The average red giant in the night sky is about 1,000 times more luminous than the average main-sequence star. If both kinds of stars have about the same brightness, how much farther away are the red giants compared to the main-sequence stars?
A) 32 times farther
B) 1,000 times farther
C) 65 times farther
D) 5.6 times farther
E) The red giants and main sequence stars have approximately the same distances.

Question

Quizplus · Accepted Answer

The luminosity of a star is related to its brightness and distance by the inverse square law. If a red giant is 1,000 times more luminous but appears equally bright as a main-sequence star, it must be farther away. The distance is proportional to the square root of the luminosity ratio, so the red giant is √1000 ≈ 32 times farther away.

The Average Red Giant in the Night Sky Is About