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

[lanereuden]

This question is strange to me because to me it presumes knowledge that stars can change temp over time. Indeed, and thankfully, i got this right thru logic not thru knowledge . But nonetheless, from either perspective, it seems somewhat speculative to see where the argument suggests that there is cool down/heat up period....plz explain if possible. I got this right but I just don’t have much confidence about why for this reason

[James Finch]

Hi Lane,

There is an implication in the stimulus of potential change over time for stars, as it mentions how they contain lithium at formation and how that lithium may be burnt up later. So the change in temperature is a fair inference to make. But the most important takeaway here is the logic in the stimulus:

All stars contain lithium when formed.

StarFormation Lithium

Only the coolest brown dwarfs aren't hot enough to convert lithium to helium.

LithiumStar Cool Brown Dwarf

Conclusion: LithiumStar Cool Brown Dwarf

What we need is the contrapositive of the conclusion:

Cool Brown Dwarf Lithium

Thankfully, (A) gives us this. As always, we can check our answer choice with the Assumption Negation technique:

Some Cool Brown Dwarfs have been hot enough to destroy lithium A star with no lithium may be a Cool Brown Dwarf

Hope this helps!

[OneSeventy2019]

Powerscore,

Curious to get your take on an alternative method I used for eliminating answer choice B..

This is obviously a necessary assumption question, meaning that we will be tasked with creating a link between the premise(s) and conclusion in the argument via an additional premise from one of the five answer choices. Because the conclusion of the argument is a universal quantifier, some sufficient condition (in this case, a start that contains no lithium) always, and without exception, implies some necessary condition (in this case, not being one of the coolest brown dwarfs). Ergo, any existential relationship (some/most) would not be able to fill in the gap because the conclusion would 'carry' the most with it.

Thoughts on if this is a valid approach for other questions or if I just got lucky here?

[Adam Tyson]

It's a good analysis, up to a point, OneSeventy2019. It would be a great analysis IF this was a Sufficient Assumption (aka Justify the Conclusion) question. But in Necessary Assumptions, the answer doesn't have to get you all the way to the conclusion, and can be very, very weak. In this case, for example, the author must have assumed that "at least one brown dwarf was never hot enough to burn lithium." Why is that an assumption? Because the negation - "no brown dwarf was never hot enough to burn lithium" - would destroy the argument!

Assumptions can be very weak, and so do not have to strength of the conclusion. I would caution you against taking that approach for these types, although it is a solid strategy for Justify questions.