LR Question 16 Explanation
QUESTION TEXT: Brown dwarfs–dim red stars that are too cool to burn…
QUESTION TYPE: Necessary Assumption
CONCLUSION: Any star without lithium isn’t a cool brown dwarf. [or: cool brown dwarfs have lithium.]
REASONING: Cool brown dwarfs aren’t hot enough to burn lithium. All stars have lithium when first formed.
ANALYSIS: The argument implies that some warmer brown dwarfs can burn lithium.
It’s possible that cool brown dwarfs were once warmer; warm enough to have burned all of their lithium.
By the time became cool brown dwarfs, they could have no lithium left.
The argument assumes that cool brown dwarfs were never warm enough to burn lithium.
___________
- CORRECT. If this isn’t true then cool brown dwarfs were once warmer and could have burned through their lithium.
- “Most” almost never is the right answer on necessary assumptions. This phrase could mean 51%, while the negation would be 50%. That can’t hurt the argument, which is making an absolute statement about all brown dwarf stars.
The answer choice is also irrelevant. It tells us that red dwarf stars are hot enough to burn hydrogen, but that doesn’t fit in with any of the information about lithium. - We don’t know whether helium has anything to do with lithium. The part of the argument about helium is just there to mislead you.
- The argument should still be fine if stars contain differing percentages of lithium. The argument is assuming that cool brown dwarfs were never warm enough to burn it.
- Appearance is irrelevant. The first sentence was just fluff. We care about stars’ heat.
Recap: The question begins with “Brown dwarfs–dim red stars that are too cool to burn”. It is a Necessary Assumption question. Learn more about LSAT Necessary questions in our guide to LSAT Logical Reasoning question types.
More Resources for Necessary Assumption Questions
- Negations Article: Learn about negations on the LSAT.
- Conditional Reasoning Article: Learn about conditional statements.
- Negations Drill: Practice your negation skills.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Intro to Conditional Reasoning: Learn conditional reasoning basics.
- Intro Course lesson: This intro course lesson covers Necessary Assumption questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers necessary assumption questions.
Diving into the weeds here but…
I’m wondering if B would be right without the use of “most”. I’m thinking no right?
Even if B applied to all stars too cold to burn Hydrogen, we’d still need to assume there is no route to a brown star. For example, I didn’t know much about stars, but I know our Sun will eventually change into another star type. If I thought a star could become a brown star than B could still be wrong if in one of a star’s previous forms (before becoming too cold to burn hydrogen) at one point ran hot enough to burn all its lithium. For example if red dwarfs became brown dwarfs, then maybe lithium would be depleted while still a red dwarf.
Having Googled this afterwards, nothing really becomes a brown dwarf through transformation. Only way would be if you had your mass siphoned off overtime. Still, to make B complete we’d need another NA like ‘no star hot enough to burn its lithium completely eventually becomes too cool to burn hydrogen’.
I know A is 100% right so I’m overthinking it, but while ‘most’ offers an easy elim of B, B still doesn’t stand muster even excusing the use of the word ‘most’ right? Or would it be considered a genuine NA, just not enough of one to fully plug the hole in the question stem logic?
You’re right that even if we fixed the wording of B (removed “most” and made it universal), it still wouldn’t be a necessary assumption.
Even if B were rewritten as “All stars too cool to burn hydrogen are too cool to destroy lithium completely”, that still wouldn’t matter to the argument. It only guarantees the star currently can’t destroy lithium. But the conclusion (“any star without lithium isn’t one of the coolest brown dwarfs”) requires ruling out the possibility that:
– A cool brown dwarf used to be hotter
– Burned all its lithium back then
– And only later cooled to its current “too cool to destroy lithium state”
Even rewritten as a universal statement, B doesn’t preclude that possibility. Only A does that.
Not sure what you mean by being a “genuine NA” but not enough to plug the hole in the argument. A Necessary Assumption is one where if you negate it, the argument falls apart. So the truth and existence of the assumption is necessary for the argument to function.