neutron stars pulsars. neutron stars in 1967, it was believed (by some) that the first intelligent...
TRANSCRIPT
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
Neutron Stars
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).
Neutron Stars
The best current model explains the pulsations as being produced by a rotating neutron star.
Neutron Stars
The remnant of a type II supernova is a remarkably compact Neutron Star.
PropertyProperty ObservationObservation
Nuclear ReactionsNuclear Reactions CeasedCeased
PressurePressure Neutron Degeneracy PressureNeutron Degeneracy Pressure
DiameterDiameter About 20 kmAbout 20 km
DensityDensity 10101717 - 10 - 101818 Kg/m Kg/m33
Note: density of an atomic nucleus is approximately 1017 Kg/m3
Neutron Stars
The Pulsar
Neutron Stars
The Pulsar
Neutron Stars
Neutron Stars
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.
Neutron Stars
Some properties:
* High velocities can be observed
* Short period pulsations for some pulsars
* X-ray bursters
Neutron Stars
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.
Neutron Stars
High SpeedsNewton’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
Neutron Stars
High SpeedsNewton’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
Neutron Stars
High SpeedsNewton’s Third Law – For every action there is an equal and opposite reaction.
Velocity of ejected neutron star
asymmetric explosion
The action reaction pair will cause the star to move
Neutron Stars
Some properties:
* High velocities can be observed
* Short period pulsations for some pulsars
* X-ray bursters
Neutron Stars
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 !
Neutron Stars
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
= 2R/T is the angular speed in radians/second or revolutions/second
Neutron Stars
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
Neutron Stars
Neutron Stars
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( )
2i = (1/1000)i = 0.0001 rev/sec
Neutron Stars
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.
Neutron Stars
Some properties:
* High velocities can be observed
* Short period pulsations for some pulsars
* X-ray bursters
Neutron Stars
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.
Neutron Stars
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.
10000 10-8.00011 10-12
Wavelength in meters
Electromagnetic Wave Basics
The Electromagnetic Spectrum
Radio
Microwave
IR
UV
X-Ray