Self-study
Support The differences in distance from Earth between the stars in any one distant galaxy are negligible compared to the vast distance to the galaxy itself. █████ ██ ███ █████ ███ ██ ███ ████ ███████ ███████ ███ ███████████ ██████████ ██ ███ ████████ ██████████ ██ █████ █████ ███████ ████ ███████████ ██ ███ ████████ ████ ██ ████████ ████████ ██████████ ██ ██████ ██ ████ ██ █████████ ███ █ ██████ ████████ ██████ ██████████ ██████████ ████ █████ ███████████████ ██ ████████ █████ ██ ███ ████ ███████ ███████
Summary
The author concludes that by studying stars in the same distant galaxy, we can determine how a star’s relative actual brightness correlates with other characteristics of a star.
Why?
If two stars are in the same distant galaxy, the comparative distance of each star from Earth is almost nothing compared to the distance between the Earth and the distant galaxy. (For example, one star might be 1,000,000 miles away, and the other star might be 1,000,001 mile away. The comparative difference of 1 mile is much smaller than the overall 1,000,000 miles between the Earth and the distant galaxy.)
Because of this, for two stars in the same distant galaxy, any difference in how bright they appear must result from differences in how bright they actually ARE. (In other words, the brightness is a function of real differences in brightness, not just a function of how close the different stars are to Earth.)
Notable Assumptions
Notice that the conclusion brings up a new concept — “other characteristics” of stars. The premises don’t tell us anything about “other characteristics.” We know something about the brightness of a star...but what does this have to do with other characteristics and our ability to correlate those characteristics with brightness? The author’s assuming that we can identify other characteristics of stars in distant galaxies and that we can correlate those characteristics with the brightness of those stars.
19.
Which one of the following ██ ██ ██████████ ███ ████████ █████████
a
If two stars ███ ██ ███ █████████ █████████ ██ ██ ███ ████████ ██ █████████ ███████ ██ ███ ████ ███ █████████████ ███ ████ ████████ ████ ██████
Whether it’s possible to determine that two stars are the same distance from Earth is irrelevant, because we already know that any differences in the distance of those stars from Earth is negligible. The author’s reasoning is based on the distance of the distant galaxy to Earth, not on the comparative difference in distance from individual stars in that galaxy to Earth.
b
If any two █████ ███ ██ ███ ████ ███████ ███████ ███████████ ██ ███ ████████ ████ ██ ███████ ████ ██ ██████████ ████ ██████
Not necessary, because “elements” is too specific. We have no reason to think the author believes that the “other characteristics” we can correlate with brightness must involve the “elements each star is burning.” Maybe the author’s referring to characteristics besides elements.
c
The stars in ███ ███ ██████ ███ ███ ███ █████████████ ███ ████ ████████ ████ ██████
Not necessary, because the argument doesn’t concern stars in “our own galaxy.” It’s about stars in distant galaxies.
d
There are stars ██ ███████ ████████ ████ ████ ████████████████ █████ ████ ███████████ ███████████ ████ ██████
Necessary, because if this were not true — if there are NO stars in distant galaxies that have characteristics besides brightness that are detectable from Earth — then we have no basis to believe that we can correlate those other characteristics with the brightness of distant stars. For example, if we can’t tell how large the star is or how fast it’s spinning, then we can’t correlate size or spinning speed with brightness.
e
If there are ███████████ ███████████ ██ ███ ███ ████ ███ █████ ███ ████ ██████ ████ █████ █████ ████ ██████ █████████████ ██ ████████ ███████████
Not necessary, because the argument is concerned with what we can do IF two stars in a distant galaxy have a significant difference in apparent brightness. It’s entirely possible that many pairs of stars won’t have any different in apparent brightness; this doesn’t undermine the author’s reasoning, which is just based on the hypothetical situation in which we do see a difference in brightness.