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I have a problem with this infamous brown dwarf question (PT32 LR1)...I got it right first time I did it and thought the answer is pretty obvious, but when I came back to it the second time I'm not as certain as I was before. One thing that really confuses me about this question is the term 'celestial object'. Is it the same as or different from 'star’? From the first sentence it seems to suggest that they are different, since it says BD are celestial objects with 'less mass than stars', but the rest of the stimilus talks about the relationship between stars and the Sun. If BD is not a star, how can we be certain that the factors determining the presence/absence of lithium in stars also apply to brown dwarfs? Am I missing something important here?

Funny thing -- this was not the first brown dwarf question that came to mind when I saw your post. There's one from PrepTest 20 that's also a royal pain. (That one, for the record, DOES call a brown dwarf a star... go figure.)

Anyway, for your question -- you're right. The stimulus does imply that brown dwarfs are not stars. So, we can't attribute any of the star information to the brown dwarf. However, the correct answer can be determined merely by the last line: the nuclear furnace of brown dwarfs cannot consume its lithium, which would leave the lithium in the atmosphere. So, if you see a celestial object with no lithium in its atmosphere, it can't be a brown dwarf -- as (C) says in its own convoluted way.

HTH

- Chris

KaplanLSATInstructor wrote: However, the correct answer can be determined merely by the last line: the nuclear furnace of brown dwarfs cannot consume its lithium, which would leave the lithium in the atmosphere. So, if you see a celestial object with no lithium in its atmosphere, it can't be a brown dwarf -- as (C) says in its own convoluted way.

HTH

- Chris

I kind of get it but still a little confused. If the stimulus is interpreted this way, can I say the middle part where it talks about stars and the sun are pretty much irrelevant, while we can assume that in the final line, lithium not being able to be consumed = lithium present in the atmosphere? Or are we still sort of making that inference out of the description about stars? I mean, it may well be possible that even if lithium cannot be consumed, it can be eliminated in other ways, if brown dwarfs ultimately are not stars.

Or am I just being too critical about the relation since it's a "most strongly supported" question and is not supposed to be 100% correct?

But yea, like you said they do claim BD is a star in another question, I kind of take that for granted in this one, while I think it isn't so obvious. Would like some further explanation on this...

A brown dwarf is a failed star. That is, it was part of a gas cloud that had enough mass to condense into an 'object' but not enough mass to start nuclear fission.

Look at the last sentence of the stimulus:

"A brown dwarf does not have a fully functional nuclear furnace and so its lithium cannot be consumed"


Therefore, the lithium is still present. And C is correct.

Remember, if it's not stated in the stimulus, don't assume that the lithium can leave any other way.

Here's a brief explanation about the credited response that I wrote for someone else. Maybe it'll help if there was anything else about the question that gave you trouble:

The credited response, choice C, is a double-negative, so it's worth rephrasing.

Rephrased: If a celestial object lacks lithium in its atmosphere, it can't be a brown dwarf.

Rephrased again: Any brown dwarf must contain lithium in its atmosphere.

The final sentence of the stimulus says that brown dwarfs' lithium cannot be consumed due to lack of a functioning nuclear furnace. Therefore brown dwarfs must still contain lithium. The second sentence of the stimulus suggests that they contain lithium in their atmospheres. The other sentences of the stimulus are irrelevant when it comes to the credited response. They're filler.

Nasty, huh?