
Most pulsars have pulsed radio emission, some have radio through gamma ray
Fig 22.2
Crab Pulsar
Crab pulsar
Pulsar associated with SNR
Precusor, main pulse
Fig. 22.4
Pulsars
Crab and Geminga pulsars, 4.5° separation, in gamma rays
Fig. 22.5
Spin-Down
Pulsar periods- generally .3-.03 seconds (3 to 30 times per second)
Pulsar loses energy over time
Energy loss causes net slowdown in rotation, intensity of pulses
Eventually star ceases to rotates
Spin-Up
But in some binary neutron star pairs, material accretes onto neutron star through accretion disk
Accreting material adds angular momentum to star, causes star to rotate faster
Spin-up, millisecond pulsars
Period =.001 seconds
Spin-up explains how pulsars appear in much older globular clusters- neutron stars lifetime
Fig. 22.8a
X-ray Bursters
Neutron star in binary pair, emission in X-rays from hot accretion disk
Analogous to novae and white dwarfs
Fig. 22.8
Pulsar Planets
Small change in milisecond pulsar's pulses measured, 1992
Attributable to Earth-sized planets
Planets orbit at 0.2, 0.4, and 0.5 AU
Masses 3 times Earth and Moon sized
Because planets are so small and close, planets are not believed to have existed before SN explosion
Perhaps they formed from the material left behind by disruption of companion star
Gamma Ray Bursts
Bursts of gamma ray emission detected in 1960s by VELA satellites (satellites designed to detect violations of Nuclear Test Ban Treaty)
Quickly it was realized that signals were from outside solar system
Light curves complicated and varied
Gamma Ray Burst Light Curves
Fig. 22.11a
Location of Gamma Ray Bursts
Compare morphology (appearance) of Fig. 22.11b and 5.35
Possible Models of Gamma Ray Bursts
Radio and optical counterparts to gamma ray bursts finally found in 1997
Optical shows that gamma ray bursts are associated with distant galaxies
Fig. 22.12
Possible Models of Gamma Ray Bursts
Inverse Squared Law places huge constraints on total energy
Emission is probably beamed in jets
Beaming reduces total energy needed
Fig. 22.13
Review Relativity Notes
Special vs. General
Nothing travels faster than light
All things, including light, are attracted by gravity
Mass curves spacetime
Time dilation, length contraction
What are observable effects of relativity?
Gravity Waves
EM waves arise from electric and magnetic fields
An accelerating charge emits EM waves (radiation)=photons
An accelerating mass theoretically should emit the gravity analog= gravity waves
Gravity is far weaker than the electric and magnetic fields, so gravity waves are far weaker than EM waves
Evidence of Gravity Waves
Gravity waves have not yet been detected
LIGO (Laser Interferometric Gravity-Wave Observatory) taking data now
In binary pair, orbiting stars lose energy through gravity waves
Orbit size will decrease
Observed by Taylor and Hulse 1974
Change in orbit is on order of diameter of Sun
In tens or hundreds of million years, stars will merge
Black Hole Formation
If neutron core exceeds ~3 solar masses (MM), core continues to collapse = black hole
Exact mass depends on rotation and magnetism
Black holes are defined by three properties
Mass, charge, angular momentum
Event horizon -the imaginary "surface" of a black hole
Event horizon occurs at the radius where the escape speed exceeds the speed of light
Schwarzschild radius
Gravitational Redshift
Photon gives up energy as it leaves gravitational "well"-photon's speed cannot change, but wavelength does
E=hc/l = hf
Fig. 22.18 and 22.17
Tidal Effect of Black Hole
What happens when object approaches the event horizon of the black hole?
Fig. 22.16
Inside the Event Horizon
What is inside a black hole?
Simple answer- We don't know.
Mathematics predicts a singularity, zero size
But a singularity is the signal that our knowledge physics is breaking down
Physics can describe a collapsing object to a size less than any elementary particle
Thus, changes to physics will be on quantum scales, not macroscopic (km)
Evidence for Black Holes
A black hole has never been directly observed
However one should see the accretion disk
Matter is very hot, emit X-rays
Black holes in binaries
Supermassive black holes in center of galaxies
Fig. 22.21
Cygnus X-1
Binary star: Main sequence blue supergiant-20 MM , unseen companion
Total mass of binary is 35MM
Spectroscopy:
Period = 5.6 days
Evidence of hot gas flowing, mass transfer
Rapid time variability, 1ms implies size less than 300 km
Summary of Neutron Stars and Black Holes
Formation of Neutron Stars and Black Holes
Properties of neutrons stars
Neutron degeneracy, pulsed emission, magnetic field, spin-up, spin down
Gamma ray bursts - models, observed emission
Properties of black holes
Schwarzschild radius, event horizon, mass, tidal stretching
Relativistic effects- gravitational radiation and redshift
Observable effects of relativity