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. PropertyObservation Nuclear Reactions Ceased Pressure Neutron Degeneracy Pressure Diameter About 20 km Density Kg/m 3 Note: density of an atomic nucleus is approximately Kg/m 3

Neutron Stars The Pulsar

Neutron Stars The Pulsar

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 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

Neutron Stars 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

Neutron Stars High Speeds Newton’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 = seconds In order for the pulsar of pulse at a period of 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 MR 2  Where L is the angular momentum I = 2/5 MR 2 is called the moment of inertia  = 2  R/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 MR 2  = constant If R decreases as the core collapses,  must increase

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 MR 2  = constant R i 2  i = R f 2  f  f = RiRi RfRf () 2 ii = (1/1000)  i = rev/sec

Neutron Stars Note: The shortest period pulsar is around 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.

Wavelength in meters Electromagnetic Wave Basics The Electromagnetic Spectrum Radio Microwave IR UV X-Ray