Two elliptical galaxies have the same apparent size and brightness. Galaxy A appears lumpy, and it is possible to observe individual stars and star clusters. The other one, galaxy B, appears very smooth, with no fine detail. Which one is likely to be more luminous?
A) Galaxy A. It has to contain more luminous objects if we can see them individually, and so the galaxy as a whole has to be more luminous.
B) Galaxy A. The lumpiness means that there is a lot of dust in the galaxy, which absorbs some of the starlight. Therefore the total amount actually emitted must be larger, if it is to have the same brightness as the other galaxy.
C) Neither. If they have the same apparent size and brightness, they must be at equal distances.
D) Galaxy B. This galaxy appears smooth because it is farther away and the light from its stars all blends together. Because it is farther away, it has to be more luminous.
E) Galaxy B. The smooth light distribution means that the stars are moving around very quickly. Therefore the galaxy has more mass and is more luminous.

Question

Quizplus · Accepted Answer

The smooth appearance of Galaxy B indicates that its stars are blending together due to distance. This means that it is farther away, and since it has the same apparent brightness as Galaxy A, it must be more luminous. The other choices do not provide convincing arguments for their respective answers.

Two Elliptical Galaxies Have the Same Apparent Size and Brightness