During the past 40 years, the star FG Sagittae has changed color from

Question

Source :  GMATPrep EP 2

This passage is based on an article written in the mid-1990s.

During the past 40 years, the star FG Sagittae has changed color from blue to yellow—direct evidence that its temperature has dropped dramatically. Its cooling is probably due to a rapid increase in its size, an inference that is indirectly supported by  two independent pieces of observational evidence . Like many stars, FG Sagittae rhythmically pulsates, beating like a heart, and over the past 30 years the period of this pulsation has noticeably lengthened. Given that a star's pulsation period is determined by its radius, FG Sagittae must have gotten larger. Moreover, FG Sagittae's total luminosity—the amount of light it emits over all wavelengths—has been observed to remain constant despite the star's plummeting temperature. It is known that a star's luminosity is the product of its temperature and its radius multiplied together: if one of the two factors has decreased while over the same period luminosity has remained constant, the other factor must have increased.

One interesting sidelight: even though the total amount of light FG Sagittae emits has not changed, the star now appears brighter in the sky than it did 40 years ago. Then, being a blue star, it emitted most of its light at ultraviolet wavelengths, and ultraviolet light is invisible to human eyes.   1 
 A 
1. According to the passage, if the luminosity of a given star has remained constant while its temperature has decreased, what will also necessarily be true of the star?
 
(A) The star will have increased in size.
 
(B) The star's apparent brightness will have substantially increased.
 
(C) The star will have changed from blue to yellow in color.
 
(D) The star's pulsation period will have shortened somewhat.
 
(E) The star will have begun to emit less visible light than it once did.

2 
 C 
2. It can be inferred from the passage that the phrase “two independent pieces of observational evidence” (see highlighted text) refers to observations made of which of the following?
 
(A) FG Sagittae's apparent brightness in the sky and its temperature
 
(B) FG Sagittae's color and its temperature
 
(C) The amount of light FG Sagittae emits across all wavelengths and its pulsation period
 
(D) FG Sagittae's radius and its color
 
(E) The amount of light FG Sagittae emits across all wavelengths and the amount it emits at ultraviolet wavelengths

3 
 B 
3. The author of the passage is primarily concerned with
 
(A) analyzing the effects of a particular phenomenon
 
(B) presenting evidence in favor of a given explanation
 
(C) reporting new data that undermines an accepted view
 
(D) evaluating the arguments in favor of a given hypothesis
 
(E) presenting an interesting sidelight on a complex phenomenon

New Scientist, Magazine issue 1786, published 14 September 1991

Life and times of a chameleon star: Since 1955, astronomers have watched the star FG Sagittae turn from blue to yellow. This rapid change has given them the chance to study a star’s evolution in their own lifetime

GMATNinja · Accepted Answer

Question 3

To untangle question #3, let's first recall what we've read and why the author wrote it:

The author first tells us that FGS has changed color, which means its temperature has dropped. 
  Then the author claims that this drop in temperature is probably due to a rapid increase in size. 
  Then the author devotes over half of the passage to discussing two pieces of observational evidence in support of that claim. 
  Lastly, the author adds a fun fact about the color of stars, mentioning it as "one interesting sidelight."

So why did the author write this thing? To walk us through evidence supporting the author's explanation of why FGS' temperature dropped. Let's see which answer choice matches up, and eliminate anything that veers away from our understanding of the purpose.

Sure, there's some analysis here, but is the author's reason for writing this passage to analyze  the  effects  of a particular phenomenon ?

No. The phenomenon being addressed here is FGS' drop in temperature. The author is writing to analyze the  causes , not the effects, of this phenomenon. So eliminate choice (A).

This is an extremely good match for what we know about the passage. The author offers an explanation that points to a rapid increase in size as the cause of FGS' drop in temperature:

"Its cooling is probably due to a rapid increase in its size"

The author devotes most of the passage presenting evidence to support this explanation. Let's keep (B) around.

What's the accepted view, and how is it being undermined? Nothing like this ever appears in the passage, so eliminate (C).

Is the author evaluating  arguments ? No. The author did not sit down and write this passage in order to address multiple existing arguments; rather, the author sets out to present a single argument and provide the evidence supporting that one argument. At best, the author is evaluating  evidence . This distinction is enough for us to eliminate (D).

The very phrase "sidelight" implies that this is a bonus piece of information that's not relevant to the core of the passage. If we remove the final paragraph entirely, the overall point of this passage remains totally unchanged and perfectly clear. Delivering this side fact is not the primary reason this passage was written, so eliminate (E).

(B) remains the best answer choice.

GMATNinja · Answer

Let's be absolutely sure that we're answering the question we were asked:

Here's the full statement where this highlighted text is written:

We can't settle for an answer choice just because it happens to include two things mentioned by the author anywhere in the passage. The correct choice will specifically give us the  two independent pieces of observational evidence  that the author brings up  in order to support the claim that FG Sagittae's cooling is due to a rapid increase in size.

So what are those two independent pieces of observational evidence? Well, the author presents them immediately after the highlighted text:

1. " Like many stars,   FG Sagittae rhythmically pulsates,   beating like a heart,   and over the past 30 years the period of this pulsation has noticeably lengthened.   Given that a star's pulsation period is determined by its radius, FG Sagittae must have gotten larger. "

2. " Moreover,   FG Sagittae's total luminosity—the amount of light it emits over all wavelengths—has been observed to remain constant   despite the star's plummeting temperature. It is known that a star's luminosity is the product of its temperature and its radius multiplied together: if one of the two factors has decreased while over the same period luminosity has remained constant, the other factor must have increased. "

Now let's see which choice matches the text.

A star's brightness is something we can generally observe, and a star's temperature is a factor of its luminosity. But neither of these are the specific pieces of observational evidence that were presented by the author. Eliminate (A).

FG Sagittae's changes in color and temperature are  outcomes  that the author is trying to explain, not  evidence  that the author uses to support the explanation. We weren't asked to repeat the final observations of what has happened to FG Sagittae. We were asked to identify the observational evidence that led to a change in color, a drop in temperature, and an increase in size. That's why we eliminate (B).

This is a spot-on match for what the author's written. The author infers that FG Sagittae's cooling is probably due to a rapid increase in size, and this phenomenon is supported by measuring the star's pulsations (which are determined by their radius) and the star's total luminosity (which is a product of radius and temperature). Let's keep (C) and see if any other choices could possibly be a better option.

Again, color is one of the outcomes the author seeks to explain, not one of the pieces of evidence that supports the author's explanation. And FG Sagittae's radius is a smaller component of luminosity. Either of these are good reasons on their own to reject (D) and stick with (C).

This choice correctly identifies one of the pieces of observational evidence. Then it repeats a smaller component of that same evidence. It completely misses the observation of FG Sagittae's pulsation.

(C) remains the best answer choice because it's the only one that directly and accurately answers the question.

I hope this helps!

GMATNinja · Answer

Passage breakdown 
In the first paragraph (P1), the author:

Makes a claim: FG Sagittae's decrease in temperature was caused by its increase in size.  
 Provides two pieces of indirect, observational evidence to support this claim.

In the second paragraph, the author:

Describes a side effect of a change mentioned in P1.

For more on the process of breaking down RC passages, check out  this article  and  our live RC videos .

Explanations for individual questions 
   Question 1  
  Question 2  
  Question 3

SajjitaKundu · Answer

Can someone explain how is the correct answer to Q2, C? I got B. The previous line to the one highlighted says that the reasons why the temperature has decreased rapidly is because of the increasing radius. So I am inferring it is something to do with the temperature?

SonalSinha803 · Answer

Hello, I am not an expert, but i would be happy to help

Just after the highlighted text, the author puts the observation. The author talks about the pulsation period which has been lengthened and after that about its luminosity-the amount of wavelengths......- which has remained constant. Thus, answer is C. B talks about color and temperature, which only comes into light when talking about luminosity remaining constant and temperature increasing. Thus, B is too vague and narrow to be the answer.

Terabyte · Answer

Can someone please explain question #3? How to differentiate between "Presenting" and "Evaluating" in this case? 
Why can't we say that he is evaluating pieces of evidence?

pandeyashwin · Answer

in line 3 : "Its cooling is probably due to..."

And , "evaluating the argument" would mean to agree/disagree with an already given explanation which is not happening here.

Pranalirewdekar · Answer

3. The author of the passage is primarily concerned with

(A) analyzing the effects of a particular phenomenon

(B) presenting evidence in favor of a given explanation

(C) reporting new data that undermines an accepted view

(D) evaluating the arguments in favor of a given hypothesis

(E) presenting an interesting sidelight on a complex phenomenon

can someone please explain why E is not valid answer.

Nikhil · Answer

I think option E is a trap answer. The author presents an interesting sidelight only at the start of the paragraph 2. This cannot the primary purpose of the passage.

In first paragraph, the author talks about a phenomenon("star FG Sagittae has changed color from blue to yellow") and provides an explanation for the phenomenon i.e "direct evidence that its temperature has dropped dramatically". He then presents more evidence that supports the given explanation in the rest of the paragraph.

So, the primary purpose "presenting evidence in favor of a given explanation" makes more sense than the rest of the answer choices.

dabaobao · Answer

Solution

Got 3/3 correct in 7 min.

hassu13 · Answer

GMATNinja 
In the qstn 2 , can you pl explain how option D is not correct answer. In the passage it is clearly written that the star`s cooling is probably due to a rapid increase in its size, an inference that is indirectly supported by two independent pieces of observational evidence. Hence the observational pieces must say something about the size(radius). Also, In the first few lines points regarding the change of colour of star is also written.

hassu13 · Answer

GMATNinja 
Also, can you pl explain me how A and D are wrong for Primary purpose question?

hassu13 · Answer

For the primary purpose question, can experts pl explain why option A is not the answer.
How can we differentiate that no analysis is presented in the passage. How would the passage be different if A were to be the correct answer. Also, in the correct option B it has been said given explanation. How can we firmly conclude that the explanation is given. Isnt this modifier making this option B wrong.

NehaJainGMAT · Answer

GMATNinja  can you explain the answer to the first question?

GMATNinja · Answer

Sure! Here's the question again:

We're basically being asked to locate where in the passage the author describes the relationship between the luminosity and temperature of a star, then either fill in or infer the missing information.

Well, here's the portion of the passage where the author describes the relationship between the luminosity and the temperature of a star:

OK, so we've confirmed the following:

Luminosity = temperature x radius. 
  If temperature has decreased while over the same period luminosity has remained constant, then radius must have increased. 
  If radius has decreased while over the same period luminosity has remained constant, then temperature must have increased.

Going back to the question: We're told right away that temperature has decreased, while luminosity has remained constant.

Therefore, according to the passage, the radius of this star must have increased. Answer choice (A) ("The star will have increased in size") is the only choice that implies an increase in the radius of the star, while every other answer choice is not backed up by evidence from the passage.

I hope this helps! (Or if you like awful jokes: I hope this, um, sheds light on the question?)

Will2020 · Answer

GMATNinja Can I eliminate D because of the word "evaluating"? Many tks!

Zubeen · Answer

We can't settle for an answer choice just because it happens to include two things mentioned by the author anywhere in the passage.  The correct choice will specifically give us the two independent pieces of observational evidence that the author brings up in order to support the claim that FG Sagittae's cooling is due to a rapid increase in size.

So what are those two independent pieces of observational evidence? Well, the author presents them immediately after the highlighted text:

1. "Like many stars, FG Sagittae rhythmically pulsates, beating like a heart, and over the past 30 years the period of this pulsation has noticeably lengthened. Given that a star's pulsation period is determined by its radius, FG Sagittae must have gotten larger."

2. "Moreover, FG Sagittae's total luminositythe amount of light it emits over all wavelengthshas been observed to remain constant despite the star's plummeting temperature. It is known that a star's luminosity is the product of its temperature and its radius multiplied together: if one of the two factors has decreased while over the same period luminosity has remained constant, the other factor must have increased."

Now let's see which choice matches the text.

Quote:
(A) FG Sagittae's apparent brightness in the sky and its temperature

A star's brightness is something we can generally observe, and a star's temperature is a factor of its luminosity. But neither of these are the specific pieces of observational evidence that were presented by the author. Eliminate (A).

Quote:
(B) FG Sagittae's color and its temperature

FG Sagittae's changes in color and temperature are outcomes that the author is trying to explain, not evidence that the author uses to support the explanation. We weren't asked to repeat the final observations of what has happened to FG Sagittae. We were asked to identify the observational evidence that led to a change in color, a drop in temperature, and an increase in size. That's why we eliminate (B).

Quote:
(C) The amount of light FG Sagittae emits across all wavelengths and its pulsation period

This is a spot-on match for what the author's written. The author infers that FG Sagittae's cooling is probably due to a rapid increase in size, and this phenomenon is supported by measuring the star's pulsations (which are determined by their radius) and the star's total luminosity (which is a product of radius and temperature). Let's keep (C) and see if any other choices could possibly be a better option.

Quote:
(D) FG Sagittae's radius and its color

Again, color is one of the outcomes the author seeks to explain, not one of the pieces of evidence that supports the author's explanation. And FG Sagittae's radius is a smaller component of luminosity. Either of these are good reasons on their own to reject (D) and stick with (C).

Quote:
(E) The amount of light FG Sagittae emits across all wavelengths and the amount it emits at ultraviolet wavelengths

This choice correctly identifies one of the pieces of observational evidence. Then it repeats a smaller component of that same evidence. It completely misses the observation of FG Sagittae's pulsation.

(C) remains the best answer choice because it's the only one that directly and accurately answers the question.

I hope this helps!

armaankumar · Answer

P1:  Presenting evidence for FG’s temp. drop (Color change, change in size-> Pulses and Luminosity)
 P2:  Provides an observation connected to total luminosity evidence in P1
All this passage is about is presenting evidence for a star’s change in temperature and size

1. According to the passage, if the luminosity of a given star has remained constant while its temperature has decreased, what will also necessarily be true of the star?
Type:  Detail
 Focus of the question:  Luminosity and temperature decrease

(A) The star will have increased in size.
 The last sentence of the first paragraph says “if one of the two factors has decreased while over the same period luminosity has remained constant, the other factor must have increased.”
The other factor is size=>Radius has increased

(B) The star's apparent brightness will have substantially increased.
 Even though the second paragraph says “the total amount of light FG Sagittae emits has not changed, the star now appears brighter in the sky than it did 40 years ago.”
It is not necessarily true for the case of every star.

(C) The star will have changed from blue to yellow in color.
 Even though FG Sagittae changed color from blue to yellow, it is not neccesarily true that every star will change from blue to yellow

(D) The star's pulsation period will have shortened somewhat.
 It is true that a star's pulsation period is determined by its radius. However, we know that the star’s pulsation period increasing when it’s size increases, and the radius of the star increases if the luminosity of a given star has remained constant while its temperature has decreased.

(E) The star will have begun to emit less visible light than it once did.
 Even though the second paragraph says “the total amount of light FG Sagittae emits has not changed, the star now appears brighter in the sky than it did 40 years ago.”
This answer choice may or may not be true for any star.

armaankumar · Answer

2. It can be inferred from the passage that the phrase “two independent pieces of observational evidence” (see highlighted text) refers to observations made of which of the following?
Type:  Detail
 Focus of the question:  Observational Evidence

We can see that the first paragraph says “Its cooling is probably due to a *rapid increase in its size*, an inference that is indirectly supported by two independent pieces of observational evidence”

The two observational evidences are the star’s pulsation period and its total luminosity.

(A) FG Sagittae's apparent brightness in the sky and its temperature
 Trap: Distortion
The brightness of the star makes up a part of total luminosity but is not what the observational evidence is directly referring to.

(B) FG Sagittae's color and its temperature
 The observational evidence is supporting an inference about the size of the star not the color.

(C) The amount of light FG Sagittae emits across all wavelengths and its pulsation period
 This answer choice covers both the observational evidences.

(D) FG Sagittae's radius and its color
 Trap: Distortion
The observational evidences refer to the pulsation period and the total luminosity from which scientists infer about the star’s radius and its color.

(E) The amount of light FG Sagittae emits across all wavelengths and the amount it emits at ultraviolet wavelengths
 Trap: Half Right answer choice
This answer choice starts of correct but the observation evidence is not referring to the amount of uv wavelengths emitted.

During the past 40 years, the star FG Sagittae has changed color from

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GMAT Reading Comprehension (RC) Questions

During the past 40 years, the star FG Sagittae has changed color from

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards