Astronomers theorize that a black hole forms when a massive object

Got 3 out of 4
This one was incorrect.

1. 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.

Hi

Let me try to address your query:

4. The passage is primarily concerned with

With a preliminary reading of the passage, we can gather that the first paragraph talks about the existence of black holes and also mentions a possible specific black hole. The second paragraph gives another instance of a possible black hole occurrence, considers an alternative explanation and then rejects it.

With this, let us examine the answer options.

(A) explaining why a particular phenomenon is so rare Neither paragraph talks about black holes being "rare" or otherwise. Eliminate.

(B) criticizing a method used to gather data about a natural phenomenon The second paragraph can be considered a negative view on a topic, however, neither paragraph really criticizes "method used to gather data". Eliminate.

(C) considering possible instances of a particular phenomenon This appears the most promising. The passage considers possible instances of black holes in different galaxies.

(D) distinguishing among several different kinds of natural phenomena The entire passage deals with black holes and, potentially, star clusters only. This cannot be considered "several" kinds of natural phenomena. Eliminate.

(E) questioning a widely accepted explanation for an unusual phenomenon The passage does present an alternate explanation to that of a black hole in a couple of instances. However, we do not know if black holes are a "widely accepted explanation" from the passage. Eliminate.

Hope this helps.

Hi CrackVerbalGMAT - I have doubts about your explanation for E. The passage states the following: "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" This clearly mentions "MOST astronomers believe...black hole..." How is this not widely accepted?

Can someone please provide the explanation of the answers of the first 4 questions

Hi Sam

This (the statement about "most astronomers" believing) is specific to the galaxy M87. The passage goes on to mention another galaxy NGC 4258 and considers an alternative explanation for that particular galaxy. Then it goes on to state that alternative explanation to be unlikely.

Question (4) talks about the primary idea of the passage as a whole. The passage does not only deal with the M87 galaxy and nor is it the primary idea being discussed in the passage. If we were to consider the passage as a whole, it does not clarify whether black holes, in general, are a widely accepted phenomenon. If we were to consider the alternative explanation of star clusters, there too the passage is unclear about whether it is widely accepted. Hence, whichever explanation we consider, the part about it being widely accepted, in general, is unclear.

Hope this helps.

Hi CrackVerbalGMAT GMATNinja

In question 3, can you plz. help me understand what is referred to as "certain other astronomical phenomena that have been observed at the center of M87 in the correct option B?

I’d love to hear any feedback from ppl.

Is there anyone who approaches RC passages and does not write anything down? I’ve tested the following method out on the last few passages I’ve done: I read very carefully and slowly, summarizing each paragraph in my head when I finish.

On several of the last few passages, I’ve observed that my accuracy has increased significantly.

Does anyone else fall in this category? I know GMATNinja mentioned how certain ppl learn and absorb information differently (kinesthetic learners??).

Just wants to know if I’m the only one or perhaps there is at least someone who is in my boat. Every time I take any notes, it takes my focus away from the passage.

Posted from my mobile device

Hi Shanks

The passage does not mention any "other astronomical phenomena that have been observed at the center of M87". However, Q3 asks us to "undermine the possible explanation for the whirling gas in M87 that is mentioned in the last sentence of the first paragraph". This explanation is that, "the concentration in M87 might be a cluster of a billion or so dim stars".

As per option (B), this star cluster would rule out other phenomena which have been observed at the center of M87. Even though we do not know what exactly these phenomena are, we do know (from the answer option) that they have been "observed". Hence, they must definitely be true/exist. If the star cluster would rule these phenomena out, then it clearly "undermines" the presence of the "cluster of a billion or so dim stars", which is only a hypothesized explanation as given in the passage (" the concentration in M87 might be..."). Hence, it is not necessary for us to know what exactly these "phenomena" are, as long as we are sure that they exist.

Hope this clarifies.

Hi CrackVerbalGMAT

Sorry, but i did not get the logic behind it. Let me explain.
The passage hypothesize that the center mass might be cluster of stars.
Now this option B mentions that THIS star cluster rules out CERTAIN other phenomenon observed at the center of M87.
If i look closely, it does not mention any other phenomenon or all phenomenon. That means that "Certain" could be anything.
Moreover, it does not rule out that that cluster of stars are not those which are causing the whirling of gas. In fact, it strenghtens it by stating something else is not there/so it is like ruling out alternate causes.
AndrewN may be you can provide your thoughts too!!!
But the question asks to undermine the statement.

Hi

You are absolutely right that that "Certain" could be anything. However, we do know something for sure about it - that it has been "observed", and therefore must exist for sure. Now, let's call these "certain phenomena" X.

We know that X exists for sure. But, as per option (B), if there were a "cluster of stars at the center of M87" (which, let's note, is only a hypothetical scenario), then X should not exist ("...would preclude the existence of certain other astronomical phenomena..."). Therefore, this undermines the theory that there is a cluster of stars at the center of M87. It takes this simple structure:

If A --> B then (not B) --> (not A)

As per option (B), if A (ie; cluster of stars), then (not X) (ie; no "certain phenomena")
We know X exists [ie; not (not X) is true], therefore (not A) (no cluster of stars) must be true

Also, no alternative explanation is being ruled out. The "cluster of stars" hypothesis is presented to explain the swirling gas. What is being ruled out in option (B) is X - which are not related to the swirling gas in any way.

Hope this clarifies.

In Q3, can you please help explain what exactly option B) means in context of the passage?

Thanks,
Sagar

A few skeptics have argued that the concentration of mass necessary to explain the speed of the whirling gas is not necessarily a blackhole: the concentration in M87 might be a cluster of a billion or so dim stars.

What I could conclude from the skeptics hypothesis:

BLACKHOLE not necessarily CAUSES --> WHIRLING GAS

Hypothesis: A CLUSTER (of a billion or so dim stars) MIGHT CAUSE --> WHIRLING GAS

We need to weaken this hypothesis,
What choice (B) says,

(B) CLUSTER PREVENTS OTHER ASTRONOMICAL PHENOMENA (such as WHIRLING GAS)

i.e. (CLUSTER DOESN'T CAUSE WHIRLING GAS BUT PREVENTS IT FROM HAPPENING.)

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.

Basically we need to weaken the hypothesis that the concentration in M87 might be a cluster of a billion or so dim stars (Let’s say it cluster theory). In other words, if we can infer that it is black hole then also it undermines the cluster theory.

The last lines of passage 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.

It means that starts can be closely spaced due to black hole. If no black hole then cluster would be torn apart

With this understanding when we read A, it says stars could exert a strong gravitational force without tearing cluster apart. So it means it is not due to concentration of billion or so dim stars but because of black hole or other phenomenon; but definitely not due to cluster theory.

With this understanding, why should I not choose A?

Please suggest .



Can I say E supports the conclusion drawn in the last lines of 1st passage instead of weakening? It is actually supporting the cluster theory , is not it? Kindly give your comments.


CrackVerbalGMAT GMATNinja @AndrewN : please give your opinion.
( sorry AndrewN for tagging you again for another passage in a day)

Hello again, mSKR. Both (A) and (E) would strengthen the notion that the whirling gas in M87 could be the result of a cluster of a billion or so dim stars, because each answer choice names a star cluster as the agent that corroborates the astronomical observations. You are correct in saying that you want to weaken the star cluster hypothesis, but you are looking in the wrong place.


If a star cluster would preclude or rule out the possibility of other established astronomical observations, then we can deduce that a star cluster must not exist at the center of the galaxy, as speculated by a few skeptics. If you did not understand the word preclude, I can see how you would overlook this one, but this answer choice is a clear weakener to what we are being asked about, the star cluster hypothesis.

- Andrew

Passage Map:
1. A: Gravty→MO shrink→BH
Eg M87; gvty of conc of mass→whirling gas cloud,
but Skeptics say it cud be stars

2. NGC 4258; speed of rotatn undermines skeptics theory
b/c Closely spaced stars→ cluster torn.
Summary:
Astronomers infer that shrinking of mass under its own gravity leads to formation of Black hole eg M84
but skeptics this concentration of mass could be cluster of stars
eg of NGC 4257 undermines skeptics theory as it's core density was so high that collision of star would tear the cluster.

Question 1.
"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."

Both of them believe that there is large concentration of mass at the core of M87; however they give different reasons for that concentration of mass. Astronomers believe that its due to black hole while skeptics believe that its due to cluster of stars.
ABC eliminated because they quote either of the two believes.
E is out of scope of the passage.
Correct: D

Question 2.
A. out of scope; not given anywhere in the passage that it was first speculation by astronomers about existence of BH.
C: The passage never confirms either of the two theories, it just mentions astronomer's view and reasoning and skeptics counter reasoning.
D. out of scope; consistency with Law of physics not talked anywhere.
E. Close but some words that sounds odd to me are: "recent evidence: no clue that the evidence is recent of very old"
"Many astronomers are skeptical: though passage talks about skeptics but its not necessary that those are
astronomers. They could be anyone with interest and knowledge about space, say a space journalist"
B: Astronomers already had a theory and with example of M87 they presumed that since there is concentration of mass at core and its gravitational effect on near by object, it must be a black hole.
Correct: B

Question 3.
"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."

According to last sentence:
Whirling of gas is due to concentration of mass which need not be a black hole; concentration of mass could be because of cluster of stars.
Lets find an option that undermines above explanation.
A: so it could be due to cluster of stars; its does not undermines rather strengthens.
C: Out of scope; no comparison b/w M87 and NGC.
D: Out of scope, "Only one-?" may or may not be true.
E: Strengthens
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.
if a Cluster of stars would be present at the center than they would be very densely packed, leading to collision and tearing of cluster so apparently concentration of mass would not be present as theorized at the center of M87.
So cluster of stars would prevent existence of concentration of mass
Correct: B

Question 4.
Passage describes a theory and presents its Skepticism and gives examples, considering various reasons for its occurrence
Correct: C

Hi mSKR

You are absolutely correct when you say that we are required to weaken the cluster theory. However, you are incorrect in your assessment of the following:

"It means that starts can be closely spaced due to black hole. If no black hole then cluster would be torn apart"

We are dealing here with two alternate hypotheses which are mutually exclusive ie; it is either a cluster of stars or a black hole. It cannot be that the cluster and black hole both exist and hence "stars can be closely spaced due to black hole" is incorrect.

You are also correct in assessing that option (A) states that a cluster of stars can co-exist. However, the reason that the passage rejects cluster theory is because the stars would rip the cluster itself apart (now, let's call this rip theory). Now, if option (A) is true, rip theory is false and hence cluster theory becomes strengthened, whereas we are required to weaken cluster theory. Therefore, (A) cannot be the correct answer.

You are absolutely correct in the reason for rejecting option (E) - it, too, acts as a strengthener of cluster theory and hence cannot be the correct answer.

Hope this helps.

Thanks svasan05 ( you have changed the name )
I am much clearer now.

When I re-read the passage, I also realized that cluster theory has dependency on density of stars. As we don't know the density of stars in M87, B would make more sense.