At the tip of the red giant branch,a star is half as hot as it was on the main sequence and 100 times larger.How does its luminosity compare to its value on the main sequence?
A) It is now 10,000 times larger.
B) It is now 600 times larger.
C) It is now 16 times larger.
D) It is now 16 times smaller.

Question

Quizplus · Accepted Answer

The luminosity of a star is proportional to the square of its radius and the fourth power of its temperature (L ∝ R^2 * T^4). If the star is 100 times larger and half as hot, its luminosity is (100^2) * (0.5^4) = 10,000 * 0.0625 = 625 times larger, which is approximately 600 times larger.

At the Tip of the Red Giant Branch,a Star Is