Passage Summary: Scientists can identify celestial objects by mass and lithium. Big stars and brown dwarfs have lithium, but stars smaller than the Sun don't have any left.

Reasoning: Brown dwarfs have lithium because their furnaces don't burn it. Large stars (Sun-sized or bigger) keep lithium because they don't mix it into their furnaces. Small stars (smaller than the Sun) have no lithium because they mix it and burn it all.

Analysis: This is a fact-heavy stimulus that requires us to synthesize the rules provided. We know that stars smaller than the Sun consume all their lithium, while brown dwarfs (which are also less massive than the Sun) do not. Therefore, if we find a celestial object that is less massive than the Sun but still contains lithium, it cannot be a star; it must be a brown dwarf. Look for an answer that draws a logical boundary between these objects based on the presence of lithium and their relative mass.