AST 104 : Midterm 3

Wednesday March 28, 2001

Answer all questions.

This is a closed book and closed notes exam. All you are allowed is your pen and calculator.

This exam has 2 questions, each consisting of several parts. Each question is worth a total of 20 points.

The first part of each question is worth 10 points, and requires you to write a one or two paragraph prose answer describing a particular concept or process from the course. It is very important that you approach this carefully. You should write an answer that logically connects the necessary concepts, and which would provide a good explanation that you would understand if it were presented to you. Include only the information that you think is necessary and that you can logically connect to the other concepts in your answer. Any information that you write down that is not connected to the chain of your argument may lead to points being deducted.

The second part of each question involves one short calculation, worth 5 points. Once again, it is extremely important that you explain your reasoning logically. If you just write down a numerical answer you will not receive many points, regardless of whether it is correct. If you carefully explain how you got a particular numerical answer then even if the answer is incorrect, you may receive quite a lot of partial credit.

Finally, the third part of each question requires you to write a very brief conclusion based on your calculation, and is also worth 5 points. Again, be careful, clear, precise and logical. This will save you time, and will help you to get the most points you can.

This exam should take a very short time to answer, but I suggest you take the extra time to make your answers as high quality as possible. It is these answers that will earn the highest scores.

Things You Might Need

.

Mass of an iron (Fe) nucleus = 92.73 x 10^-27 kg

Mass of a Syracusium (SU) nucleus = 185.7 x 10^-27 kg

Speed of light: c=3 x 10⁸ m/s

s=5.6705 x 10^-8 Watts/m²/degree⁴

Question 1 (20 points)

a) (10 points) Give a brief, one or two paragraph description of what happens when a solar-mass star leaves the main sequence.

b) (5 points) The star Rigel has a surface temperature twice that of the Sun, and a luminosity 350,000 times greater than that of the Sun. How does the radius of Rigel compare numerically to that of the Sun?

c) (5 points) Make a sketch of where Rigel sits on the main sequence of the H-R diagram, compared to where the sun sits. On the same diagram indicate where red giant and white dwarf stars lie.

Question 2 (20 points)

a) (10 points) Give a brief, one or two paragraph explanation of why high mass stars typically live a shorter time on the main sequence than lower mass stars.

b) (5 points) In the core of a very massive star, iron eventually forms. A friend claims that the next thing that happens is that iron nuclei fuse to make a new element called Syracusium (SU).

Fe + Fe ® SU + energy

Perform a short calculation to show that this cannot happen.

c) (5 points) Because this reaction cannot occur, what happens next in the life of such a massive star, and why?