An astronomer observes two stars. Star X is 100 light-years away and appears very bright. Star Y is 10 light-years away but appears much dimmer. What is the most likely explanation for this observation?

A star’s apparent brightness depends both on its intrinsic luminosity and its distance. Since Star X appears brighter even though it is ten times farther away, it must give off much more light than Star Y to overcome the effect of distance. Thus, the most likely explanation is that Star X is much larger and more luminous than Star Y.

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