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Astronomers theorize that a black hole forms when a massive

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Astronomers theorize that a black hole forms when a massive [#permalink] New post 17 Feb 2013, 03:20
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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.
Q10: 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?
A. The speed of the gas whirling around the center of M87 is caused by a dense object that is not a black hole.
B. The concentration of mass at the center of M87 is probably a large cluster of dim stars.
C. The presence of a black hole at the center of M87 is the most likely explanation for the speed of the gas whirling about the galaxy’s core.
D. The speed of the gas whirling around the center of M87 is caused by a large concentration of mass at the core of M87.
E. The gravitational influence of a star cluster would not be strong enough to account for the speed of the gas whirling around the core of M87.
[Reveal] Spoiler:
D


Q11: The passage asserts which of the following about the existence of black holes?
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.
[Reveal] Spoiler:
B


Q12: 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.
[Reveal] Spoiler:
B


[Reveal] Spoiler:
DBB
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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 18 Feb 2013, 18:08
Hi Roopika2990,

A good way to approach such questions would be to read the questions first and then read the passage. It might save you time and also help you pinpoint the right answer during the first read itself.

10. We need to find out whether the skeptics would agree with the astronomers on something? If so, what would that be.
A. The speed of the gas whirling around the center of M87 is caused by a dense object that is not a black hole. // This is believed by the skeptics. However, the astronomers don't agree with this. They believe this is a black hole.
B. The concentration of mass at the center of M87 is probably a large cluster of dim stars. // This is again the belief held by skeptics and not by astronomers
C. The presence of a black hole at the center of M87 is the most likely explanation for the speed of the gas whirling about the galaxy’s core. // This is the belief of the astronomers and not skeptics.
D. The speed of the gas whirling around the center of M87 is caused by a large concentration of mass at the core of M87. // This is not directly mentioned as anyone's belief in particular. However this can be inferred from the passage. The passage suggests that the astronomers believe that this large concentration is due to a black hole while the skeptics believe that this large concentration is due to a cluster of stars located at the center. This is the right answer.
E. The gravitational influence of a star cluster would not be strong enough to account for the speed of the gas whirling around the core of M87. // This was proved by the astronomers with reference to NGC 4258

11. We need to find the fact that is asserted by the passage.
A. Astronomers first speculated about the existence of black holes when they observed gas whirling around the center of a particular galaxy. // The paragraph mentions that the astronomers believe that the gas whirling around the center of a particular galaxy is because of the presence of black holes at the center. However, this is not the first reason for speculation.
B. Evidence used to argue for the existence of black holes is indirect, coming from their presumed effects on other astronomical bodies. // This is the right answer. This is suggested in the lines 6-10.
C. Recent observations of certain astronomical bodies have offered proof. // The statement does not confirm what the proof is for. So we cannot be sure about this option or what the option mentions.
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. // The movement of dust and gases suggest this. There is no considerable body of evidence. Hence, wrong
E. Many astronomers are skeptical about certain recent evidence that has been used to argue for the existence of black holes. //astronomers are skeptical about the existence of black holes.

Let me know if you have any more doubts.

roopika2990 wrote:
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.


Q10:
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?
A. The speed of the gas whirling around the center of M87 is caused by a dense object that is not a black hole.
B. The concentration of mass at the center of M87 is probably a large cluster of dim stars.
C. The presence of a black hole at the center of M87 is the most likely explanation for the speed of the gas whirling about the galaxy’s core.
D. The speed of the gas whirling around the center of M87 is caused by a large concentration of mass at the core of M87.
E. The gravitational influence of a star cluster would not be strong enough to account for the speed of the gas whirling around the core of M87.

Q11:
The passage asserts which of the following about the existence of black holes?
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.

Q12:
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.

[Reveal] Spoiler:
DBB

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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 24 Feb 2013, 12:19
Can u ( or ANYONE) pls answer the 12th question ?
need help :( :( :(
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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 25 Feb 2013, 10:46
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Hello,

I am unable to understand the point where the first paragraph ends. I see two possible explanations for the whirling gas in M87, either the presence of a black hole at the center or the presence of a cluster of stars at the center. Could you please clarify where the first paragraph ends so that I could help you?

My guess would be that it ends at "the concentration in M87 might be a cluster of a billion or so dim stars." If this is so, the option that undermines this explanation would be b.
Reason: B states that if the center of the whirling gas consisted of a cluster of stars, then it would prevent certain other known astronomical phenomena from occuring at the center of the M87.

Could you please throw some more light on the question so that I could help you further?

swarman wrote:
Can u ( or ANYONE) pls answer the 12th question ?
need help :( :( :(

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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 26 Feb 2013, 13:30
You rightly spotted the line Kris!! and made me understand well :)
THANKKKKK QQQQQ sooo much <3 !!
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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 26 Feb 2013, 19:06
Kris01 wrote:
Hello,

I am unable to understand the point where the first paragraph ends. I see two possible explanations for the whirling gas in M87, either the presence of a black hole at the center or the presence of a cluster of stars at the center. Could you please clarify where the first paragraph ends so that I could help you?

My guess would be that it ends at "the concentration in M87 might be a cluster of a billion or so dim stars." If this is so, the option that undermines this explanation would be b.
Reason: B states that if the center of the whirling gas consisted of a cluster of stars, then it would prevent certain other known astronomical phenomena from occuring at the center of the M87.

Could you please throw some more light on the question so that I could help you further?

swarman wrote:
Can u ( or ANYONE) pls answer the 12th question ?
need help :( :( :(


Why cant the 12th answer be A?
This statement clearly resolves the doubt which is cast in the passage right!
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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 26 Feb 2013, 20:41
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Hello Aditya,

Q12:
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.

Notice the word "could". So there exist two possibilities.
1. They exert such a gravitational pull in which case, the cluster would be town apart. Since M87 has not been torn apart, this cluster does not exist.
2. They do not exert such a gravitational pull in which case M87 would not be torn apart and would exist. Since, it does exist, a star cluster could be located at its center.

In GMAT, you should pay special attention to words like could,might,may etc which give rise to a yes/no possibility. Often these statements have hidden information.

Hope this helps!

aditya111 wrote:
Kris01 wrote:

I am unable to understand the point where the first paragraph ends. I see two possible explanations for the whirling gas in M87, either the presence of a black hole at the center or the presence of a cluster of stars at the center. Could you please clarify where the first paragraph ends so that I could help you?

My guess would be that it ends at "the concentration in M87 might be a cluster of a billion or so dim stars." If this is so, the option that undermines this explanation would be b.
Reason: B states that if the center of the whirling gas consisted of a cluster of stars, then it would prevent certain other known astronomical phenomena from occuring at the center of the M87.

Could you please throw some more light on the question so that I could help you further?

swarman wrote:
Can u ( or ANYONE) pls answer the 12th question ?
need help :( :( :(


Why cant the 12th answer be A?
This statement clearly resolves the doubt which is cast in the passage right!

_________________

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Re: Astronomers theorize that a black hole forms when a massive [#permalink] New post 16 Mar 2013, 11:52
Pretty interesting question! Thanks for the explanations !
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Re: Astronomers theorize that a black hole forms when a massive   [#permalink] 16 Mar 2013, 11:52
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