If an answer is numerical you must show the equation for credit.

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Question 1: About what fraction of stars have planets?
D) 90%

Question 2: Which star is the coolest?
B) Red


Question 3: What is the temperature of a star with a peak in its spectrum at 250 nm?
2.9x10⁶/250 = 11,600K

A cloud collapses to form 4 stars. Star A is 12 solar masses, Star B is 3 solar masses, Star C is 0.8 solar masses and star D is 0.4 solar masses. These stars all begin the main sequence at the same time and would be roughly the same distance from us.

Question 4: Which star is the hottest?
Star A

Question 5: Which star is the faintest?
Star D

Question 6: What is the total luminosity of these 4 stars (in solar units) on the main sequence?
This is L_MS=M^3.5 for each star and then added together.
12^3.5=5986, 3^3.5=47, 0.8^3.5=0.46, .04^3.5=0.04
Totals to 6034L_Sun

Question 7: After 3 billion years, how many stars are still on the main sequence?
This is t_MS=1x10¹⁰/M^2.5 and if we work clever, we can work less. Massive stars evolve very fast, so begin with Star B.
t_MS=1x10¹⁰/3^2.5=6.4x10⁸=640 Million years, so already less than 3 billion. This means Star A would also be gone. Since Star C is less massive than our Sun and we know our Sun will last 10 billion years on the MS, we really are done-Stars C and D will be around after 3 billion years. But just to complete, let's check Star C
t_MS=1x10¹⁰/0.8^2.5=1.75x10¹⁰=17.5 Billion years, so it will be around.
So the answer is 2 stars will be left.

Question 8: After 15 billion years, how many stars are still on the main sequence?
We already know star C will be around, so Star D will be too. The answer is still 2 stars.

Question 9: Why must stars evolve?
They are using energy to be luminous and that's a limited supply, so they must evolve.

Question 10: What is the source of energy for our Sun?
Fusion of H into He

Question 11: I see a red star and a blue star in a binary. The red star is brighter, what else do I know about these two stars?

A few things:
The blue star is hotter.
They are at the same distance
They are the same age (began at the same time) and formed from the same gas cloud.
The blue star is smaller than the red star

Question 11: How will our Sun end?
C) As a white dwarf.