Many stars are born in clusters of hundreds or thousands crowded

Explanation for Question 1

This question is definitely a hard question, and part of the reason why is that, to identify the correct answer, we have to use information from both the third paragraph and the fifth paragraph.

1. It can be inferred from the passage that if the hypothesis about the sun's origins is correct, then most of the sun's siblings probably

A. have more heavy elements than is normal for stars born so far from the galactic center

Scanning the passage for "heavy elements," we find it in the third paragraph:

This hypothesis could explain the puzzling levels of heavy elements in the sun. Generally, the farther a star's birthplace is from the galactic center, the poorer the star is in heavy elements. However, a nearby supernova that seeded the meteorites with iron 60 could also have enriched the sun with these elements.

We see that the passage indicates that the Sun is not poor "in heavy elements." Rather, it has "puzzling levels of heavy elements," meaning that it has higher levels of heavy elements than would be expected given the distance of its birthplace from the galactic center.

Then, in the fifth paragraph, the passage says the following about the Sun's siblings:

These stars should have a composition much like the sun's

We see that, if the Sun has "puzzling levels of heavy elements" that are higher than would be expected given how far the Sun's birthplace is from the galactic and the Sun's siblings "should have a composition much like the Sun's," then it follow that the Sun's siblings probably "have more heavy elements than is normal for stars born so far from the galactic center."

So, this choice is supported by the passage.

Keep.

B. are surrounded by comets and meteorites with odd, skewed orbits

Regarding "comets and meteorites with odd, skewed orbits," the passage says the following:

The odd, skewed orbits of many comets also suggest that the sun once belonged to a cluster. The internal dynamics of the solar system cannot account for these orbits, since the comets are beyond the gravitational influence of the major planets.

To eliminate this choice, we have to notice that the what the passage says is solely about the orbits of comets associated with "the solar system," in other words, comets around the Sun.

That information does not indicate that "most of the Sun's siblings" also are surrounded by comets with odd, skewed orbits. For all we know from what the passage says, most of those stars have no comets around them.

Also, the passage doesn't say anything about "meteorites with odd skewed orbits." Regarding meteorites, it discusses that "scientists analyzed two meteorites" and the implications of what the scientists found, which does not involve "skewed orbits."

Eliminate.

C. will be identified as the stars along an arc through the galactic center

The first part of this choice, "will be identified as the stars along an arc," could be correct since the passage does say the following:

astronomers are looking for stars similar to the sun in an arc along which the hypothetical cluster would have dispersed

However, this choice fails because the passage does not indicate that the "arc" would be "through the galactic center." On the contrary, what the passage says in the third paragraph, "This hypothesis could explain the puzzling levels of heavy elements in the sun. Generally, the farther a star's birthplace is from the galactic center, the poorer the star is in heavy elements," tends to indicate that the Sun and its siblings are far from the galactic center.

Eliminate.

D. remain crowded within a cluster with a diameter of a few light-years

This choice is contrary to what the passage says, which is that the Sun is now "isolated," and thus is not within a few light years of the other stars in the cluster because those stars "have scattered."

So, while the passage says, "Many stars are born in clusters of hundreds or thousands crowded within a diameter of a few light-years," it does not indicate that the Sun's siblings "remain crowded within a cluster with a diameter of a few light years."

Eliminate.

E. contain chemical compounds that include nickel 60 as well as iron 60

This choice is a little tricky to eliminate because we could get the impression that it's supported by the following:

a nearby supernova that seeded the meteorites with iron 60 could also have enriched the sun with these elements

However, notice that the plural "these elements" would not refer to the singular "iron 60."

To find what "these elements" refers to, we have to look further above in the passage to find "the puzzling levels of heavy elements in the sun." So, the point that the passage is making is that the supernova enriched the sun with various "heavy elements," not that it enriched the Sun with iron 60, or with nickel 60.

So, when the passage says in the fifth paragraph, "These stars should have a composition much like the Sun's," it is not saying that the Sun's siblings likely "contain chemical compounds that include nickel 60 as well as iron 60" but rather is saying that the Sun's sibling likely contain "heavy elements" in general.

Eliminate.

The correct answer is (A).

Explanation for Question 2

2. The second paragraph mentions the nickel 60 in two meteorites in order to support the conclusion that

A. the meteorites date from the formation of the solar system

The passage describes the two meteorites as "dating from the formation of the solar system," but it does not mention the nickel 60 in the meteorites to support that description. After all, the passage says nothing that suggests that the presence of nickel 60 in the meteorites indicates that they date from the formation of the solar system.

Eliminate.

B. nickel 60 undergoes radioactive decay even after it has been incorporated into chemical compounds

This choice is a little tricky to eliminate because the passage says the following:

They detected nickel 60, the product of radioactive decay of iron 60, in chemical compounds where iron would normally be found. This indicates that the compounds originally formed from iron, which then metamorphosed into nickel.

That information could seem to support what this choice says.

However, there is a key reason why it doesn't.

The passage does not indicate that "nickel 60 undergoes radioactive decay." Rather, it mentions "radioactive decay of iron 60" into nickel 60.

Eliminate.

C. the solar system contains more heavy elements than would be expected given its probable birthplace

This choice is tricky to eliminate since the passage does indicate that the Sun "contains more heavy elements than would be expected given its probable birthplace."

However, the first red flag we might see about this choice is that "the solar system" and "the Sun" are not the same thing. So, what the passage says about what the Sun contains may not mean that the solar system contains more heavy elements than would be expected.

At the same time, there is another issue, which is that the passage doesn't use the fact that the meteorites contained nickel 60 to support the conclusion the solar system or the Sun contains more heavy elements than would be expected. Rather, the passage simply indicates that the Sun contains "puzzling levels of heavy elements" without any support.

If we think about it, the above all makes sense since the fact that two meteorites contain heavy elements doesn't mean that the solar system as a whole contains more heavy elements than expected.

Eliminate.

D. our solar system was seeded with heavy elements by a comet about 1.8 million years ago

This choice combines what the passage says in the second paragraph, "a supernova explosion within five light-years of the sun when the sun was about 1.8 million years old," with what the passage says about comets in the fourth paragraph to say something that the passage simply does not say. After all, the passage does not say anything about a comet that "seeded the solar system with heavy elements or about anything that happened "1.8 million years ago."

Eliminate.

E. the newborn sun was part of a cluster in which a supernova occurred

Scanning the passage for "supernova," we see the following in the second paragraph:

Therefore the iron must have originated nearby, probably from a supernova explosion within five light-years of the sun when the sun was about 1.8 million years old. If the sun had been as secluded as it is today, a supernova so close by would have been extremely unlikely. However, it would have been far more probable if the exploding star and the newborn sun were packed close together as part of a cluster.

Before that, we see information indicating that the iron that "originated nearby" metamorphosed into the nickel found in the meteorites.

Thus, the passage is saying that the nickel came from iron and that the iron probably came from a supernova "within five light-years of the sun" and that therefore it is likely that "the exploding star (the supernova) and the newborn sun were packed close together as part of a cluster."

So, we see that the passage uses the fact that nickel 60 was found in the meteorites to support the conclusion that "the newborn sun was part of a cluster in which a supernova occurred."

The correct answer is (E).

Explanation for Question 3

3. To support the claim that a supernova once exploded near the sun, the passage mentions that

A. our sun was born in a particularly large cluster of stars packed close together

This choice is a tricky trap because this choice indicates basically the exact opposite of what the passage does. So, it's easy to get the impression that this choice is correct if we don't catch that the relationship indicated by this choice is the opposite of what goes on in the passage.

The passage indicates that the fact that scientists "detected nickel 60" in meteorites indicates that a supernova once exploded near the sun, which in turn indicates that our sun was born in a cluster of stars packed close together.

In other words, the passage indicates that "the claim that a supernova once exploded near the sun" supports what this choice says, "our sun was born in a particularly large cluster of stars packed close together," rather than this choice supports that claim.

Eliminate.

B. the levels of heavy elements in the sun are unusually high, given how far it was from the galactic center when it was born

Scanning the passage for "levels of heavy elements," we see the following:

This hypothesis could explain the puzzling levels of heavy elements in the sun.

Looking above in the passage for what "this hypothesis" refers to, we see the following:

Therefore the iron must have originated nearby, probably from a supernova explosion within five light-years of the sun

The fact that "this hypothesis" could explain the puzzling, or high, levels of heavy elements indicates that the hypothesis may be valid.

Also, after "This hypothesis could explain the puzzling levels of heavy elements in the sun," we see this:

a nearby supernova that seeded the meteorites with iron 60 could also have enriched the sun with these elements

So, we see that the passage uses the fact that "the levels of heavy elements in the sun are unusually high, given how far it was from the galactic center when it was born" to support the hypothesis that "a supernova once exploded near the sun."

Keep

C. many comets have odd, skewed orbits

Scanning the passage for "comets" and "skewed orbits," we find the following:

The odd, skewed orbits of many comets also suggest that the sun once belonged to a cluster.

So, the passage uses the fact that "many comets have odd, skewed orbits" to support the claim that "that the sun once belonged to a cluster," not the claim that a supernova once exploded near the sun.

Eliminate.

D. the internal dynamics of the solar system cannot account for the odd, skewed orbits of many comets

Scanning the passage for "internal dynamics of the solar system," we find the following:

The odd, skewed orbits of many comets also suggest that the sun once belonged to a cluster. The internal dynamics of the solar system cannot account for these orbits

We see that the passage uses the fact that "the internal dynamics of the solar system cannot account for the odd, skewed orbits of many comets" to support the claim that "that the sun once belonged to a cluster," not the claim that a supernova once exploded near the sun.

Eliminate.

E. stars with compositions similar to the sun form an arc along which a hypothetical star cluster would have probably dispersed

The passage indicates that "stars similar to the sun in an arc" would be "further evidence" that "the sun once belonged to a cluster," not that "a supernova once exploded near the sun."

Eliminate.

Explanation for Question 4

4. The main purpose of the passage is to

A. describe how a supernova explosion near the newborn sun altered the composition of the solar system

The passage discusses how a supernova explosion likely altered the composition of the sun, not "the composition of the solar system."

Also, the discussion of the supernova explosion is represents just a portion of the passage. So, this choice does not capture the main purpose of the passage as a whole.

Eliminate.

B. present evidence that our sun was born in a cluster of stars that has since dispersed

Considering the main points of the passage, we see the following:

The passage begins by introducing the idea that "Many stars are born in clusters" and then stating the point "New evidence suggests that our sun also originated in a cluster, and thus has many sibling stars that have scattered, leaving our sun relatively isolated."

Then, in the paragraphs that follow the first paragraph, the passage presents evidence that "our sun also originated in a cluster": nickel 60 in meteorites, puzzling levels of heavy elements in the sun, odd, skewed orbits of many comets, and stars similar to the sun in an arc.

Thus, we see that the passage as a whole does "present evidence that our sun was born in a cluster of stars that has since dispersed."

Keep

C. explain the unusually high level of heavy elements in meteorites and in the sun

The passage does not say that there is an "unusually high level of heavy elements in meteorites." Regarding meteorites, it says only that nickel 60 was found in two of them.

Also, the discussion of "unusually high levels of heavy elements" represents only a portion of the passage. So, this choice does not capture the main purpose of the passage as a whole.

Eliminate.

D. assess the claim that stars with compositions much like the sun's are distributed in an arc through the galaxy

The passage does not "assess" that claim. Rather, it only mentions that "astronomers are looking for stars similar to the sun in an arc."

Also, the passage mentions an "arc through the galaxy" only in the final paragraph. So, this choice does not capture the main purpose of the passage as a whole.

Eliminate.

E. discuss the hypothesis that a star cluster accounts for the odd, skewed orbits of many comets

The discussion of the hypothesis that a star cluster accounts for the odd, skewed orbits of comets represents only a portion of the passage. So, this choice does not capture the main purpose of the passage as a whole.

Eliminate.

The correct answer is (B).