RC : Astronomers theorize that a black hole forms

[chetan86]

Astronomers theorize that a black
hole forms when a massive object
shrinks catastrophically under its own
Line gravity, leaving only a gravitational
(5)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.
(10)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
(15)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
(20)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,
(25)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
(30)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
(35)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?
Which of the following, if true, would most clearly undermine the possible explanation for the whirling gas in M87 that is mentioned in the last sentence of the first paragraph?

A The stars in a star cluster at the center of M87 could exert a strong gravitational force without tearing the cluster apart.
B A cluster of stars at the center would preclude the existence of certain other astronomical phenomena that have been observed at the center of M87.
C The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87.
D Only one other galaxy has been observed to contain gas clouds whirling about its center as they do about the core of M87.
E The gravitational force of a cluster of a billion or so dim stars would be sufficient to cause a whirling ring of gas and dust to collect around the center of a galaxy.

Could you please let me know whether my understanding is correct?

Skeptics are basically saying that concentration of mass could be other than black hole such as a cluster of a billion or so dim stars.

To undermine this claim, B is saying that if there was a cluster then gravity causing the gas to whirl could not be available; In other words, a cluster would prevent gas clouds whirling unusually fast about the galaxy’s center.

[RonPurewal]

choice (b) says that a cluster would "preclude other ... phenomena ... that have been observed".

explain to a 10-year-old:
if there's a cluster, then stuff we've actually seen would be impossible.

well, the stuff we've actually witnessed clearly isn't impossible--since we've witnessed it-- so that would rule out the whole cluster hypothesis.

[RickyH486]

choice (b) says that a cluster would "preclude other ... phenomena ... that have been observed".

explain to a 10-year-old:
if there's a cluster, then stuff we've actually seen would be impossible.

well, the stuff we've actually witnessed clearly isn't impossible--since we've witnessed it-- so that would rule out the whole cluster hypothesis.

Hi Ron, thanks for the great analogy. Your analogy has helped me to understand how OA:B makes sense. Could you kindly help me explain what is wrong with choice C? When I first attempted this problem, I have eliminated ADE, and was struggling between B and C. I incorrectly chose C at the end and I could not understand where is the logical flaw in C.

Here is my reasoning, please point out where my mistake is. In the second paragraph, there is a part that says "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.". How I interpreted this part is "if center were star clusters then stars would be closely spaced". C says "The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87." Choice C tells us that "stars within the core of M87 is LESS closely spaced than are stars within NGC4258". Because the stars within the core of M87 are LESS closely spaced than are stars within NGC4258, we can therefore conclude that the centre of M87 cannot be star clusters as the skeptics suggested.

Hopefully you can understand how my reasoning worked out, if there is anything that is not clear, I will be happy to come back and add more details.

[RickyH486]

I have also encountered another problem on this same passage.

The passage asserts which of the following about the existence of black holes? OA: B
A. Astronomers first speculated about the existence of black holes when they observed gas whirling around the center of a particular galaxy.
B. Evidence used to argue for the existence of black holes is indirect, coming from their presumed effects on other astronomical bodies.
C. Recent observations of certain astronomical bodies have offered proof.
D. A considerable body of evidence suggests the existence of black holes, even though their behavior is not completely consistent with the laws of physics.
E. Many astronomers are skeptical about certain recent evidence that has been used to argue for the existence of black holes.

When I first attempted this one, I eliminated D and E right away and was stuck among ABC. Now I think about it, I kind of understand why C is wrong since there isn't solid proof in passage that says black hole exists (please let me know if my thinking is correct). I chose A at the end, and later found out OA is B. I get why B is the right answer, but still having doubt about A, namely I don't know what is wrong with A. Master Ron, could you let me know how would eliminate answer choice A?

[RonPurewal]

the text that supports choice B is near the beginning:
Astronomers must infer the existence of black holes, which are invisible, from their gravitational influence on the visible bodies surrounding them.

__

choice A says
Astronomers first speculated about the existence of black holes when...

this passage says nothing about how (or why) astronomers ORIGINALLY may have speculated about the existence of black holes.

there's no text that has ANY connection—no matter how vague—with the history of the idea of black holes, or with how scientists first came up with that idea. so, A should be a straightforward elimination.

what evidence did you think supported choice A?

[RickyH486]

the text that supports choice B is near the beginning:
Astronomers must infer the existence of black holes, which are invisible, from their gravitational influence on the visible bodies surrounding them.

__

choice A says
Astronomers first speculated about the existence of black holes when...

this passage says nothing about how (or why) astronomers ORIGINALLY may have speculated about the existence of black holes.

there's no text that has ANY connection—no matter how vague—with the history of the idea of black holes, or with how scientists first came up with that idea. so, A should be a straightforward elimination.

what evidence did you think supported choice A?

Interesting. I think I did not fully understand the context of choice A when I first attempted this problem (even when I reviewed the problem on my second attempt, I failed to notice the significance of the part that you have highlighted for me-"first speculated"). It just did not occur to me that choice A was talking about "the history of the idea of black holes". If you are interested to know how I wrongly interpreted choice A, here it goes: I naively thought A was just talking "when astronomers see gas whirling around a particular galaxy, they speculated about the existence of black holes". Thanks for the great explanation, now I understand this problem perfectly.

Would you be so kind as to also help me out with the problem that I have in this post?

choice (b) says that a cluster would "preclude other ... phenomena ... that have been observed".

explain to a 10-year-old:
if there's a cluster, then stuff we've actually seen would be impossible.

well, the stuff we've actually witnessed clearly isn't impossible--since we've witnessed it-- so that would rule out the whole cluster hypothesis.

Hi Ron, thanks for the great analogy. Your analogy has helped me to understand how OA:B makes sense. Could you kindly help me explain what is wrong with choice C? When I first attempted this problem, I have eliminated ADE, and was struggling between B and C. I incorrectly chose C at the end and I could not understand where is the logical flaw in C.

Here is my reasoning, please point out where my mistake is. In the second paragraph, there is a part that says "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.". How I interpreted this part is "if center were star clusters then stars would be closely spaced". C says "The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87." Choice C tells us that "stars within the core of M87 is LESS closely spaced than are stars within NGC4258". Because the stars within the core of M87 are LESS closely spaced than are stars within NGC4258, we can therefore conclude that the centre of M87 cannot be star clusters as the skeptics suggested.

Hopefully you can understand how my reasoning worked out, if there is anything that is not clear, I will be happy to come back and add more details.

[RonPurewal]

choice C is not talking about the stars at the center of galaxies, so it's irrelevant to any of this stuff.

MUCH MORE IMPORTANTLY—you are trying to CONNECT the information in the bottom paragraph with the information in the top paragraph to pick that choice. the answers to detail questions DO NOT EVER do this!
RC DETAIL QUESTIONS will NEVER require you to PUT TOGETHER two different pieces of information from different parts of the passage!
if you are even LOOKING in two different areas of the passage, STOP—you are using a kind of reasoning that is NOT used in these questions.

[RickyH486]

choice C is not talking about the stars at the center of galaxies, so it's irrelevant to any of this stuff.

MUCH MORE IMPORTANTLY—you are trying to CONNECT the information in the bottom paragraph with the information in the top paragraph to pick that choice. the answers to detail questions DO NOT EVER do this!
RC DETAIL QUESTIONS will NEVER require you to PUT TOGETHER two different pieces of information from different parts of the passage!
if you are even LOOKING in two different areas of the passage, STOP—you are using a kind of reasoning that is NOT used in these questions.

Good to know! Thanks!

[RonPurewal]

you're welcome.

[Gui]

Astronomers theorize that a black
hole forms when a massive object
shrinks catastrophically under its own
Line gravity, leaving only a gravitational
(5)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.
(10)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
(15)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
(20)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,
(25)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
(30)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
(35)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?
Which of the following, if true, would most clearly undermine the possible explanation for the whirling gas in M87 that is mentioned in the last sentence of the first paragraph?

A The stars in a star cluster at the center of M87 could exert a strong gravitational force without tearing the cluster apart.
B A cluster of stars at the center would preclude the existence of certain other astronomical phenomena that have been observed at the center of M87.
C The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87.
D Only one other galaxy has been observed to contain gas clouds whirling about its center as they do about the core of M87.
E The gravitational force of a cluster of a billion or so dim stars would be sufficient to cause a whirling ring of gas and dust to collect around the center of a galaxy.

Could you please let me know whether my understanding is correct?

Skeptics are basically saying that concentration of mass could be other than black hole such as a cluster of a billion or so dim stars.

To undermine this claim, B is saying that if there was a cluster then gravity causing the gas to whirl could not be available; In other words, a cluster would prevent gas clouds whirling unusually fast about the galaxy’s center.

Why D is wrong?

Even if there is just on other galaxy containing gas clouds whirling its center, it does not rule the cluster of dim stars as an explanation. Is that it?

Many tks!

[Sage Pearce-Higgins]

I'm having trouble understanding your post. There seem to be two different questions being asked here: please could you post the question and answer choices (no need to post the passage again) that you'd like help with?

[Gui]

Hi Sage,

Which of the following, if true, would most clearly undermine the possible explanation for the whirling gas in M87 that is mentioned in the last sentence of the first paragraph?

A) The stars in a star cluster at the center of M87 could exert a strong gravitational force without tearing the cluster apart.

B) A cluster of stars at the center would preclude the existence of certain other astronomical phenomena that have been observed at the center of M87.

C) The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87.

D) Only one other galaxy has been observed to contain gas clouds whirling about its center as they do about the core of M87.

E) The gravitational force of a cluster of a billion or so dim stars would be sufficient to cause a whirling ring of gas and dust to collect around the center of a galaxy.

Why D is wrong? Tks!

[Sage Pearce-Higgins]

It seems like your reasoning may be as follows: it's a really rare occurrence, so that it's unlikely to be the case for M87, so the explanation is undermined.

Notice how, in my reasoning above, I was really vague about what referred to what. I simply said 'it', or 'the explanation', without actually identifying what the problem was really dealing with. In your study time, take time to slow down and figure out exactly what's going on.

To be clear, the question refers to an explanation: that there might be a cluster of stars at the center of M87. Those stars, rather than a black hole, could explain the whirling gases observed in M87.

Answer D states that Only one other galaxy has been observed to contain gas clouds whirling about its center as they do about the core of M87. So it's the whirling gas clouds that are the rarity, not the explanation. We have no information about what explains these strangely whirling gases in the single other case - perhaps it has a black hole, or perhaps a cluster of stars. And, even with that information, we'd need to know a bit more information to compare the two galaxies.