Q16 - Brown dwarfs—dim red stars

[iridium77]

Can someone please help me with diagramming this question ?

[timmydoeslsat]

I would not diagram this stimulus.

There is a lot of stuff going on here, but lets get to the heart of the matter.

All stars contain a lot of L.

All stars but the coolest of the brown dwarfs are hot enough to destroy L by converting it to H.

Therefore, if a star has ~L, it is not the coolest of brown dwarfs.

Two things should be jumping out at you.

1) We know that the coolest of brown dwarfs are not hot enough to destroy L. But just because they are not hot enough to destroy L, can't there be another way to destroy L? Perhaps there is some other ability other than heat that can accomplish this.

2) OK, the coolest of the brown dwarfs are not hot enough to destroy L, but did they have the ability to do so in the past?

Answer choice A gets at this. Negate A. Some of the coolest brown dwarfs have been hot enough to destroy L.

This would ruin the conclusion.

[ManhattanPrepLSAT1]

Great explanation Timmy!

There are two tracks to think about when working on Necessary Assumptions; those that ask you to link together a chain of reasoning (diagram), and those that ask you to defend the argument from something devastating (the approach Timmy took). The one is of the defending form. Timmy's explanation of the argument is right on, so I'll just walk through the incorrect answers.

Incorrect Answers

(B) is too broad and would apply to stars other than brown dwarfs when the argument is only about brown dwarfs.
(C) is irrelevant. We do not need to guarantee that the brown dwarfs also need to have helium, just that they have lithium.
(D) is irrelevant. While it is necessary that brown dwarfs do contain lithium, it is not necessary that they all have roughly the same percentage of lithium.
(E) is out of scope. Appearance plays no role in this argument.

[mxl392]

What is discomforting to me is that answer choice A directly restates something that is already in the question. It's like saying:

Oranges are tasty. They also have orange-colored skin.

What is the assumption:

- Oranges contain sugar.
- Oranges have orange skin.
- etc.

I chose answer D because I thought the answer choice would be something like: "The argument assumes that all starts contain Lithium."

A star could form without lithium, yet be hotter than the coolest brown dwarf, right?

[austindyoung]

I chose answer D because I thought the answer choice would be something like: "The argument assumes that all starts contain Lithium."

So, that assumption you put would simply be a premise booster. We already know that when stars are first formed they contain lithium. Furthermore, it doesn't matter, in regards to the core of this argument, how much lithium they have.

We have to take the premises for granted. Even the conclusion for that matter. Our job is to evaluate their connection. Easier said than done.

[johnscottwilsonsr]

I keep having an objection to A being the correct answer. Please help correct my understanding.

1) There is a difference between being able to destroy Lithium and accomplishing the feat of having destroyed ALL the lithium.

2) The conclusion requires the assumption that ALL the lithium is destroyed.

For A to be the correct answer, is it not required to read "None of the coolest brown dwarfs has ever been hot enough to destroy lithium COMPLETELY"?

Could not the coolest brown dwarfs have been hot enough to destroy some of the Lithium for a time, but then cooled down to where they could not destroy all?

That is the sole reason I rejected A.

I would not diagram this stimulus.

There is a lot of stuff going on here, but lets get to the heart of the matter.

All stars contain a lot of L.

All stars but the coolest of the brown dwarfs are hot enough to destroy L by converting it to H.

Therefore, if a star has ~L, it is not the coolest of brown dwarfs.

Two things should be jumping out at you.

1) We know that the coolest of brown dwarfs are not hot enough to destroy L. But just because they are not hot enough to destroy L, can't there be another way to destroy L? Perhaps there is some other ability other than heat that can accomplish this.

2) OK, the coolest of the brown dwarfs are not hot enough to destroy L, but did they have the ability to do so in the past?

Answer choice A gets at this. Negate A. Some of the coolest brown dwarfs have been hot enough to destroy L.

This would ruin the conclusion.

[rtrombley]

This question has left me somewhat flummoxed, precisely for the reason that John mentioned.

I would appreciate it if an instructor addressed his post.

I think it is clear that BCD and E are incorrect.

I understand how temporal differences are a factor that should be taken into account in the correct answer choice, and that A addresses this consideration.

However, if we apply the negation test to answer choice A and rewrite it as the following, "Some of the coolest brown dwarfs have (at times) been hot enough to destroy lithium", this would not completely destroy the argument, as it leaves open the possibility that lithium could remain in the brown dwarfs.

I keep having an objection to A being the correct answer. Please help correct my understanding.

1) There is a difference between being able to destroy Lithium and accomplishing the feat of having destroyed ALL the lithium.

2) The conclusion requires the assumption that ALL the lithium is destroyed.

For A to be the correct answer, is it not required to read "None of the coolest brown dwarfs has ever been hot enough to destroy lithium COMPLETELY"?

Could not the coolest brown dwarfs have been hot enough to destroy some of the Lithium for a time, but then cooled down to where they could not destroy all?

That is the sole reason I rejected A.

I would not diagram this stimulus.

There is a lot of stuff going on here, but lets get to the heart of the matter.

All stars contain a lot of L.

All stars but the coolest of the brown dwarfs are hot enough to destroy L by converting it to H.

Therefore, if a star has ~L, it is not the coolest of brown dwarfs.

Two things should be jumping out at you.

1) We know that the coolest of brown dwarfs are not hot enough to destroy L. But just because they are not hot enough to destroy L, can't there be another way to destroy L? Perhaps there is some other ability other than heat that can accomplish this.

2) OK, the coolest of the brown dwarfs are not hot enough to destroy L, but did they have the ability to do so in the past?

Answer choice A gets at this. Negate A. Some of the coolest brown dwarfs have been hot enough to destroy L.

This would ruin the conclusion.

[rtrombley]

After discussing this with my instructor (Emily), I feel confident in being able to address John's question.

First off, just a re-cap of the conditional reasoning in the stimulus.

CBD ---> ~HDLC
__________

~L ---> ~CBD

This conclusion follows perfectly if we assume ~HDLC ---> L.

_ _ _ _ _ __ _ _ _ _ _ _

CBD ---> ~HDLC ---> L

The contrapositive gives us

~ L ---> HDLC ----> ~CBD

_______

The key to this question involves correctly interpreting the answer choices in a way that parallels the conditional relationships in the stimulus.

If we interpret answer choice A, "None of the coolest brown dwarfs has ever been hot enough to destroy lithium", according to common sense, it can be properly inferred that, if a CBD has never been hot enough to destroy any lithium at all, then it has never been hot enough to destroy lithium completely.

If we interpret answer choice "A" as implying ~HDLC, it works perfectly within the context of the stimulus.

_______

However, if we apply the negation test to the answer choice, and rewrite the statement as "Some of the CBD have been hot enough to destroy lithium", the question is, would this necessarily wreck the argument?

Not necessarily. There is somewhat of a jump from the idea of being able to destroy something, and being able to destroy it completely. For example, a hand sanitizer could destroy 99.9% of a type of bacteria, regardless, the .1% of remaining bacteria precludes it from completely[/i] destroying the bacteria to the extent that a substance contains none[i] of that bacteria.

The same can hold true for the stimulus, the CBD could destroy lithium, while still upholding the condition of ~HDLC (failing to destroy all of the lithium present in the star).


In essence, for these questions, while the language of certain answer choices may not perfectly mirror the conditional relationships in the stimulus, making (reasonable) inferences that allow the correct answer choice to function within the context of the stimulus is essential

[ohthatpatrick]

Have you ever heard the expression, "Don't let the perfect be the enemy of the good?"

It means that sometimes you should take something that's good, even though it's not perfect.

(A) is better than all the other choices.

Everyone is correct to say that the negation of (A) does not REFUTE the argument.

But that's not really the standard to which we hold the negation test.

You want to think about the negation test as,
"the correct answer, if negated, would SERIOUSLY cripple/weaken the argument."

For example, consider this argument:

All boys like sports.
Thus, Andy likes sports.

What are we assuming?
"Andy is a boy".

If I negate that and say that "Andy is NOT a boy", have I refuted the conclusion? i.e., have I proven that Andy does NOT like sports? No, maybe she does like sports.

But ... what the negation HAS done is severely weakened the argument by showing that the premise is not applicable to Andy.

Consider this example:
Bob is crying.
Thus, he must be cutting onions.

What does the author need to assume?

Here's one possible correct answer:
(A) Bob did not just break up with a girl he loves very much.

If we negate this and get "Bob DID just break up with a girl he loves very much", we have severely weakened the argument by supplying a very plausible alternative explanation for why Bob is crying.

But have we proven that Bob is NOT cutting onions? Of course not.

So don't think of the Negation Test as "the correct answer, if negated, will DISPROVE the conclusion". Sometimes it will, but all it really needs to do is strongly weaken the argument.

Hope this helps.

[WaltGrace1983]

I'd like to chime in with a little bit of a more simple approach in case any of you guys still need help. What makes this question hard is that the background information is super tempting to incorporate into the argument. However, the background information (the first sentence) is not really that important here.

All stars contain a lot of lithium
+
All stars, except the coolest brown dwarfs, destroy Lithium by converting it to Helium (S & ~CBD → ~L)
→
If a star has ~Lithium, it is not amongst the coolest of the brown dwarfs

The assumptions are that (1), converting Lithium to Helium is the ONLY WAY to get rid of Lithium. If it isn't, then maybe the coolest drown dwarfs got rid of Lithium in a different way than simply burning it. And (2) - one that I didn't see - is that maybe the coolest brown dwarfs have at one point been able to get rid of lithium. Maybe the coolest brown dwarfs used to be super hot but then they eventually became quite cool.

(B) is super close. However, we only know something about these one particular types of stars, brown dwarfs. Now we know that brown dwarfs are too cool to burn hydrogen but that doesn't mean they are the only stars that are too cool to burn hydrogen. Thus, we absolutely have to assume that "at least one star is both too cool to burn hydrogen and too cool to destroy lithium completely." That "star" would be amongst the coolest of the brown dwarfs. However, we definitely cannot assume anything about "most." "Most" is a very common wrong answer choice on NA questions.

(C) We don't need to assume anything about helium. All we know about helium is that the stars that are NOT the coolest brown dwarfs convert lithium to helium. That's it.

(D) We get "most" again, which is problematic. Then it says that most stars contain about the same % of lithium. I would see no reason why we have to assume that.

(E) Appearance is talked about in the background information as simply a description. We don't need to assume anything about it.

(A) is the only one left. If we negate it, we get "At least one of the coolest brown dwarfs has been hot enough to destroy lithium." If this is true, then we can assume that there is at least one brown dwarf that has no lithium. This would absolutely not make the conclusion follow from the premises.

This question is an absolute bear. However, I think this is one of those times when it is more important to eliminate than to fully see the argument for its intricacies.

[SJK493]

I understand the problem conceptually, but would like to try diagramming the conditional reasoning behind it.
Premise: All stars but the coolest of the brown dwarfs are hot enough to destroy lithium completely by converting it to helium.
Not Coolest Brown Dwarf → Destroy Lithium
Conclusion: Accordingly, any star found that contains no lithium is not one of these coolest brown dwarfs.
No Lithium → Not Coolest Brown Dwarf

Would this be acceptable? How would the assumption be drawn here?