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## Any thoughts on this RC???

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ssgmatter Legendary Member
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#### Any thoughts on this RC???

Sat Jul 10, 2010 8:40 am
Astronomers theorize that a black
hole forms when a massive object
shrinks catastrophically under its own
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 gal-
axyâ€™s center. Most astronomers
believe that the large concentration
(15)of mass at the galaxyâ€™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 clusterâ€™s
existing in NGC 4258 was severely
undermined when astronomers mea-
sured the speed of a ring of dust and
gas rotating close to the galaxyâ€™s
(30)center. From its speed, they calcu-
lated that the coreâ€™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.

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.

I have made that part black for which this question is asked for

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kevincanspain GMAT Instructor
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Sat Jul 10, 2010 2:11 pm
We have to look for a reason to rule out the possibility that there is a cluster of dim stars at the center of the galaxy. B implies that the existence of such a cluster would be incompatible with the phenomena that have been observed. Thus to accept the possibility would be to deny the validity of these observations. B is correct

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ssgmatter Legendary Member
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Sat Jul 10, 2010 7:37 pm
kevincanspain wrote:
We have to look for a reason to rule out the possibility that there is a cluster of dim stars at the center of the galaxy. B implies that the existence of such a cluster would be incompatible with the phenomena that have been observed. Thus to accept the possibility would be to deny the validity of these observations. B is correct
Thankyou Kevin.

However, I dont understand what other phenomena it is talking about...B says that existence of such cluster would be incompatible with other phenomena that have been observed..is it the blackhole??

Please explain a little more in details

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rockeyb Legendary Member
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Sat Jul 10, 2010 8:19 pm
The passage suggests that there are two reasons for whirling gas :

1. Gravitational force exerted by black hole .

2. Clusters of stars .

We have to find reasons that would undermine the conclusion the reason 2 .

B dose this clearly by saying that if Clusters of stars exist then other phenomenon can not exist .

This other phenomenon is reason number 1.

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kevincanspain GMAT Instructor
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Sat Jul 10, 2010 11:25 pm
ssgmatter wrote:
kevincanspain wrote:
We have to look for a reason to rule out the possibility that there is a cluster of dim stars at the center of the galaxy. B implies that the existence of such a cluster would be incompatible with the phenomena that have been observed. Thus to accept the possibility would be to deny the validity of these observations. B is correct
Thankyou Kevin.

However, I dont understand what other phenomena it is talking about...B says that existence of such cluster would be incompatible with other phenomena that have been observed..is it the blackhole??

Please explain a little more in details
I have no idea what other phenomena to which they are referring either. However, it doesn't matter: these phenomena should be taken as fact according to B, and they would not occur if this cluster existed

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Optimus Prime Senior | Next Rank: 100 Posts
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Mon Jul 26, 2010 11:02 pm
Kevin
i am confused between B and E

Wouldnt E be a better option as it explains that the gravitational force of the cluster will stop the whirling of the gas and make it (gas) collect at the centre of the Galaxy.

Please explain

surfer123 Junior | Next Rank: 30 Posts
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Thu Nov 11, 2010 2:43 pm
Optimus Prime wrote:
Kevin
i am confused between B and E

Wouldnt E be a better option as it explains that the gravitational force of the cluster will stop the whirling of the gas and make it (gas) collect at the centre of the Galaxy.

Please explain
(E) would help prove the statement rather than undermine.

frank1 Legendary Member
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Fri Nov 12, 2010 2:16 am
ssgmatter wrote:
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.

I have made that part black for which this question is asked for
Like enquility data sufficiency question rephrase
Rephrase of question is
we have to show it is indeed black hole not a cluster...
A:It says cluster can do it with out being torn,which take part of people who say it is cluster.
B:B tries to say if it were cluster something is would have been not possible...so it may not be cluster
C:Netural in this case...donot add any info.
D easy kill out of scope
E:It takes the part of people who beleive it is cluster.....saying when many star come together to form custler this phoenomena could happen(it should not be black hole for this to happen..)

So IMO B

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maddy2u Senior | Next Rank: 100 Posts
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Mon Dec 20, 2010 9:02 am
IMO, the answer is B .

navami Legendary Member
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Sat Mar 05, 2011 9:38 am
OA please. I picked A

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shrikantpatel Newbie | Next Rank: 10 Posts
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Fri Apr 15, 2011 6:48 am
IMHO B.

MM_Ed Senior | Next Rank: 100 Posts
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Sat Jul 02, 2011 6:10 pm
B, of course. If the cluster hypothesized at the end of the first paragraph were to preclude any phenomena already observed, then the hypothesis would fail.

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olegpoi Senior | Next Rank: 100 Posts
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Fri Aug 26, 2011 1:03 pm
Imo D

To prove that black hole make gases whirl we need to find similarity between m87 and NGC 4258

parul9 Master | Next Rank: 500 Posts
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Thu Sep 29, 2011 9:57 am
OA please!
Why can't it be C - The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87?

parul9 Master | Next Rank: 500 Posts
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Thu Sep 29, 2011 9:58 am
I was almost settled with B.. and then as always, I over analysed! X-(

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