Transcript Neutron Stars

Neutron Stars
Pulsars
Neutron Stars
In 1967, it was believed (by some)
that the first intelligent signal from
outer space had been discovered.
A graduate student by the name of
Jocelyn Bell observed highly periodic,
well defined pulsations of
electromagnetic energy.
The pulses lasted 0.01 sec, and
repeated regularly with a period of
1.34 sec.
WOW
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Bell’s thesis advisor, Anthony Hewish, explained the observation as being
related to a small, rotating, radiating object.
The object had to be undergoing regular rotation to explain the well-defined
periodicity.
The object had to be small in order to explain the sharpness of the emission
(a large radiating object would would produce the radiation over a longer
time).
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The best current model explains the pulsations as being produced by
a rotating neutron star.
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The remnant of a type II supernova is a remarkably compact Neutron
Star.
Property
Observation
Nuclear Reactions
Ceased
Pressure
Neutron Degeneracy Pressure
Diameter
About 20 km
Density
1017 - 1018 Kg/m3
Note: density of an atomic nucleus is approximately 1017 Kg/m3
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The Pulsar
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The Pulsar
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Lighthouse Model – the periodic emission of radiation as the pole of the
neutron star rotates periodically into the view of the earth.
Note: The reception of the pulsating radiation
requires that the radiating “hot spot” be directed
toward the earth. Otherwise, the pulsations will not
be seen.
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Some properties:
* High velocities can be observed
* Short period pulsations for some pulsars
* X-ray bursters
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High Speeds
Doppler shift measurements show that many neutron stars are moving at
very high speeds.
Explanation:
The KABLOOEY of the outer shells in the SN event exert a force back on
the neutron core (Newton’s Third Law – For every action there is an equal
and opposite reaction.
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High Speeds
Newton’s Third Law – For every action there is an equal and opposite reaction.
Symmetric explosion
The action reaction
pair will not cause the
star to move
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High Speeds
Newton’s Third Law – For every action there is an equal and opposite reaction.
asymmetric explosion
The action reaction
pair will cause the
star to move
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High Speeds
Newton’s Third Law – For every action there is an equal and opposite reaction.
asymmetric explosion
Velocity of
ejected
neutron star
The action reaction
pair will cause the
star to move
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Some properties:
* High velocities can be observed
* Short period pulsations for some pulsars
* X-ray bursters
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Millisecond Pulsar – a pulsar with a period an the order of a millisecond.
1 millisecond = 0.001 seconds
In order for the pulsar of pulse at a period of 0.001
seconds, the neutron star must be rotating a thousand
times a second !
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Explanation:
Conservation of Angular Momentum: The angular momentum of a collapsing
core must be conserved.
For a rotating sphere, the angular momentum is given by
L = I = 2/5 MR2 
Where
L is the angular momentum
I = 2/5 MR2 is called the moment of inertia
 = 2R/T is the angular speed in radians/second or revolutions/second
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Explanation:
Conservation of Angular Momentum: The angular momentum of a collapsing
core must be conserved.
L = I = 2/5 MR2  = constant
If R decreases as the core collapses,  must increase
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Example:
A star is rotating with  = 1 rev/sec. If the core begins wit ha radius equal to
the radius of the sun, and collapses to a final radius of 0.01 R, what will its
final angular speed become after the collapse? Assume the mass stays a
constant.
Solution: use proportions
L = I = 2/5 MR2  = constant
Ri 2  i = Rf 2  f
f =
Ri
Rf
( )
2
i = (1/1000)i = 0.0001 rev/sec
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Note: The shortest period pulsar is around 0.001 seconds (millisecond pulsar).
Periods shorter than this do not exist (such as in the
previous calculation) because the object would have to
rotate so fast it would overcome gravity and fly apart.
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Some properties:
* High velocities can be observed
* Short period pulsations for some pulsars
* X-ray bursters
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X-Ray Burster – Neutron star that experiences a large increase in
luminosity, emitting radiation in the X-ray region of the electromagnetic
spectrum.
Explanation: The mechanism is basically the same as that of a nova. A
neutron star that is part of a binary system will accrete material from the
companion. Once enough material is accumulated o nthe surface of the
neutron star, hydrogen fusion will occur.
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X-Ray Burster – Neutron star that experiences a large increase in
luminosity, emitting radiation in the X-ray region of the electromagnetic
spectrum.
Explanation: Because the gravity is so strong at the surface of the neutron
star, compression will happen rapidly, and the fusion will occur violently,
producing high energy photons.
Recall E = hf, so high energy photons will have a very high frequency.
Electromagnetic Wave Basics
The Electromagnetic Spectrum
Wavelength in meters
10000
1
.0001
10-8
IR
Radio
Microwave
10-12
X-Ray
UV