#19 - The difference in distance from Earth between the stars

[Administrator]

Complete Question Explanation

Assumption. The correct answer choice is (D).

Answer choice (A):

Answer choice (B):

Answer choice (C):

Answer choice (D): This is the correct answer choice.

Answer choice (E):

This explanation is still in progress. Please post any questions below!

[letsdothis]

I guessed B. struggled to understand the stimulus here so if anyone could help?

[Adam Tyson]

The stimulus is saying that because those galaxies are so far away, any big differences in their brightness can't be because one is much further away than the other. It's like seeing two people who are far away from you but you know they are in the same yard as each other. If one of them looks a lot taller than the other it can't be because the taller one is a lot closer to you, so it must be that they are actually taller.

The author uses that evidence to support a claim that we could determine other things about those distant stars by looking at their brightness. For example, maybe we could determine what percentage of the star was made of helium? Or perhaps we could compare how fast it is spinning to its brightness, and learn something about the relationship of spin speed to brightness?

The problem with that is that we might not be able to actually know anything else about those stars! What if we have no way to tell how much helium a distant star has, or calculate how fast it is spinning? If we can't determine any other characteristics but its apparent brightness, then this argument would fall apart, because we won't be able to determine how that brightness correlates to anything else.

To avoid that problem, then, the author must have assumed that we CAN determine some other things about those stars. Looked at that way, this is what we would call a "Defender" assumption, because the answer defends the argument against a potential objection. It fixes a problem in the argument. And when you negate that answer, we are back to having that problem and the conclusion of the argument is not valid.

[Bruin96]

If the conclusion brings in some new information that is unsupported in the stimulus kind of like this question and the part about "other characteristics" will the correct answer mention the new part?

When reviewing D I noticed it was the only one that mentioned that they have other characteristics and picked it for that reason. Although I was also torn with answer choice B because of "elements". Is the wording of elements why B is wrong?

[Adam Tyson]

That's absolutely right, Bruin96! New information in the conclusion should always catch your attention, and when that is coupled with a Justify the Conclusion, Assumption, or Strengthen question the correct answer will almost always have to touch on the new idea. I say "almost always" because I am a cautious type, but it's about as close to "always" as anything on the LSAT can ever be. Answers that ignore the new idea are wrong; the right answer will connect the premises to the new thing.

And you're also right about answer B: we cannot say that the author must assume anything about the elements a star is burning because the argument did not discuss that issue! While those elements MIGHT be the sort of thing they are talking about, it doesn't have to be.

[g_lawyered]

Hi P.S.,
This was also a hard argument for me to understand because of the consistent double negatives in the premises. Can someone please better explain the stimulus and why answer B isn't correct? Here's my reasoning:

Conclusion- Last sentence. Has a rogue term "other characteristics"
Premise 2: "How Brightly each star is actually burning" (rogue term).
I made assumption between both rogue terms of: there's a similarity between how bright star burns and other characteristics in the galaxy. With this prediction, I was able to have answer choice B and D as contenders and chose B. Can someone please clarify if the assumption I made is correct or not? Also, Can someone please explain in detail what makes answer choice B incorrect? and what makes answer choice D correct?
Thanks in advance!

[Robert Carroll]

GGIBA003,

I don't see any double negatives in the premises myself, and I think Adam's explanation of the argument is about the best available. I'll try to explain it again, though.

The author is saying that distant galaxies are so far away that the distance from the galaxy to Earth is much larger than the distances between those stars. This isn't quite what's said, but I'm oversimplifying to make it more clear. The author thinks that, because of that, if two stars in such galaxies look different in brightness from each, it must be due to actual differences in how brightly they're burning - apparent differences because of distance really can't matter that much. The conclusion states that we can tell how well brightness correlates with other characteristics. But...we don't have any mention of other characteristics in the premises. If I don't know what they are nor how to detect them, I don't know how the author is going to correlate them with brightness. So we want an assumption that those other characteristics are at least somewhat known already.

Answer choice (B) is way too specific. This is an Assumption question, recall. We do want some other characteristics to be detectable, or knowable, but answer choice (B) talks specifically about the elements a star is burning. We do not need to assume something about that one specific characteristic. So this is too specific and too strong an assumption to have to make for an Assumption question.

Answer choice (D) does not have the specificity problem.

Robert Carroll

[g_lawyered]

Thanks for your explanation Robert. It helped me a lot to realize the premise and different language the stimulus uses compared to the other answer choices.

[hope0922]

Hi, can anyone break down the explanation for the correct answer choice (D) while I initially chose B I understand why it is incorrect just wondering what makes D correct

thanks

[Luke Haqq]

Hi hope0922!

You might try applying the Assumption Negation technique to the correct answer to understand why it is correct. Negated, answer choice (D) would be, 'There [no] are stars in distant galaxies that have characteristics, other than brightness, discernible from Earth."

If that were true, then the conclusion that "we should be able to determine how a star's relative actual brightness correlates with other characteristics by studying stars in the same distant galaxy" would not follow from the premises. As Robert note above, we don't have any mention of these "other characteristics" in the premises. Answer choice (D) bridges that gap.