Homework Assignment #5
Main topics: Galaxies
Solutions are available in PDF format.
This assignment is due at the beginning of class on Friday, April 18. Late homework will not be accepted.
Homework should be written on paper. Please copy the problem statement onto the page above your solution. Remember to put your name at the top of the page. If you require more than one page, multiple pages must be stapled together. I will take off three points if you do not staple the homework. The homework will be returned once it has been graded in the boxes on the ninth floor of the Physics building, opposite the elevator.
Problems involve either a calculation, or a short answer. Where a calculation is called for, show your work. Where a short answer is called for, a few sentences is sufficient. In either case, be sure to make your reasoning clear, and draw diagrams if that helps you explain your answer.
Consider the following two facts: (1) Nearby elliptical galaxies have formed stars over a range of times (i.e. the stars weren't all formed right once, or right at the beginning of the Universe). However, they haven't formed any stars in the last 1-2 billion years. (2) The speed of light is not infinite. For instance, we are eight light minutes from the Sun, so when we look at light from the Sun, we're seeing it as it was eight minutes ago. Given these two facts, what might you expect to observe about the colors of elliptical galaxies observed billions of light-years away compared to the colors of nearby elliptical galaxies?
Chapter 17, Question 15. Also answer: if the Galaxy is 14 billion years old, how many times has the Sun been around the Galaxy?
Chapter 17, Question 16. We haven't talked about Quasars in class, but they are fascinating galaxies which are extremely bright right at the center. Like our galaxy, Quasars have supermassive black holes at their center. (Quasar black holes are probably a billion solar masses, whereas our Galaxy's black hole is only 2.6 million solar masses.) The other key difference is that there is a source of food for Quasar black holes: material is being dumped into the black hole. As you know, when things get closer together potential energy is released; some of that potential energy released as gas gets very close to the event horizon of the black hole comes out as radiation. This problem gives you a sense of the rate at which one must feed a black hole to power a quasar, by comparing the energy released to the amount of energy in the mass of a star. (Think back to when we talked about converting matter to energy many weeks ago!)
Chapter 18, Question 6.
Chapter 19, Question 6.