Astronomers theorize that a black hole forms when a massive object shrinks catastrophically under its own gravity, leaving only a...
GMAT Reading Comprehension : (RC) Questions
Astronomers theorize that a black hole forms when a massive object shrinks catastrophically under its own gravity, leaving only a gravitational field so strong that nothing escapes it. Astronomers must infer the existence of black holes, which are invisible, from their gravitational influence on the visible bodies surrounding them. For example, observations indicate that gas clouds in galaxy M87 are whirling unusually fast about the galaxy's center. Most astronomers believe that the large concentration of mass at the galaxy's center is a black hole whose gravity is causing the gas to whirl. A few skeptics have argued that the concentration of mass necessary to explain the speed of the whirling gas is not necessarily a black hole: the concentration in M87 might be a cluster of a billion or so dim stars.
The same hypothesis might have been applied to the galaxy NGC 4258, but the notion of such a cluster's existing in NGC 4258 was severely undermined when astronomers measured the speed of a ring of dust and gas rotating close to the galaxy's center. From its speed, they calculated that the core's density is more than 40 times the density estimated for any other galaxy. If the center of NGC 4258 were a star cluster, the stars would be so closely spaced that collisions between individual stars would have long ago torn the cluster apart.
The skeptics mentioned in the first paragraph would be most likely to agree with the astronomers mentioned in line 13 about which of the following statements concerning the galaxy M87?
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Astronomers theorize that a black hole forms when a massive object shrinks catastrophically under its own gravity, leaving only a gravitational field so strong that nothing escapes it. | • What it says: Scientists think black holes are created when huge space objects collapse and become so dense that their gravity traps everything. • What it does: Introduces and defines the central concept of black holes • Source/Type: Scientific theory (astronomers' hypothesis) • Connection to Previous Sentences: This is our starting point - no previous information to connect to • Visualization: Regular massive star → Collapses under gravity → Black hole with inescapable gravitational field • Reading Strategy Insight: Start here with confidence - this is just a definition to ground us |
| Astronomers must infer the existence of black holes, which are invisible, from their gravitational influence on the visible bodies surrounding them. | • What it says: Since black holes can't be seen, scientists have to detect them by watching how their gravity affects nearby visible objects. • What it does: Explains the detection method/challenge for black holes • Source/Type: Scientific methodology explanation • Connection to Previous Sentences: This builds directly on sentence 1 - now we know WHAT black holes are (sentence 1) and HOW we find them (sentence 2) • Visualization: Black hole (invisible) → Gravity affects → Nearby visible stars/gas → Scientists observe these effects • Reading Strategy Insight: This is logical progression, not new complexity. We're just learning the detection method for the concept we already understand. |
| For example, observations indicate that gas clouds in galaxy M87 are whirling unusually fast about the galaxy's center. | • What it says: In one specific galaxy (M87), gas clouds are spinning around the center much faster than normal. • What it does: Provides a concrete example of the detection method just described • Source/Type: Observational evidence/fact • Connection to Previous Sentences: This is exactly what sentence 2 promised! Sentence 2 said scientists detect black holes by observing effects on visible bodies - this is that observation in action. • Visualization: Galaxy M87 center (suspected black hole) ← Fast-spinning gas clouds orbiting around it Normal galaxy: gas spins at speed 100 M87 galaxy: gas spins at speed 300+ (unusually fast) • Reading Strategy Insight: Feel relieved here - this is just an example, not a new concept! The pattern from sentence 2 is being demonstrated. |
| Most astronomers believe that the large concentration of mass at the galaxy's center is a black hole whose gravity is causing the gas to whirl. | • What it says: Most scientists think there's a black hole at M87's center, and its strong gravity is making the gas spin so fast. • What it does: Explains the mainstream scientific interpretation of the M87 observation • Source/Type: Scientific consensus/interpretation • Connection to Previous Sentences: This completes the logical chain: - Sentence 1: What black holes are - Sentence 2: How we detect them - Sentence 3: Specific observation - Sentence 4: Scientific conclusion from that observation • Visualization: M87's center: Massive black hole → Strong gravity → Gas clouds spin fast → Scientists observe fast spinning • Reading Strategy Insight: This is the payoff sentence where everything connects. The author just walked us through a complete example of black hole detection. |
| A few skeptics have argued that the concentration of mass necessary to explain the speed of the whirling gas is not necessarily a black hole: the concentration in M87 might be a cluster of a billion or so dim stars. | • What it says: Some scientists disagree and think the mass at M87's center could be a huge cluster of faint stars instead of a black hole. • What it does: Introduces an alternative explanation/counterargument to the mainstream view • Source/Type: Minority scientific opinion/alternative hypothesis • Connection to Previous Sentences: This contrasts with sentence 4 - Most astronomers say "black hole" BUT a few skeptics say "star cluster" • Visualization: M87's center - Two possibilities: Option 1 (Most astronomers): Single black hole Option 2 (Skeptics): Cluster of ~1,000,000,000 dim stars packed together Both could create enough gravity to spin gas fast • Reading Strategy Insight: Don't panic - this isn't abandoning the black hole concept. It's just showing there's a debate about one specific case. • What We Know So Far: Black holes, how to detect them, M87 example, scientific disagreement about M87 • What We Don't Know Yet: How this debate gets resolved |
| The same hypothesis might have been applied to the galaxy NGC 4258, but the notion of such a cluster's existing in NGC 4258 was severely undermined when astronomers measured the speed of a ring of dust and gas rotating close to the galaxy's center. | • What it says: Scientists could have argued the same "star cluster" explanation for another galaxy (NGC 4258), but new measurements of spinning dust/gas made that explanation much less believable. • What it does: Introduces a second example where the star cluster hypothesis was tested and found wanting • Source/Type: Observational evidence that challenges the alternative hypothesis • Connection to Previous Sentences: This builds on the debate from sentence 5 by testing the skeptics' star cluster idea in a different galaxy • Visualization: NGC 4258 galaxy center: Skeptics might say: "Star cluster here too" But new dust/gas speed measurements → Evidence against star cluster idea • Reading Strategy Insight: The author is about to resolve the debate introduced in sentence 5. This is setup for the resolution, not new complexity. |
| From its speed, they calculated that the core's density is more than 40 times the density estimated for any other galaxy. | • What it says: Based on how fast the dust/gas was spinning, scientists figured out that NGC 4258's center is more than 40 times denser than the center of any other galaxy they've studied. • What it does: Provides the specific calculation result that undermines the star cluster hypothesis • Source/Type: Quantitative scientific evidence/calculation • Connection to Previous Sentences: This gives the specific data from sentence 6's "measurements" • Visualization: Galaxy density comparison: Typical galaxy center density: 100 units NGC 4258 center density: 4,000+ units (40x more) This extreme density = problem for star cluster idea • Reading Strategy Insight: This is concrete evidence, not theory. The numbers make the abstract concept tangible. |
| If the center of NGC 4258 were a star cluster, the stars would be so closely spaced that collisions between individual stars would have long ago torn the cluster apart. | • What it says: If NGC 4258's center really were a cluster of stars, those stars would be packed so tightly together that they would have crashed into each other and destroyed the cluster long ago. • What it does: Explains why the high density measurement rules out the star cluster hypothesis • Source/Type: Logical scientific reasoning/conclusion • Connection to Previous Sentences: This is the final logical step: - Sentence 6: Measurements undermined star cluster idea - Sentence 7: Showed extreme density (40x normal) - Sentence 8: Explains WHY extreme density kills star cluster idea • Visualization: Hypothetical star cluster at NGC 4258's density: Stars packed 40x tighter than normal → Stars collide frequently → Cluster destroys itself → Star cluster can't exist Therefore: Must be black hole instead • Reading Strategy Insight: This is the resolution! The debate from sentence 5 is now settled in favor of black holes. The passage has come full circle to support the original black hole theory. • Final Understanding: The author used two galaxy examples to show that while skeptics propose alternatives to black holes, the evidence strongly supports black hole explanations. |
2. Passage Summary:
Author's Purpose:
To demonstrate how scientific evidence can settle debates about black holes by showing that alternative explanations fail when tested against observational data.
Summary of Passage Structure:
In this passage, the author builds a case for black hole detection by walking us through the scientific process from theory to evidence:
- First, the author explains what black holes are and how scientists detect them since they can't be seen directly.
- Next, the author presents a specific example (galaxy M87) where scientists found evidence of a black hole, but notes that some skeptics proposed an alternative explanation involving a cluster of stars.
- Then, the author introduces a second galaxy (NGC 4258) where the same debate could have occurred, but new measurements provided strong evidence against the star cluster idea.
- Finally, the author explains why the measurements prove that a star cluster couldn't exist at such high density, effectively ruling out the alternative explanation.
Main Point:
While skeptics can propose alternative explanations for what appears to be black hole evidence, careful scientific measurements and analysis ultimately support the black hole explanation over competing theories like star clusters.
3. Question Analysis:
This question asks us to identify what the skeptics and astronomers would AGREE on regarding galaxy M87. The key insight is that we need to find common ground between two groups that disagree about the nature of the mass at M87's center.
Connecting to Our Passage Analysis:
From our passage analysis, we know that:
- Most astronomers believe M87's center contains a black hole
- Skeptics argue it might be a cluster of dim stars instead
- Both groups are trying to explain the same observed phenomenon: unusually fast-spinning gas clouds
- Both groups agree that SOMETHING with significant gravitational pull is causing this spinning
The passage analysis shows us that the debate isn't about WHETHER there's a massive object at M87's center, but about WHAT KIND of massive object it is.
Prethinking:
Both skeptics and astronomers are responding to the same observational evidence: gas clouds whirling unusually fast around M87's center. They both accept this observation as fact and both agree that such fast spinning requires a large concentration of mass to generate the necessary gravitational force. Therefore, the correct answer should identify something both groups accept as true about the relationship between the observed gas motion and the presence of concentrated mass.
Why It's Wrong:
- This represents only the skeptics' view, not a point of agreement
- The passage shows astronomers believe the dense object IS a black hole
- This choice takes sides in the debate rather than finding common ground
Common Student Mistakes:
- Did I confuse "skeptics would agree with astronomers" with "skeptics have this opinion"?
→ Remember the question asks for SHARED beliefs, not individual positions - Am I focusing on the disagreement instead of the agreement?
→ Look for what both groups accept as factual, not their competing theories
Why It's Wrong:
- This is specifically the skeptics' alternative hypothesis
- Most astronomers reject this explanation in favor of the black hole theory
- This represents the point of disagreement, not agreement
Common Student Mistakes:
- Did I think this was a compromise position both sides accept?
→ Re-read carefully - this is the skeptics' competing theory, not shared ground - Am I misunderstanding what "agree with" means in the question?
→ It means find identical beliefs, not similar reasoning processes
Why It's Wrong:
- This represents the astronomers' mainstream view, not the skeptics' position
- The skeptics explicitly argue AGAINST this explanation
- This is the opposite of what skeptics believe
Common Student Mistakes:
- Did I accidentally choose what astronomers believe rather than what both groups believe?
→ Check that your choice represents BOTH perspectives, not just one - Am I confusing the majority opinion with the consensus opinion?
→ "Most astronomers" doesn't mean "all scientists including skeptics"
Why It's Right:
- Both groups accept the observational evidence of fast-spinning gas
- Both groups agree this spinning requires a gravitational explanation
- Both groups acknowledge that significant mass concentration is necessary
- This represents the factual foundation both sides build their theories upon
Key Evidence: "Most astronomers believe that the large concentration of mass at the galaxy's center is a black hole whose gravity is causing the gas to whirl. A few skeptics have argued that the concentration of mass necessary to explain the speed of the whirling gas is not necessarily a black hole" - both groups accept that concentrated mass causes the gas motion.
Why It's Wrong:
- The skeptics specifically propose that a star cluster COULD account for the spinning gas
- This contradicts the skeptics' fundamental argument
- The passage shows skeptics believe star clusters can generate sufficient gravitational force
Common Student Mistakes:
- Did I confuse this with the NGC 4258 evidence that works against star clusters?
→ The question specifically asks about M87, where star clusters remain a viable alternative in the skeptics' view - Am I applying evidence from one galaxy to another galaxy?
→ Keep the examples separate - M87 and NGC 4258 have different evidence profiles