PrepTest 105, Section 2, Question 16
Brown dwarfs—dim red stars that are too cool(67 more words not shown)
The argument depends on assuming(5 more words not shown)
- ANone of the(11 more words not shown)
- BMost stars that(13 more words not shown)
- CBrown dwarfs that(13 more words not shown)
- DMost stars, when(9 more words not shown)
- ENo stars are(11 more words not shown)
Reading the argument
What if the star used to be hotter? Being too cool to destroy lithium now doesn't tell us that it has always been too cool.
The similar names can make this harder to see. A brown dwarf is a dim red star, but that doesn't make it a red dwarf. More importantly, the argument singles out the coolest brown dwarfs. Those are the stars described as too cool to destroy all their lithium; other stars can destroy it.
Every star starts with a substantial amount of lithium. The author therefore treats missing lithium as a way to rule out membership in that coolest group. But imagine a star that destroyed its lithium during an earlier, hotter period and later cooled down. It would now be one of the coolest brown dwarfs, yet have no lithium left. That would defeat the proposed test. We don't need to know whether stars actually follow that history. The argument hasn't ruled it out.
What we’re looking for
The question asks for a required assumption. I'd look for something closing that temperature-history gap: a star in the coolest group couldn't previously have burned away its lithium.
Working the answers
- ACorrect
This rules out the history we were worried about. If none of the coolest brown dwarfs has ever been hot enough to destroy its lithium, we cannot explain a lithium-free member of that group by saying it burned the lithium earlier and then cooled.
The word “ever” does the work. The premises describe which stars are hot enough; the argument needs that distinction to hold across the relevant history of the currently cool stars. Otherwise, finding no lithium would not reliably exclude a star from the coolest group.
- B
The argument concerns the coolest brown dwarfs, not the majority of all stars too cool to burn hydrogen. The coolest ones could be a small part of that broader group. We don't need to know how most members of the broader group handle lithium. More importantly, this doesn't address whether a currently cool star could have destroyed its lithium earlier.
- C
Destroying helium is a different issue. The reasoning uses whether a star could have destroyed its lithium to classify it. It doesn't require lithium-preserving stars to be capable of destroying some other element. We should not invent a connection between those processes.
- D
A substantial initial amount isn't the same as an equal initial percentage. One star could begin with more lithium than another, and the argument's distinction between retaining lithium and destroying it completely could still work. Matching starting percentages isn't required.
- E
The resemblance to red dwarfs helps introduce the stars, but appearance doesn't drive the conclusion. Some other kind of star could look even more like a red dwarf without changing whether the coolest brown dwarfs can lose all their lithium. This comparison adds nothing the proposed lithium test needs.
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