Neutron Stars
• A neutron star forms during the supernova
explosion if the mass of the collapsing core
exceeds the Chandrasekhar limit of 1.4 M
• Degenerate neutron pressure count-balances
the gravitation
• A neutron star is a dense stellar corpse
consisting primarily of closely packed
degenerate neutrons
• Proposed in 1930’s, not verified until 1960’s
Properties of Neutron Stars
• Diameter of about
20 km
• Mass less than 3 M
• Magnetic field 1012
times stronger than
that of the Sun
• Rotation period of
roughly 1 second
A Model of Neutron Star Structure
• A neutron star consists of
a superfluid,
superconducting core
surrounded by a
superfluid mantle and a
thin, brittle crust
• There is evidence for an
atmosphere
The discovery of pulsars in the 1960s
• A pulsar is a source of periodic pulses of radio radiation
Discovery - Jocelyn Bell Burnell
She was graduate student at the University of Cambridge
Discovery - Jocelyn Bell Burnell
Her PhD supervisor Anthony Hewish was granted the
Nobel Prize in 1974. She was not even mentioned.
Pulsar Found in the Crab Nebula
• Crab nebula was created by the supernova explosion occurred on
July 4, 1054, recorded by Chinese astronomer
• A fast rotating pulsar, with period of 0.033 second, or 30 times
per second, is discovered at the center of the Crab nebula
• Such fast spin can not be from a white dwarf
Pulsars are rapidly rotating neutron stars
• A neutron star can spin very fast
because of its small size
• Magnetic field is strong because
all magnetic field in the
progenitor star is squeezed and
concentrated into the neutron
star size
• Magnetic axis is inclined to the
rotation axis
• Charged particles from the
surface are accelerated along the
intense magnetic field, and
radiate electromagnetic radiation
Pulsars are beamed radiation sweep the Earth
• Because charged particles
move along the magnetic
field lines
• The radiation is also along the
magnetic field lines, forming a
radiation beam in parallel
with the magnetic axis
• The beam sweeps around the
sky as the star rotates
• If the Earth happens to lie in
the path of the beam, the
pulsar can be detected
Pulsars is like a lighthouse beacon
• Pulsar is a
rotating neutron
star whose
radiation beam
happens to
sweep the Earth
Periods of Pulsars
• Radio telescopes have found more than 1000 pulsars
• Their rotation period is in a wide range from 1 ms (milisecond
or 0.001 sec) to 10 second
• An isolated pulsar slows down as it ages, so its period
increases
What powers the Crab Nebula?
• The ultimate energy source
for the luminous nebula is the
spinning of the neutron star
• The spinning or rotation
energy is transferred into the
surrounding nebula, which
results in the gradual slow
down of the star
• The nebula shines due to the
radiation from the energetic
electrons accelerated along
the magnetic fields
Pulsars gradually slow down, but have
glitches
• The pulse rate of many pulsars is slowing steadily
• Sudden speedups of the pulse rate, called glitches, may
be caused by interactions between the neutron star’s
crust and its superfluid interior
Gravity Equivalent of Curvature of Space
• The curved space not only acts on the object with mass
• The curved space also acts on the light, even though light does not
have mass
• The light seeks to move across the shortest distance between tow
points; in a curved space, the light bends instead of moving in a
straight line
1. During the solar eclipse, the starlight is deflected by
the Sun’s gravity by an amount of 1.75 arcsec (1919)
2. Mercury, the closest planet to the Sun, shows an
excessive precession that perfectly fits the slightly
curved space near the Sun.
Proof of Theory of Relativity
Gravitational Red Shift
• Because of the time dilation, the period of light wave
from the surface of a strong gravity becomes longer,
and thus the frequency becomes smaller
• Or equivalently, wavelength becomes longer; this is so
called gravitational red shift
• On the surface of a white dwarf, red shift (Δλ/λ) is a
factor of 10-4
• On the Sun, the gravitational red shift is negligible
Theory of Relativity Predicts Black Holes
• If a stellar corpse has a mass greater than about 2 to 3 M,
gravitational compression will overwhelm all forms of internal
pressure, including degenerate neutrons and nuclear forces
• The stellar corpse will collapse to a singularity, immediately
around which the escape speed exceeds the speed of light
Stellar Black Hole
• Far away from the black
hole, the space is the
same as in the case of a
normal main sequence
star
Certain binary star systems probably
contain black holes
• Black holes have been
detected using indirect
methods
• Some binary star
systems contain a black
hole
• In such a system (e.g.,
Cygnus X-1), gases
captured from the
companion star by the
black hole emit
detectable X raysCygnus X-1
Stellar Black Hole