PrepTest 158, Section 2, Question 19
The differences in distance from Earth between the(72 more words not shown)
Which one of the following(6 more words not shown)
- AIf two stars(22 more words not shown)
- BIf any two(19 more words not shown)
- CThe stars in(12 more words not shown)
- DThere are stars(12 more words not shown)
- EIf there are(20 more words not shown)
Reading the argument
The first sentence is easier if we picture the scale. Imagine a distant galaxy a million units away, with two of its stars differing in distance from us by only one unit. Those are made-up numbers, but they show the point: the difference between the stars' distances is tiny compared with the whole trip from Earth to that galaxy.
The argument uses that fact to set aside distance as an explanation for a significant brightness difference. If one of those stars looks much brighter, the author says it is actually burning more brightly. “Relative” brightness just means how bright one star is compared with another.
Then we get a new goal: find out how brightness goes along with other characteristics. What other characteristics can we observe? Knowing which star is brighter isn't enough to compare brightness with something else. We also need information about that other feature.
What we’re looking for
That's the assumption I'd look for. To study a relationship between brightness and other traits in distant stars, we must be able to distinguish at least some of those other traits. Otherwise we'd have only the brightness side of the comparison.
Working the answers
- A
The argument doesn't require us to be unable to compare distances across different galaxies. Its proposed method focuses on stars within the same distant galaxy. That method could work even if other distance comparisons were possible too. A adds a limitation the argument doesn't need.
- B
The elements the stars are burning might be one possible characteristic to study, but the argument doesn't name that particular feature. It needs some other observable characteristics, not a guarantee that differences in chemical elements will be detectable for any two stars. B is more specific and more sweeping than the required assumption.
- C
The stars in our own galaxy aren't the subjects of the proposed study. The argument could use the distant-galaxy comparison whether our local stars were at similar distances or very different ones. C doesn't supply anything needed to connect brightness with other observable characteristics.
- DCorrect
At least some distant stars must have other characteristics we can discern from Earth. Without them, we'd have nothing observable to compare with brightness. D doesn't need to name every characteristic or say we can measure every star. It supplies the basic possibility on which the proposed comparison depends.
- E
This concerns stars with substantially different distances, while the method deliberately compares stars whose distance differences are negligible. It also doesn't follow that a distance difference must produce an apparent brightness difference: differences in actual brightness might offset it. The argument doesn't require this broader rule.
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