Chapter 20

A \(2 \mathrm{M}_{\odot}\) core of a star contracts after using its nuclear fuels. Explain why we can be sure that the star will not become a white dwarf.

Suppose two white dwarf stars have the same surface temperature. Why is the more massive of the two white dwarfs the less luminous?

Main sequence stars of \(5 \mathrm{M}_{\odot}\) are thought to evolve into \(1 \mathrm{M}_{\odot}\) white dwarfs. What happens to the \(4 \mathrm{M}_{\odot}\) that do not become part of the white dwarf?

What is the relationship between the central stars of planetary nebulae and white dwarf stars?

Suppose AGB stars did not lose mass through cool winds. What effect would this have on the number of white dwarf stars in the galaxy?

What is the difference between the spectra of type I supernovae and those of type II supernovae?

The core of a massive \(A G B\) star consists of iron and nickel surrounded by shells of successively lighter elements. What does this structure have to do with the history of consumption of nuclear fuels by the star?

Why does the process of neutronization reduce the ability of the degenerate electrons in the core of a massive AGB star to support the weight of the star?

What are the processes that reverse the in fall of matter during a type II supernova explosion and blast material out of the star?

What is the ultimate origin of the energy released in a type II supernova?

Only a small percentage of the energy of a type II supernova is carried away by radiation and the shell of matter blasted outward. What happens to the rest of the energy released in the explosion?

Why are many supernova remnants bright in the radio part of the spectrum?

Why does the expansion of a supernova remnant slow as time passes?

What effect do supernova explosions have on the chemical makeup of interstellar gas?

What are the similarities and differences between the mass-radius relationships of white dwarfs and neutron stars?

What does the concept of conservation of angular momentum have to do with the rapid rotation of neutron stars?

Why would no pulses be observed from a rotating neutron star if its magnetic axis and spin axis were aligned?

What happens to the rotation rate of a pulsar as time passes?

Why are all pulsars not found within supernova remnants?

How many coordinates are used in spacetime?

Why is a stationary body represented by a vertical line rather than a point in a spacetime diagram?

Why can't a body move horizontally in a spacetime diagram?

What is the difference between space-like trips and time like trips?

Suppose you lived in a two-dimensional world. Describe a way you could use geometry to determine whether your world was flat or curved.

Suppose you made a triangle in your backyard. You used a stretched string to make three geodesics to form the sides of the triangle. You then used a protractor and found that the sum of the angles of the triangle was \(180^{\circ} .\) You know that the surface of the Earth is curved, so why didn't your triangle contain more than \(180^{\circ}\) ?

Suppose an astronaut falling toward a black hole used a watch to check her pulse rate. She notes that her pulse rate remains constant as she falls closer to the black hole. (Obviously, this astronaut is tough.) Why if clocks run more slowly in strongly curved regions of spacetime, does the astronaut find that her pulse rate doesn't change?

What effect does the curvature of spacetime have on the frequency of radiation passing through it?

The spectra of white dwarfs usually show large redshifts. Why doesn't this imply that most white dwarfs are moving rapidly away from us?

What would happen to the distance of the Earth from the Sun and the length of the year if the Sun were instantly replaced by a \(1 \mathrm{M}_{\odot}\) black hole?