Brown dwarfs—dim red stars that are too cool to burn hydrogen—are very similar in appearance to red dwarf stars, whic...

Veda on August 5 at 09:25PM

Confused about A

While I saw that A was a strong enough answer choice to support the strong conclusion in the question, I hesitated to pick it because it says none of the coolest brown dwarfs have even been hot enough to destroy lithium, but I didn't feel like that was necessary because they could have been hot enough to destroy some lithium, but maybe just not completely like the question says. Could someone please explain this?

1 Reply

Shunhe on August 6 at 05:10PM

Hi @Veda-Bhadharla,

Thanks for the question! So (A) is certainly strongly worded, and that’s a good thing to be wary for on strengthen with necessary premise questions, but note also that the language in the stimulus is strongly worded (for example, any star found that contains no lithium is not one of these coolest brown dwarfs.)

So let’s sum up this argument first. Well, brown dwarfs look like red dwarf stars. Red dwarf stars are just hot enough to burn hydrogen. All stars but the coolest of the brown dwarfs are hot enough to destroy lithium completely by converting it to helium. And then finally, we conclude that stars without lithium aren’t one of these coolest brown dwarfs.

What’s the necessary assumption here? Well, we know that the coolest brown dwarfs can’t destroy lithium now, but what if they used to be able to before the cooled down? The argument seems to be assuming that that can’t happen. Let’s take a look at (A), which tells us that none of the coolest brown dwarfs has ever been hot enough to destroy lithium. Let’s try the negation test on this one. Negating (A), we get that “some of the coolest brown dwarfs have been hot enough to destroy lithium at some point.” Well, if that’s true, then there might be a coolest brown dwarf that is a star that contains no lithium! And then it wouldn’t be true that any star found with no lithium isn’t a cool brown dwarf. So if (A) is negated, the argument falls apart, and so (A) is a necessary assumption of the argument and the correct answer.

Hope this helps! Feel free to ask any other questions that you might have.