#24 - Science journalist: Brown dwarfs are celestial objects

[sarae]

Please explain why C is correct

[Ron Gore]

Hi, Sara!

Thanks for your question. The dense facts and strange wording of the answer choices definitely make this question difficult.

This question is categorized as Must Be True. Your Prephrase for the correct answer is that it will be either a restatement of one of the facts in the stimulus, or it will be an inference permissible by a combination of the facts.

The set of facts in this stimulus is lengthy and dense. When combined with the awkward phrasing of the correct answer choice, (C), LSAC has made a fairly direct question difficult.

Among the facts we're provided in the stimulus is that a brown dwarf's lithium cannot be consumed because it does not have a fully functional nuclear furnace. To be certain, no fact in the stimulus expressly tells us that a brown dwarf must necessarily have lithium in its atmosphere. However, the combination of the second sentence (i.e., brown dwarfs are identified by...whether or not lithium is present in their atmospheres) and the last sentence (i.e., so its lithium cannot be consumed) permits us to infer that a brown dwarf must have lithium in its atmosphere.

Answer choice (C) tests us on this definitional aspect of a brown dwarf, that it must have lithium in its atmosphere. In conditional terms:

brown dwarf lithium in atmosphere

The contrapositive of this relationship would be that if there is no lithium in the atmosphere, then it is not a brown dwarf:

lithium in atmosphere brown dwarf

Hidden beneath its awkward language, answer choice (C) expresses this conditional relationship.

If I may help you further, please let me know.

Ron

[sarae]

thank you!

[Ron Gore]

You're welcome.

[lsat2016]

Hi, Sara!

Answer choice (C) tests us on this definitional aspect of a brown dwarf, that it must have lithium in its atmosphere. In conditional terms:

brown dwarf lithium in atmosphere

Ron

Hello,
Could you explain how you knew that this statement could be translated into a conditional statement? I just read it as "a brown dwarf has lithium" and missed an important part of the question.

Thank you!

[Clay Cooper]

Hi Lsat 2016,

Sure! Thanks for your question.

Definitions are always conditional statements. If the sentence says 'a brown dwarf has lithium,' then one characteristic of a brown dwarf has been defined: that it has lithium. Therefore, we know it must be the case that:

if it's a brown dwarf, then it has lithium.

Or,

brown dwarf lithium

For another example, if we said it is a characteristic of mammals that they give birth to live young, that would be part of the definition of mammal, and we could then state this relationship conditionally:

mammal gives birth to live young

In other words, being a member of a group (brown dwarfs, mammals, etc) that is defined by having a certain characteristic is sufficient to prove that you necessarily have that characteristic as well.

Does that clarify it some?

[LSAT2018]

I have read through your posts and I understand why Answer (C) is correct.
But if I were to be given a similar problem, I don't think I`ll be able to think in such a way. So I would just like to ask why the statements about 'stars at least as massive as the sun' and 'stars with less mass than the sun' are irrelevant to the inference? Also how would this question figure in the Premise/Conclusion structure?

[James Finch]

Hi LSAT2018,

This is a Must Be True question, so there has to be some inference available to be made. The key to stimuli like these is to think about what the stimulus is trying to prove, which will be a missing element of the stimulus itself. The stimulus will set up premises that we can then ourselves draw a conclusion from. More difficult versions will often throw in irrelevant information to try and throw test takers off track. Be aware of this and always keep your eyes on the prize!

Here, the first and second sentences in the stimulus set up the necessary conditions for a brown dwarf: they are celestial objects with less mass than stars but more than planets, and either have or don't have lithium in their atmospheres. The uncertainty of the second sentence about whether brown dwarfs' atmospheres contain lithium should immediately jump out at you as a red flag. A science journalist attempting to explain what defines a brown dwarf would never actually leave that ambiguity, but would clearly state that brown dwarfs have lithium in their atmospheres (or vice versa). So most likely we need to infer whether or not brown dwarfs have lithium in their atmospheres to correctly answer this question.

In fact, the final two sentences of the stimulus give us the premises to infer whether or not brown dwarfs have lithium in their atmospheres, albeit in a confusing way: by creating a distinction between stars as large as the Sun, and stars that are smaller than the Sun, and whether they have lithium in their atmospheres, the test makers are trying to throw you off track. But we're concerned with brown dwarfs, which are smaller than any stars, so this distinction about stars is irrelevant. We're only concerned with the information in the final sentence, from which we can infer that brown dwarfs have lithium in their atmospheres, as they lack a nuclear furnace capable of consuming all the lithium. So having lithium in the atmosphere is a necessary condition of being a brown dwarf.

Using this inference as a prephrase, we can attack the answer choices. Answer choice (C) says exactly this, albeit with an odd double negative construction. Puzzle the actual meaning out, and we have an answer choice that lines up with our prephrase, and thus a correct answer.

Hope this clears things up!

[LSAT2018]

Can I ask how you would eliminate the remaining answers? And to clarify, any celestial object refers to stars right?

[lilmissunshine]

Could you explain how (A) and (D) are different and why they are incorrect?

My diagram for the stimulus is:

[Star >= Sun] Lithium
Contrapositive: No Lithium [Star < Sun]

[Star < Sun] No Lithium
Contrapositive: Lithium [Star >= Sun]

According to this logic, (A) and (D) seem right. Is it because we can't assume "any celestial object" is "a star"? Another confusing part is that a brown dwarf is less mass than a star but the science journalist talks about lithium, he/she refers to stars' mass compared to that of the Sun. Is the matter of mass completely irrelevant?

Many thanks!