9. Proxima Centauri, the nearest star to the sun, is a Main Sequence star that is cooler than the sun. Where on the H-R diagram would it be found?
a. above and to the right of the sun
b. above and to the left of the sun
c. below and to the right of the sun
d. below and to the left of the sun

We asked this one to make sure you remember the H-R diagram. The key to this question is to remember that the temperature scale runs "backwards"- i.e. hotter stars are on the left, cooler on the right. Also, remember that the Main Sequence runs from the top left to bottom right.

For questions 10, 11, and 12: Alpha Centauri is a binary, consisting of two Main Sequence stars, Alpha Centauri A and Alpha Centauri B. Alpha Centauri A is a one-solar-mass star, and Alpha Centauri B is a one-half-solar-mass star. They are separated by about 10 AU, the distance between the sun and Saturn.

10. Alpha Centauri A is yellow. What color is Alpha Centauri B?
a. orange
b. yellow
c. blue
d. a one-half-solar-mass star is too cool to fuse hydrogen, so it doesn't emit any visible light

A less massive Main Sequence star will be cooler than a more massive one. If it's cooler, its surface color moves toward the red end of the spectrum.

11. Alpha Centauri A takes 30 years to orbit the binary's center of mass. How long does Alpha Centauri B take to orbit the center of mass?
a. 15 years
b. 30 years
c. 60 years
d. 900 years

If you took the quiz that covered binary stars, you should remember this. The center of mass of a binary must always be on the line connecting the two stars, and the two stars are always the same distance from the center of mass. Think of it as two stars on the ends of a stick, being twirled like a baton. The two ends of the stick both take the same amount of time to go around.

12. Which star will leave the Main Sequence first?
a. Alpha Centauri A, because it is more massive
b. Alpha Centauri B, because it is less massive
c. they will both leave at the same time, because they were born together
d. there is no way to tell, because the stars are touching each other and exchanging mass

The more massive the star, the shorter its life. You know the stars are not touching each other, because a one-solar-mass Main Sequence star is much smaller than 10 AU in diameter (fortunately for us on Earth). In fact, a red giant with one solar mass can only get to the size of the orbit of Mars (1.5 AU), so the two stars will never be touching each other.