LSAT 158 | Section 2 | Logical Reasoning: Q19

LR Question 19 Explanation

QUESTION TEXT: The differences in distance from Earth between the stars …

QUESTION TYPE: Necessary Assumption

CONCLUSION: We can determine the relationship between a star’s brightness and other characteristics by studying other stars in the same galaxy.

REASONING: Stars in the same galaxy are roughly the same distance from Earth, so differences in how bright they appear must reflect real differences in brightness.

ANALYSIS: The argument sets up a fairly simple idea: if distance is basically the same, then apparent brightness reflects actual brightness. That part works.

But, then, it suddenly jumps to how brightness relates to other characteristics. How did we get there? It never tells us that we can observe anything about these stars besides their brightness.

We would need to assume that we can actually detect and measure other features of those stars from the Earth. Otherwise, we’d have brightness, but nothing to compare it to.

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1. The argument only deals with stars in the same distant galaxy, so this doesn’t matter.
2. This is too specific. Detecting elements isn’t required and doesn’t impact the argument.
   Negation: We can’t detect the difference in elements each star is burning from Earth.
3. Irrelevant. The entire argument is about stars in distant galaxies, not in our own galaxy.
4. CORRECT. If we can’t observe other characteristics, we can’t study their relationship to brightness.
   Negation: There are no stars that have other discernible characteristics than brightness from Earth.
5. Sure, but the argument already accounted for this by comparing stars from the same distant galaxy, where differences in distance from Earth are negligible.