#24 - Science journalist: Brown dwarfs are celestial objects

[Jonathan Evans]

Hi, LMS,

Good question. Your difficulty arises from the negations you are using for your contrapositives.

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

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

Let's look at each of these statements:

• [Star >= Sun] Lithium This is correct.
  Contrapositive: No Lithium [Star < Sun] This is not quite right.

When you're making a contrapositive (or any time that you logically negate a statement) be both careful and conservative in your negation. The valid contrapositive for the statement above would be:

• No Lithium NOT [Star >= Sun]

Let's clarify what this means. The negation of "stars larger than or equal to stellar mass" is not "stars less massive than the sun." The negation is simply "not a star larger than or equal to the sun." This object need not even be a star. It could be a baked potato for all we know. All we know is that an object that has no lithium in its atmosphere is not a star with mass equal to or greater than the sun.

The same principle applies to your second conditional:

• [Star < Sun] No Lithium This is correct.
  Contrapositive: Lithium [Star >= Sun] This is not quite right.

Following the previous instructions, the contrapositive should be:

• Lithium NOT [Star < Sun]

This works for brown dwarfs because this object with Lithium is not a star at all! It is a brown dwarf, an object less massive than any star.

There is a conditional in the stimulus for brown dwarfs:

• Brown Dwarf Lithium
  Contrapositive: No lithium Not a Brown Dwarf

You can see that by combining the brown dwarf stimulus with the other valid conditionals, we may infer:

• No lithium Not a brown dwarf & not a star with mass greater than or equal to that of the sun

Notice that this combined conditional directly addresses the attempted, incorrect inference in answer choice (A). The correct inference would be as follows:

• Any object without lithium is neither a brown dwarf nor a star with mass greater than or equal to that of the sun.

Likewise, answer choice (D) in fact directly contradicts the information in the passage. The passage establishes that brown dwarfs do have lithium in their atmosphere.

Good work here. I hope this helps!

[Jennifer Janowsky]

Lilmissunshine and LSAT2018,

In this prompt, stars are a form of celestial objects--just like LSAT2018 said.

However, though stars are celestial objects, not all celestial objects are stars. Therefore, both (A) and (D) are incorrect for the same reason: because they refer too broadly to celestial objects rather than stars.

Hope that helps!

[lilmissunshine]

Thank you Jonathan and Jennifer!! That was incredibly helpful! Much appreciated!

[lbevins]

I got this answer wrong because "No Celestial Objects" at the beginning of the answer choice confused me. I understand why the others are wrong after reviewing the posts, but I'm still not 100% confident in my interpretation of this answer choice. Can someone diagram the answer (including the celestial objects part) or explain how I should be interpreting this form of answer choice? Thank You!!

[Jonathan Evans]

Hi, LBevins,

Good question!

The premises stipulate that:

1. The lithium in brown dwarfs cannot be consumed (last sentence).
2. We also know from the first sentence that brown dwarfs are celestial objects.

Statement (1) above is the one that matters to prove answer choice (C). We could diagram it as follows:

• BD L
• Contrapositive L BD

Essentially, if there's something with no lithium in it, it cannot be a brown dwarf. The "celestial objects" part of answer choice (C) does not make a difference because we literally know that anything without lithium is not a brown dwarf.

The double negative might make (C) difficult to parse. It might help to think of answer choice (C) like this:

• "Nothing with no lithium in its atmosphere is a brown dwarf." "Anything with no lithium is not a brown dwarf."

Does this help?

[sdb606]

I'd like to suggest a different take because I don't see support for BD requiring Lithium. Just because a BD cannot consume Lithium does not mean it has Lithium in the first place. If I said, "Dogs are unable to grow wings longer than 6 feet," that statement is true and at the same time does not mean that dogs have wings. Additionally, if an object is characterized by its presence or absence of something (as in the second sentence), then there must exist objects that have that something and some that do not. I would never say, "Dogs are characterized by whether or not they have wings."

My reasoning for C is as follows:

The stimulus says BD L OR L
Answer choice (C) says: BD L

C is supportable because it refers to a subset of the set of BD described in the stimulus. If the stimulus said, "All dogs are either black or white" and an AC said, "All dogs are black," this is compatible with the stimulus. Is this sort of reasoning allowed?

[Rachael Wilkenfeld]

There's a small problem with your reasoning, sdb. Because of the last statement in the stimulus, we can infer that brown dwarfs DO have lithium. It says "....so its lithium cannot be consumed." That structure indicates that the dwarfs do have lithium, because otherwise it wouldn't be able to refer to the brown dwarfs' lithium.

Hope that helps!

[ivan.l99]

I chose C but almost chose B. There were a lot of scientific terms in this Q overall. Any good way to rule out choice B?

[Rachael Wilkenfeld]

Hi ivan,

Good question. We aren't sure that answer choice (B) has to be true. In fact, we have some reasons to think that answer choice (B) is not always true. Brown dwarfs are celestial objects with lithium in their atmosphere, and they don't have a fully functional nuclear furnace. We couldn't say if they have a nuclear furnace that has incompletely mixed elements because we don't know if they have a nuclear furnace that can mix at all.

It's mostly wrong though because it isn't proven. Wrong answer choices in Must Be True questions are sometimes things that are really likely to be true. But that isn't the same as something that Must Be True. Whatever answer choice we select has to be something that we can fully support with the information in the stimulus. That's what makes answer choice (C) a winner. If it has no lithium in the atmosphere, it can't be a brown dwarf because we know brown dwarfs can't consume the lithium in the atmosphere.

Hope that helps!