Measurements indicate that a certain star has a very high luminosity (100,000 times as bright as the Sun) and yet is relatively cool (3500 K). How can this be?
A) The star must be quite small.
B) The star must be very large.
C) There must be an error in observation because no star can have these properties.
D) The star must be in the upper part of the main sequence.

Question

Quizplus · Accepted Answer

The star's high luminosity despite its relatively cool temperature suggests that it has a large surface area from which it is emitting light. According to the Stefan-Boltzmann law, a star's luminosity is directly proportional to its surface area and the fourth power of its surface temperature. Therefore, for a star to be very luminous while having a cool surface temperature, it must have a very large surface area, indicating that the star is very large in size. This description fits with characteristics of red giants or supergiants, which are indeed very large and luminous stars with cooler surface temperatures compared to smaller, main-sequence stars.

Measurements Indicate That a Certain Star Has a Very High