Brown dwarfs—dim red stars that

Answer choices

1. A

   Correct61% picked this

   None of the coolest brown dwarfs has ever been hot enough

   Why this is right

   If we negate this, it's saying that "some of the coolest brown dwarfs were at some point hot enough to destroy lithium". Does this hurt the argument? Yes, it gives us a pathway to argue that "just because we see a coolest brown dwarf doesn't guarantee there's no lithium ... some of them might have burned up all their lithium back when they were hot enough to destroy lithium." This doesn't feel like the most powerful type of Weaken idea, when negated, because it just creates a possibility of contradicting the conclusion. But that still counts as Weakening. When an author's conclusion is sure of itself, then presenting the possibility that the conclusion could be false is a gamechanger.

   Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

   61%
2. B

   Too Specific: most19% picked this

   Most stars that are too cool to burn hydrogen are too cool to

   "Most" is wrong 99% of the time we see it on Necessary Assumption, because negating most is essentially like toggling between 51% and 49%. Does it make any difference to this author whether 51% vs. 49% of stars that are too cool to burn hydrogen are also too cool to destroy lithium? Nope. It only matters to this author that the coolest brown dwarfs are both too cool to burn hydrogen and do not destroy their lithium completely.

   19%
3. C

   Irrelevant Relationship9% picked this

   Brown dwarfs that are not hot enough to destroy lithium are hot enough

   If we negate this, it says that brown dwarfs that aren't hot enough to destroy lithium (like the coolest ones in our conclusion) are not hot enough to destroy helium either. Does that weaken? Nope. If the coolest brown dwarfs are cool enough that they can't burn hydrogen, lithium, or helium, that's fine. It's actually weirder for the author to think that cool brown dwarfs are not hot enough for hydrogen / lithium but are hot enough for helium.

   9%
4. D

   Too Specific10% picked this

   Most stars, when first formed, contain roughly the same percentage

   Too Specific: most Too Strong: roughly the same "Most" is wrong 99% of the time we see it on Necessary Assumption, because negating most is essentially like toggling between 51% and 49%. Does it make any difference to this author whether 51% vs. 49% of stars begin with a roughly equivalent percentage of lithium? Nope. It only matters to this author that whatever percentage of lithium the coolest brown dwarfs start out with, it's big enough that it never gets fully burned off (i.e. whatever lithium they start with, the author just needs it to be true that they never fully deplete it).

   10%
5. E

   Too Strong: nothing is more similar1% picked this

   No stars are more similar in appearance to red dwarfs than

   Our spidey-sense should tell us that this is just trying to derive an inference from the very first claim in the paragraph. It has nothing to do with whether we can draw the conclusion in the last sentence. If we negated this and said, "Even though red dwarfs are very similar in appearance to brown dwarfs, there is another type of star that is even more similar in appearance", it wouldn't affect the author's argument at all.

   1%