## AST 248: Frequently Asked Questions

Actual questions sent in by actual students.

CONTENTS

"Go not to the elves for advice, for they will say both no and yes"
-- J.R.R. Tolkein

### What Do We Need To Know (and when do we need to know it)?

1. Q. Are the grades curved? What is the equivalent letter grade?

A. The grades posted on the grades page are not curved. They are the raw grades you would have if the semester were to end today. Grades will not be curved until after the semester is over. In general, you should be in the top 10-15% of the class to expect an A, and the top 40% to expect a B (this isn't Harvard: you don't get an A as a reward for hard work in high school). The 50th percentile is near the B-/C+ boundary. It is rare that a student who faithfully does the work, takes all tests and the final, and participates in discussions, will get less than a C.

Curving never reduces your raw grade: if everyone scores 90% or better, everyone gets an A.

2. Q. Are the quizzes and tests cumulative?

A. In general, yes. Quizzes will tend not to be cumulative, and will mostly be based on the material since the last quiz. Tests will also concentrate on the material since the last test, but as we build up to answering the question (What is N?), you will need to remember the early material.

3. Q. Do we have to know all the laws such as "inverse-square law," "black body law," "Wein's law," etc.? Or do we have to know that these laws are related to certain reasonings (i.e. - we should know that the reason for our believing stars are black bodies is because of Wein's law) ?

A. Yes and No. You do need to know the inverse square law. You should have a vague idea what a black body is. Wein's law is not critical. You need to know that we know stellar temperatures and other parameters not by guessing but because they obey physical law. You should know what gravity is, and that it is an inverse-square law. Review Newton's laws and Kepler's laws. But do not memorize formulae. I may ask you to explain what a law is, or why it is important, but I will not ask you to write down a formula.

4. Q. Do we have to know numbers like the solar constant or the luminosity of the sun with respect to the earth?

A. No. I will give you those kinds of numbers if you need them.

5. Q. Do we have to reason things like you do, such as
"The inner and outer radii of the habitable zones is proportional to the square root of the stellar luminosity. The luminosity is roughly proportional to the cube of the stellar mass. Therefore the habitable zone radii are proportional to the mass to the 3/2 power. "
Do we have to go through this reasoning process for the exam?

A. If you can. This is all a part of estimation. In the example above, I would not expect you to determine that the radii are proportional to mass3/2 from scratch. But if I told you that the radii are proportional to the square root of the luminosity, and the the luminosity is proportional to mass cubed, I would hope that you could come up with the mass to the 3/2 power yourself.

6. Q. Is it true that we can bring a crib sheet to the exam?

A. Yes it is. You are allowed one sheet of paper, no larger than 8 1/2 x 11 inches in size. You may write anything you want on that sheet. Or you may type anything you want on the sheet. you may even use both sides, if you think it will help. You may not use a magnifying glass, microscope, or other optical aid to read it.

7. Q. OK, so what do I really need to know?

A. Flippant answer: Everything. Knowledge is good. But concepts are much more important than facts.

8. Q. Can you give me my PIN number?

A. Yes, but you must come to office hours, and show an ID to retrieve it. The TA can also give it to you.

### I Didn't Understand...

1. Q. How long is one galactic year?

A. about 200 million years. This is the time our Sun takes to orbit the Galaxy once.

Feel free to send more general questions in...