#16 - Brown dwarfs--dim red stars that are too cool to burn

[Administrator]

Complete Question Explanation

Assumption. The correct answer choice is (A)

The stimulus begins by contrasting similar stars-- brown dwarfs and red dwarfs. Brown dwarfs are too cool, whereas red dwarfs are just hot enough, to burn oxygen. That particular contrast is actually irrelevant to the argumentation, which takes place after that first sentence. Since all stars upon formation contain lithium, and all stars except the coolest brown dwarfs can destroy lithium completely, any star that lacks helium is not one of the coolest brown dwarfs.

The argument doesn't make sense, because stars do not necessarily stay the same over their histories. For instance, there is no reason given to suppose that one of the hottest brown dwarfs couldn't destroy its lithium and then cool down later, thus becoming one of the coolest brown dwarfs, while lacking lithium.

You are asked on which assumption the argument depends, so you should focus on that unwarranted assumption.

Answer choice (A): This is the correct answer choice. The argument must assume that those coolest brown dwarfs have not, in the past, been hot enough to destroy their lithium.

Answer choice (B): Importantly, this incorrect choice treats the first sentence as if it plays a part in the argument. Since the first sentence merely supplies extraneous information, it is not connected to the main line of reasoning, and you should not believe that any connection is necessary. Also, since the stimulus makes conclusions about certainty, assumptions about majorities may not be helpful, and in any case this choice supports a conclusion about most brown dwarfs, but the stimulus only made a conclusion about the coolest brown dwarfs.

Answer choice (C): Since the stimulus makes no conclusion about the destruction of helium, this choice is irrelevant and incorrect. You should not have assumed that since helium is the result of the destruction of lithium, the stimulus claims that the helium persists. A result can in turn be destroyed, and the actual fact that helium requires a pretty high incineration temperature is irrelevant.

Answer choice (D): The stimulus states that stars are born containing substantial amounts of lithium, and it is in no way necessary to assume that they contain roughly equal percentages. You should not assume that detection capability is relevant to the argument, and this choice is wrong. You need to differentiate between the structure of an argument and the building blocks and results of the argument. When criticizing an argument or finding assumptions, you need to be interested in whether the premises presented a complete picture, and whether the premises definitely lead to the conclusion. You are not actually interested in whether the premises and conclusion are correct.

Answer choice (E): Since the comparison between brown and red dwarfs was utterly irrelevant to the main line of reasoning, this incorrect choice cannot represent a necessary assumption.

[Sophia123]

Hi!

I understand why the first sentence of the stimulus is irrelevant to the argument. However, when I was working through this problem I thought about this more on a spectrum:

Cool Brown Dwarfs - Normal Brown Dwarfs - Red Dwarfs

If answer choice (c) was rewritten to say " brown dwarfs that are not hot enough to destroy lithium are not hot enough to destroy helium" would this statement be accurate since it would basically be referring to the coolest brown dwarfs? I don't think it would still be the correct answer since its not an assumption that the argument depends on, but is it a correct statement at least?

Thank you in advance!

[Ricky_Hutchens]

Hi Sophia,

I agree that it still wouldn't be the answer, but I can't say if that would be accurate or not because the stimulus never says anything about stars being hot enough to destroy helium. It also never tells me anything about which stars actually contain helium. It is possible that all stars can destroy helium once lithium has been converted, or it is possible that no star can destroy helium.

[taylorballou]

Hi,

I narrowed down my contenders between answer choices A and B. When I used the assumption negation technique, answer choice A became "At least one of the coolest brown dwarfs has ever been hot enough to destroy helium," which I thought was the correct answer choice initially. When I compared it to the passage, however, I second guessed myself because the stimulus says that all but the coolest of the brown dwarfs can destroy lithium completely, which I didn't think answer choice A addressed, since being able to destroy lithium doesn't mean the planet is able to destroy all of it.

I'm not sure if the error I made here was putting too much emphasis on the "completely" in the stimulus or underestimating the certainty of the answer choice.

Thanks,

Taylor

[Kristina Moen]

Hi Taylor,

Welcome to the Forum! Great job on negating answer choice (A). Remember, if an answer choice kills the conclusion, that means it's the correct answer. The conclusion here is "Accordingly, any star found that contains no lithium is not one of these coolest brown dwarfs." And if even ONE cool brown dwarf star has been hot enough to destroy lithium, then that means that it's possible to find a cool brown dwarf star without lithium. If it seems like the negated version of an answer choice directly contradicts a premise in the stimulus, check again. Perhaps you made an assumption or logical jump as you were reading (after all, we want arguments to make sense). You will never see an answer choice that is just a restatement of a premise on an Assumption question. The answer choices are all new pieces of information. For example, if the stimulus had the premise "All the cars are red," an answer choice on an Assumption question would not say "Every car is red" or "No cars here are not red." An assumption is an unstated premise.

Here, the logical jump or assumption that is very easy to make when reading this stimulus (and that I myself didn't catch until reading the first answer choice!) is that "All stars but the coolest of the brown dwarfs are hot enough to destroy lithium completely by converting it to helium" is a statement about the stars' entire lives. Pay attention to the verb "are." Consider this similar argument: "All people but the tallest can fit on this ride. If you find someone who has ever been on the ride, then it's not one of the tallest people." But wait! What about before the tall person was so tall?

Answer choice (A) addresses that assumption by introducing the concept that stars don't necessarily have the same temperature for their whole lives. In order for the conclusion to work, we have to assume that the coolest dwarf brown stars were never hot enough to destroy lithium.

So the word "completely" still means what you think it means. Currently, that cool brown dwarf star cannot destroy all of the lithium. But it might have already destroyed all the lithium in its younger, hotter days.

[lathlee]

hi the link says this is assumption fl, what does it mean FL? Flaw? https://www.powerscore.com/lsat/help/lr_15-hardest.cfm

[Claire Horan]

Hi lathlee,

"FL" is an acronym for Formal Logic. In this question, it refers to the use of Sufficient and Necessary relationships in the stimulus.

Cheers!

[LSAT2018]

Is the formal logic referring to the conditional reasoning in the final statement? Then would it be correct that answer (A) is a Defender Assumption, in that it defends the necessary condition, that it remains the same over time and cannot destroy lithium?

No Lithium → Not Coolest Brown Dwarfs

[Francis O'Rourke]

The formal logic present in this stimulus can be found in the final two sentences:

All stars but the coolest of the brown dwarfs are....
NOT coolest brown dwarf hot enough to destroy lithium

Accordingly any star found that contains no lithium is....
Star lacking lithium NOT coolest brown dwarf

Both of these statements are phrased in a rather torturous manner. It would have been easier if the stimulus had given us fewer negatives, such as "but the coolest" and "no lithium," to deal with, but that is sometimes the challenge of harder LR questions!

The speaker of this argument uses the fact that all stars initially contain lithium and the fact that stars other than brown dwarfs can destroy lithium to say that if you find a star without lithium, then it must be one that is hot enough to destroy lithium and thus not one of the coolest brown dwarfs. What is interesting is that the stimulus never tells us that brown dwarfs have always been unable to burn lithium. It may be the case that some brown dwarfs cooled down after destroying all the lithium they once contained.

Answer choice (A) defends the idea that the coolest brown stars never burn lithium. This was not present in the stimulus however, so I would not say that it is defending a necessary condition. It is defending against a reasonable objection to the conclusion however.

Let me know if this helps you, or if you have any other questions!

[LSAT2018]

The formal logic present in this stimulus can be found in the final two sentences:

All stars but the coolest of the brown dwarfs are....
NOT coolest brown dwarf hot enough to destroy lithium

Accordingly any star found that contains no lithium is....
Star lacking lithium NOT coolest brown dwarf

I understand that the assumption must contain information on whether or not the stars are hot enough to destroy lithium completely by converting it to helium. But given the conditional reasoning you have provided above, it becomes:
No Lithium → Not Coolest Brown Dwarf → Hot Enough to Destroy
Not Hot Enough to Destroy → Coolest Brown Dwarf → Lithium

But this doesn't seem to match up with the answer (A) none of the coolest brown dwarfs has ever been hot enough to destroy lithium. It seems like a Mistaken Reversal. Am I missing something here?