Exam 3

• Exam 3 covers Lectures 18-25.
• 50 multiple choice questions.
• You need a No. 2 pencil and an eraser.
• No calculators (or cell phones, ipods, or laptops) are allowed.
• Enter your name and your ID number.
• Turn in the answers and leave whenever you are done.
• "Re-take" option
• strictly optional
• likely to help but it can hurt your grade
• new grade = (75% original grade) + (25% "re-take" grade)
• take a new answer sheet and return it to my office (LGRT 522) by 1pm next day
• no make-up exam (a reminder)

Exam 3 Preparation

The following is a list of things to review in preparation for the exam. It is roughly in the order of importance and relevance, so start from the top and work your way down as time permits. Look at items 8-10 if you have extra time, but don't start your study by reading the whole textbook!

1. Lecture outlines on web
2. In-class quizes
3. Sample exam questions are in OWL
4. OWL prelecture quizes and HWs 5 & 6
5. extra practice quizes
6. "Test Yourself" questions at the end of each Units in the textbook
7. Quantitative concepts you should know:
• Doppler effect and redshift: how are redshifts calculated from measurements? What is the relationship between cosmological redshift and the size of the universe?
• Hubble law and the age of the universe: how do you calculate distance from recession velocity? how is the age of the universe related to the value of the Hubble constant?
• Weighing galaxies: how do you calculate the mass of a galaxy?
• Physical characteristics of the Milky Way Galaxy (e.g. size, mass, age)
• Physical characteristics of the universe (e.g. size, mass, age)
• Keg's Law (for practicing ratios)
8. "Review Problems" at the end of each chapter
9. Know what the bold-faced terms mean in the textbook
10. No need to memorize physical constants

Q & A session: May 12 (Monday) at 5:00pm in HAS 134

Answers to inclass quizes 18-25

• quiz 18: (18E) anwer=3 (from the halo, the whole galaxy would be only on one side of the sky; (18A) answer=3; (18B) answer=4 (comets are the only family of solar system objects that do not belong to the disk-like structure); (18C) answer=2 (the rotation speed of stars and gas in MW remains flat beyond the stellar disk. For this to be true, a lot of unseen mass must lie beyond the stellar disk.)
• quiz 19: (19A) answer=3; (18B) answer=3 (Hubble Law states that V = HD. V is 1400 km/s and H is 70. So, the distance D is V/H or 1400/70 = 20 Mpc.); (18C) answer=2.
• quiz 20: (20A) answer=2 (Finite speed of light means you are looking further back in time when you observe distant galaxies using a telescope.); (20B) answer=3; (20C) answer=3 (if a galaxy turns on bright only for 1/1000th of time, the you have to look at 1000 galaxies at any given time to see one lit up.)
• quiz 21: (21A) answer=4 (In Lecture 13 (and elsewhere, also see Lecture 22), I showed an example of how to calculate mass from orbital speed and size. By doing the same calculation, you will see that the correct answer is about 3 billion solar masses.); (21B) answer=4
• quiz 22: (22A) answer=1; (22B) answer=4
• quiz 23: (23A) answer=5; (23B) answer=2; (23C) answer=3 (A new definition of redshift z is that (size of universe today)/(size of universe at redshift z)=z+1. So, if z=0.5, then 1+z=1.5, or the size of the universe now is 1.5 times larger.); (23D) answer=1 (The Earth and the solar system are both held together by gravity and thus are unaffected by the expansion of space); (23E) answer=1 (light travels 15 billion light years in 15 billion years, so your cosmic horizon is 15 billion light years away in a static universe.)
• quiz 24: (24A) answer=2 (this is ideal gas law); (24B) answer=2 (The initial flash of the BB itself cannot be seen because the universe was opaque to light until it was about 400,000 yrs old. The microwave background radiation we measure is the snapshot of that moment, some 400,000 yrs after the BB; (24C) answer=4 (neutron has no charge, so neutron-proton collision does not have to fight the repulsive force of electric charges.)
• quiz 25: (25A) answer=3 (10e10 x 10e18 = 10e28) ; (25B) answer=3 (10e28/10e30=10e-2=0.02); (25C) answer=4 (the fate of the universe depends only on how much mass the universe has, so the right answer is 4)

Exam 3 Answers. Check OWL for your grade.