PT20,S1,16-Brown dwarfts-dim red stars that are too cool to

[b91302310]

The answer is (A). But I think it's the premise already mentioned in the sentence " All stars but the coolest of the brown dwarfs are hot enough to destroy lithium ...." So, why is it the assumption?

Could anyone help?

Thanks!

[bbirdwell]

This is a tricky one.

Here are the facts:
1. Brown dwarfs = dim red star too cool to burn H
2. Red dwarfs = similar to BD in appearance, hot enough to burn H
3. Stars, when first formed, contain Li
4. All but the COOLEST of the Brown Dwarfs are hot enough to destroy Li by converting it to He.

Conclusion:
Star w/no Li --> not one of the coolest Brown Dwarfs

In other words (the contrapositive):
The coolest Brown Dwarfs have Lithium.

The catch here is the little qualifier attached to statement #4: by converting it to Helium. Is that the only way to destroy Li? Perhaps not. So in order to draw the conclusion, we must essentially assume that the coolest Brown Dwarfs cannot destroy Li in some way other than converting it to He. This is what (A) says.