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A tiny fraction of binary systems belong to a curious subclass whose r

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A tiny fraction of binary systems belong to a curious subclass whose r  [#permalink]

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New post 20 Oct 2018, 01:12
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A tiny fraction of binary systems belong to a curious subclass whose radiation has a wavelength distribution so peculiar that it long defied explanation. Such systems radiate (5) strongly in the visible region of the spectrum, but some of them do so even more strongly at both shorter and longer wavelengths: in the ultraviolet region and in the infrared and radio regions.

This odd distribution of radiation is best explained by the pairing of a cool red-giant star and an intensely hot small star, known as symbiotic stars, that travel around a common center. Recently two symbiotic-star systems, the first to be detected outside our galaxy, have been observed in the Large Cloud of Magellan.

The spectra of symbiotic stars indicate that the cool red giant is surrounded by a very hot ionized gas which satellite observations finally identified as radiating from an invisible hot companion. It is possible that symbiotic stars represent a transitory phase in the evolution of certain types of binary systems in which a substantial amount of matter transfers from the larger partner to the smaller.

The exact evolutionary course that turns a binary system into a symbiotic one is unknown. The comparative scarcity of known symbiotics in our galaxy suggests that if all binaries of modest mass pass through a symbiotic phase in their evolution, the phase must be extremely brief, perhaps as short as a million years. It is suspected that the evolutionary course of binary stars is predetermined by the initial mass and angular momentum of their gas clouds. Since red giants and Mira variables are thought to be stars with a mass of one or two suns, it seems plausible that the original cloud from which a symbiotic system is formed can consist of no more than a few solar masses of gas.
1. The passage implies that symbiotic star systems differ from other binary systems in which one of the following ways?
(A) Symbiotically paired stars emit a radiation pattern different from that of most binary stars.
(B) In symbiotic star systems, one star is the center of the other's orbit.
(C) Symbiotically paired stars are the only binary stars which are capable of exchanging matter.
(D) Symbiotic star systems are more common than other binary systems.
(E) Symbiotic star systems are the only binary systems that can be detected by satellite- borne instruments.

2. The primary purpose of the passage is to
(A) argue that a great percentage of binary star systems are symbiotic
(B) criticize the theory of symbiotic stars as overly speculative
(C) describe symbiotic stars as a distinct type of binary system
(D) present evidence that binary star systems have evolved from gas clouds
(E) compare symbiotic stars to red giants and Mira variables

3. According to the passage, the radiation emitted by symbiotic stars is distinctive in that it
(A) generates standard wavelengths
(B) consists partly of visible waves
(C) is transferred from one star to its partner
(D) is strongest at the extreme ends of the spectrum
(E) emanates primarily from the larger star

4. The author suggests that
(A) the detection of radiation from an invisible hot companion star prompted scientists to investigate the peculiar ionized gas surrounding cool red giants
(B) small hot stars attach to cool red giants because red giants have a mass of one or two suns
(C) a million years is a brief period of time for the occurrence of many solar events
(D) the only symbiotic star systems to be detected outside of our galaxy are in the Large Cloud of Magellan
(E) if binary stars of modest mass passed through symbiotic phases lasting much more than a million years, it is likely that more of them would have been detected


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New post 20 Oct 2018, 01:15
1

Symbiotic Stars. What Makes It Difficult


This one has a structure common to many GMAT science passages: it introduces a mystery that has puzzled scientists for some time, and then proceeds to document various findings that support a theory meant to explain the mystery. This passage holds together a little better than does "Mitochondria"; the structure is a bit more coherent. The author states the mystery up front, and then methodically presents the theory of symbiotic stars as a possible solution. But the theory contains a ton of details, which, of course, the test makers exploit in the questions. In passages where the details are so technical and abstract, so difficult to connect to everyday experience, you have to pay extra careful attention to the reason why the author includes the details. As always, it behooves you to break the passage down into its key elements.

Key Points of the Passage Purpose and Main Idea

: The author's purpose is to describe the phenomenon of symbiotic star systems; namely, their characteristics and possible origin. The passage is mainly descriptive, but if we had to settle on a main idea, it would sound something like this: The nature of symbiotic stars helps explain certain strange radiation distribution patterns that have long puzzled scientists.

Paragraph Structure

:
Paragraph 1 introduces a mystery that had "long defied explanation": certain binary star systems exhibiting quirky radiation patterns stronger on the extreme sides of the spectrum.
Paragraph 2 gets right to the explanation. Evidently, this pattern can be explained by the pairing of a cool big star and a hot little star. These two types of stars are attached to one another through a common center; hence, the notion of symbiosis. The paragraph goes on to explain how the big and small stars are detected, and where a few of these things have been found. An 800 test taker paraphrases the author's ideas into the simplest terms possible.
Paragraph 3 throws in some more details about this partnership, and suggests that symbiotic stars represent a phase in the evolution of certain binary systems. There's no need to assimilate every detail just yet. You should simply mark this paragraph as the place where some of the mechanisms of these star systems are laid out. In the last paragraph, the author continues this speculation as to the evolution of symbiotic stars, employing a good deal of scientific terminology in the process. Again, not to panic—get the gist and you'll know where to return to reread, if a question demands it. An 800 test taker knows that all the information she needs to answer the questions is right there in the passage. She therefore jots down the paragraph topics in her roadmap to help her hunt for answers later.
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New post 20 Oct 2018, 01:19
1

ANSWERS AND EXPLANATIONS 1.A, 2.С, 3.D, 4.E



1

. (A) Paragraph 1 states that symbiotic stars are distinctive from other binary star systems in the pattern of radiation that they emit. In fact, this is the basis of the whole mystery, right? So (A) must be true. Sometimes the first question on even a difficult passage is very straightforward. (B) is wrong because the second paragraph specifically mentions that both stars in a symbiotic system travel around a common center.
(C) and (E) are way too broad; neither one is implied by anything in the passage, which doesn't speculate on what's the case out there in the universe in general. (D) contradicts the first paragraph, where we're told that symbiotic stars make up a tiny fraction of binary systems.

2

. (C) Since the gist of the passage is that symbiotic stars represent a special type of binary star system— one with a bizarre radiation pattern—choice (C) is correct. And the neutral verb "describe" fits the author's method to a T. The same, however, can't be said of choices (A) and (B), which we can dismiss on the basis of their verbs alone. The author doesn't "argue" or "criticize" anything in this passage; he simply `describes a phenomenon.
An 800 test taker pays very careful attention to the verbs in answer choices, especially in "primary purpose" questions.
(D) The theory that binary stars are born in gas clouds is a detail from the last paragraph, certainly not the passage's main point, so (D) cannot describe the primary purpose of this passage. Notice how it doesn't even mention the main concept of the passage, symbiotic stars. It's very hard to describe the author's purpose without a reference to the passage's central character. (E)'s out because the author doesn't even tell us what Mira variables are, let alone compare them to symbiotic stars.

3

. (D) Next up is a detail question that relates to the main idea. The last sentence of the first paragraph states that the type of binary system later defined as symbiotic radiates "even more strongly at both shorter and longer wavelengths" than in the middle of the spectrum. Choice (D) is a near perfect paraphrase of this.
(A) We know from paragraph 1 that the radiation emitted is remarkable because it is not standard, at least within the context of binary systems more generally.
(B) cannot be correct. Although some of the radiation from symbiotic systems is visible, that's not what makes these systems' radiation patterns "distinctive."
(С) picks up on some of the language in the third paragraph by discussing a transfer from one star to the other. The problem with this answer choice is that matter, not radiation, is what transfers between the stars. While paragraph 3 says that the "invisible hot companion" radiates gas, it does not suggest that such radiation is transferred to the cool red giant. Clearly this choice requires you to be very careful.
(E) The passage never suggests that the radiation primarily emanates from the larger star, and even if we somehow made this leap, it would still be incorrect to say that this is what makes their radiation distinctive. None of this is hinted at in the passage.

4

. (E) Next up is an open-ended Inference question with no clues as to what the test makers are after, so we have no choice but to wade into the choices, looking for the one that's supported by the hard facts of the passage. (A) bollixes up the order of things; in fact, it pretty much gets it backwards. In paragraph 3 we learn that the ionized gas surrounding the cool red giant looked weird to scientists for decades before radiation from the invisible hot companion was discovered, so it can't be the detection of radiation that prompted scientists to look into the matter.
(B) tries to fashion a causal relationship out of two facts in the passage. True, the small hot stars attach to the big cool ones, and yes, we're told in paragraph 4 that red giants have a mass of one or two suns. But what has one thing to do with the other? Nothing, as far as the passage suggests, so (B) is out.
An 800 test taker is suspicious of choices that attempt to link two or more elements from different parts of the passage.
(C) A million years may not seem so "brief" to us, but according to the passage that's a relatively short period of time for a symbiotic phase. Regardless, other solar events are outside the scope of the passage, so there's no way we can judge from the material at hand the time it takes for these to occur. For all we know, a million years is a long time for most solar events; all we know about is symbiotic stars.
(D) erroneously plays off the Magellan detail in paragraph 2. just because a few symbiotics were found over there doesn't in any way suggest that the only symbiotic stars outside our galaxy are in the Large Cloud of Magellan.
(E) That leaves (E), which must be correct. In paragraph 4, the author says that the small number of symbiotics detected in our galaxy suggests that the symbiotic phase is brief—"perhaps as short as a million years." Evidently, the author sees a link between the length of the phase and our ability to detect symbiotic stars. Therefore, it's reasonable to infer that if these phases were much longer, we'd probably detect more of them.

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