Many stars are born in clusters of hundreds or thousands crowded within a diameter of a few light-years. These clusters...
GMAT Reading Comprehension : (RC) Questions
Many stars are born in clusters of hundreds or thousands crowded within a diameter of a few light-years. These clusters usually disperse over a period of billions of years. 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.
In 2003 scientists analyzed two meteorites dating from the formation of the solar system. 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. The iron had to form, reach the solar system, and be incorporated into the meteorites fairly quickly, before it turned into nickel.
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.
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.
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. These comets were most likely stirred up by a star passing nearby, which would have been improbable unless the sun had many neighbors.
As further evidence, astronomers are looking for stars similar to the sun in an arc along which the hypothetical cluster would have dispersed. These stars should have a composition much like the sun's, since they would have been enriched by the same supernova that enriched the early solar system.
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
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Many stars are born in clusters of hundreds or thousands crowded within a diameter of a few light-years. | What it says: Stars often form in tightly packed groups. What it does: Introduces the concept of star clusters as birthplaces. Source/Type: Scientific fact Connection to Previous Sentences: This is our opening - establishing foundational knowledge about star formation. Visualization: Imagine 500-2000 stars packed into a space only 3-4 light-years across (like fitting thousands of marbles into a basketball). Reading Strategy Insight: This sentence sets up a general principle. Look for how this will connect to our sun specifically. |
| These clusters usually disperse over a period of billions of years. | What it says: Star clusters don't stay together permanently - they slowly break apart. What it does: Adds the time dimension and explains cluster evolution. Source/Type: Scientific fact Connection to Previous Sentences: This builds directly on sentence 1 by explaining what happens AFTER stars form in clusters. We're getting the complete lifecycle picture. Visualization: Those 500-2000 stars gradually drift apart over 2-5 billion years, eventually spreading across much larger distances. Reading Strategy Insight: The author is building a logical sequence: birth → dispersal. This setup suggests we'll learn about a specific example soon. |
2. Passage Summary:
Author's Purpose:
To explain how scientists use multiple types of evidence to support the hypothesis that our sun originally formed in a star cluster rather than in isolation.
Main Point:
Multiple lines of scientific evidence strongly suggest that our sun was born in a star cluster and was influenced by a nearby supernova explosion, even though the sun now appears isolated because its sibling stars have scattered across the galaxy over billions of years.
3. Question Analysis:
The question asks what we can infer about "most of the sun's siblings" if the hypothesis about the sun's cluster origins is correct. This requires us to understand what would logically follow for other stars that shared those same origins.
The passage establishes that normally, "the farther a star's birthplace is from the galactic center, the poorer the star is in heavy elements," but our sun has "puzzling levels of heavy elements" that exceed this normal pattern.
Prethinking:
If the hypothesis is correct, then the sun's siblings would all have been affected by the same nearby supernova explosion and would therefore have MORE heavy elements than normal for stars born so far from the galactic center.
Why It's Right:
- Directly supported by the passage's logic about the supernova enrichment affecting all cluster members
- Connects to the passage's explanation that our sun has "puzzling levels of heavy elements" that exceed the normal pattern for stars born far from the galactic center
- Aligns with the final sentence's prediction that sibling stars "should have a composition much like the sun's, since they would have been enriched by the same supernova"
Key Evidence: "However, a nearby supernova that seeded the meteorites with iron 60 could also have enriched the sun with these elements" and "These stars should have a composition much like the sun's, since they would have been enriched by the same supernova that enriched the early solar system."
Why It's Wrong:
- The passage only mentions our solar system's comets having odd, skewed orbits
- There's no evidence that sibling stars would necessarily have the same types of comets or orbital disturbances
- The comet orbit evidence is specific to our solar system's history, not a general characteristic of all cluster members
Why It's Wrong:
- The passage states astronomers are looking for stars "in an arc," not specifically "through the galactic center"
- The arc represents the dispersal pattern of the original cluster, which could be in any direction
- The passage doesn't suggest the search pattern necessarily goes through the galactic center
Why It's Wrong:
- Directly contradicts the passage's main premise that clusters "usually disperse over a period of billions of years"
- The entire hypothesis depends on the idea that the original cluster has scattered, leaving our sun "relatively isolated"
- If stars remained crowded together, there would be no need to search for dispersed siblings
Why It's Wrong:
- Confuses the evidence (nickel-60 found in our meteorites) with what would be true of other star systems
- Iron-60 and nickel-60 detection depends on having meteorites from the solar system formation period, which wouldn't be accessible from other star systems
- The passage doesn't suggest this specific evidence would be detectable in sibling stars