Chapter 23 Solutions:

2. Neutron degeneracy pressure arises when neutrons are so close that their quantum states begin to overlap. Since no two fermions, neutrons in this case, can occupy the same quantum state, a pressure results. The combined pressure from neutron degeneracy pressure and the strong nuclear force prevent further gravitational collapse of a neutron star if the remaining supernova core (neutron star) is less than 2-3 solar masses. In a white dwarf star it is electron degeneracy pressure that is preventing gravity from collapsing the star. In this case the upper mass limit is 1.4 solar masses which is better known than for the case of a neutron star (see 25. below)

4. In order to generate 30 pulses/sec as observed in the crab pulsar, astronomers realized that such an object must be smaller than a white dwarfs and rotating rapidly. A neutron star with its strong magnetic field (~ 10¹² gauss) tilted with respect to the rotation axis creates a beam of radiation capable of rapid regular pulses.

5. The ideas of pulsars being due to an alien civilization was soon discarded when many more pulsars were found with very similar properties.

6. Neutron stars rotate more rapidly than ordinary stars because the rate of spin increased as the stars collapsed to a small size due to conservation of angular momentum (like the ice skater who pulls her mass in closer to her rotation axis and spins faster or my demonstration in class on the stool using masses). The magnetic fields of these stars are strong because these fields became highly concentrated when the stars collapsed.

14. Superconductivity refers to the ability of electric currents to flow without resistance. Superfluidity refers to the movement of fluids without friction.

21. As a pulsating x-ray source gathers matter from its close companion star, it (the neutron star) spins up and becomes a millisecond pulsar.

22. While both a nova and a Type Ia supernova both result from white dwarf stars being in close binary systems, a Type Ia supernova probably accretes more mass, more rapidly pushing it over the Chandrasekhar limit causing thermonuclear reactions to occur throughout the star leading to a supernova explosion that destroys the star. Conversely, a nova accretes a smaller amount of mass from its companion star and thermonuclear reactions just take place on the surface, resulting in a smaller explosion and the possibility of events repeating.

23. Novae are caused by infalling matter from the companion star to spill onto the surface of a white dwarf. When the temperature becomes high enough the surface material explodes. In contrast, X-ray bursters are caused by infalling matter from the companion star to spill onto the surface of a neutron star. Periodically this matter will ignite explosively causing the X-ray burst.

25. We don’t know enough about the strong nuclear force to precisely determine the maximum mass of neutron stars.