2. Degenerate neutron pressure is what keeps the neutron star from collapsing under its own weight. At the densities found inside neutron stars, the neutrons are so close together that, if they were any closer, the Pauli exclusion principle for neutrons would be violated. So the neutrons push back to avoid this, halting any further contraction. (But if the object has a mass of more than about 3 solar masses, the neutrons can't push back hard enough, and the object will continue to collapse, becoming a black hole singularity.)

5. Their speedy rotation is due to conservation of angular momentum. The original high-mass star rotates relatively slowly on its axis. When its core contracts, its spinning is speeded up automatically. Since its final size is so small compared to its original size, its final spin-rate is much larger than its original spin-rate.

The magnetic field of the original high-mass star's core is augmented tremendously when that core shrinks to become the neutron star. This is because the core's magnetic field lines are "attached" to the core's surface. When the core contracts, its surface area is tremendously reduced, but all of its original magnetic field lines must continue to be attatched to it. So the final neutron star's field lines are very close together at its surface, meaning the magnetic field there is very strong.

7. Our Sun's core will not become a neutron star, because its mass is less than 1.4 solar masses. Therefore the collapse of the Sun's core will be halted by degenerate electron pressure, and it will become a white dwarf (a much larger, less dense object than a neutron star).

19. The relevant thing to notice from Figure 23-3 is that the two beams of radiation that are emitted from the neutron star's magnetic poles are extremely narrow. Imagine a spinning, magnetized neutron star out there in space. Since its beams are so narrow, the odds that the neutron star is spinning in such a way that one of its beams will sweep over you is very small. (The less narrow the beams, the more chance that one of them will be pointed in your direction as it rotates -- because the neutron star's "aim" doesn't have to be so precise.) Let's suppose that the odds of a beam sweeping over you are 1 in 10,000. In that case, for every 10,000 rotating, magnetized neutron stars out there, only one will appear as a pulsar. So we conclude that there are many more neutron stars out there than just the pulsars that we see.

21. According to pages 525 and 526 of your book, the supernova that made the Crab Nebula was seen in the year 1054. But that exploding star was about 2,000 parsecs away, which is equivalent to (2,000 pc)(3.26 ly/pc) = 6,520 light-years. So the explosion actually took place something like 6,520 years before it was seen in 1054 -- which would be approximately the year 5467 B.C.E. ("approximately" because the Crab Nebula is not exactly 2,000 parsecs away from us).

(By the way, if you're wondering why the answer is not 1054-6520 = 5466 B.C.E. -- it's because there was no year 0. The year before 1 C.E. was 1 B.C.E., with nothing in between. Simple subtraction assumes that there was a year 0 between the years 1 and -1. No, this will not be on the exam. ;-) )

Last edited 18 Apr 04 M. A. Weinstein.