You discover a binary star system in which one star is a 15 MSun main-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist?
A) The two stars probably were once separate, but became a binary when a close encounter allowed their mutual gravity to pull them together.
B) Although both stars probably formed from the same clump of gas, the more massive one must have had its birth slowed so that it became a main sequence stars millions of years later than its less massive companion.
C) The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.
D) The two stars are simply evolving normally and independently, and one has become a giant before the other.
C
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When the first law of thermodynamics is applied to an ideal gas that is taken through an isothermal process
A. 
B. W = 0.
C. Q = 0.
D. none of the above.
What characteristic of a star primarily determines its location on the main sequence?
a. age b. distance from the galactic center c. mass d. space motion e. radius
Which of the following statements is true of open clusters?
A) They contain 105 to 106 stars in a region about 10 pc in diameter. B) They have a transparent appearance. C) They comprise stars that are crowded together. D) They are spherical.
A charged particle with mass m is suspended in the air by an upward-directed electric field E. What is the charge on the dust particle?
a. q = mgE b. q = mg/E c. q = E/mg d. q = Em/g