Homework #5 Solutions



Chapter 14, p. 314      Problems 7-11


For each of the stars in problems 7-11, decide whether you think it is possible that it could harbor an advanced civilization, and explain your reasoning in one or two paragraphs. 



For each of 7-11, consider what is necessary for an advanced civilization to develop.  Assuming that this civilization is similar to us, an environment at a moderate temperature with an atmosphere to protect against harmful ultraviolet rays is needed.  Liquid water is a necessity, as well as relatively stable temperatures.  These things are needed for a long period of time in order for the civilization to advance technologically. 


If one is considering an advanced civilization very different from our own understanding of life on this planet, all bets are off. 


For each star, some main points are listed:

  1. A 10 MSun main-sequence star.
    1. This is a high mass star, whose main sequence lifetime is on order of millions of years instead of billions.  Could life develop in millions of years?  Probably not an advanced civilization.  The Earth is 4.6 billion years old, and we have only recently progressed technologically. 
    2. Increased luminosity
  2. A carbon star.
    1. This is a later stage of evolution of a high mass star that has already been through the red supergiant phase, where its radius greatly expands.  At what distance is the life-sustaining planet, and could life continue?  Probably would not find an advanced civilization. 
  3. A 1.5 MSun red giant.
    1. Same answer as 10, but a slightly larger star.  At what distance is the life-sustaining planet located?
    2. Increased luminosity
    3. If an advanced civilization existed, it has probably been destroyed during this stellar phase.
  4. A 1 MSun helium-burning star.
    1. Occur after red giant phase when the hydrogen core is exhausted.  Would the red giant engulf the location of the life-sustaining planet?  Also, keep in mind that the main-sequence lifetime of a low mass hydrogen-burning star is about 10 billion years.  All the later stages together are about 1 billion years.  So, could life re-develop in this shorter time?  Probably not. 
  5. A red supergiant.
    1. Same as #9, but for a high mass star, so even more pronounced expansion
    2. Increased luminosity – probably would not find an advanced civilization.

Chapter 15, p. 329      Review 10

What is the event horizon of a black hole?  How does it get its name?  How is it related to the Schwarzschild radius?


The event horizon is the location at which nothing can escape the gravitational pull of the black hole, not even radiation (light).  “The event horizon gets its name because information can never reach us from events that occur within it.”  (p. 324)  The Schwarzschild radius is the distance from the center of the Black Hole (singularity), given in Cartesian coordinates, where the event horizon is located.  It depends only on the mass of the black hole, and is tricky to compute since space-time becomes more warped as one approaches the event horizon. 



Chapter 16, p. 350      Review 1

Draw simple sketches of our galaxy as it would appear face-on and edge-on, identifying the disk, bulge, halo and spiral arms.



Edge-on View:









Face-on View: 









Chapter 17, p. 381      Review 1

Distinguish between spiral galaxies and elliptical galaxies in terms of their shapes and colors.  What are irregular galaxies?


“Spiral galaxies look like flat, white disks with yellowish bulges at their centers.  The disks are filled with cool gas and dust, interspersed with hotter ionized gas… and usually display beautiful spiral arms.  Elliptical galaxies are redder, more rounded, and often longer in one direction than in the other, like a football.  Compared with spiral galaxies, elliptical galaxies contain very little cool gas and dust, though they often contain very hot, ionized gas.  Galaxies that appear neither disklike nor rounded are classified as irregular galaxies.  The sizes of all three types of galaxies span a wide range…”


(p. 355)