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Until recently astronomers have been puzzled by the fate of red giant and supergiant stars. When the core of a giant star whose mass surpasses 1.4 times the present mass of our Sun (M⊙) exhausts its nuclear fuel, it is unable to support its own weight and collapses into a tiny neutron star. The gravitational energy released during this implosion of the core blows off the remainder of the star in a gigantic explosion, or a supernova. Since around 50 percent of all stars are believed to begin their lives with masses greater than 1.4M⊙, we might expect that one out of every two stars would die as a supernova. But in fact, only one star in thirty dies such a violent death. The rest expire much more peacefully as planetary nebulas. Apparently most massive stars manage to lose sufficient material that their masses drop below the critical value of 1.4 M⊙ before they exhaust their nuclear fuel.
Evidence supporting this view comes from observations of IRC+10216, a pulsating giant star located 700 light-years away from Earth. A huge rate of mass loss (1 M⊙ every 10,000 years) has been deduced from infrared observations of ammonia (NH3) molecules located in the circumstellar cloud around IRC+10216. Recent microwave observations of carbon monoxide (CO) molecules indicate a similar rate of mass loss and demonstrate that the escaping material extends outward from the star for a distance of at least one light-year. Because we know the size of the cloud around IRC+10216 and can use our observations of either NH3 or CO to measure the outflow velocity, we can calculate an age for the circumstellar cloud. IRC+10216 has apparently expelled, in the form of molecules and dust grains, a mass equal to that of our entire Sun within the past ten thousand years. This implies that some stars can shed huge amounts of matter very quickly and thus may never expire as supernovas. Theoretical models as well as statistics on supernovas and planetary nebulas suggest that stars that begin their lives with masses around 6 M⊙ shed sufficient material to drop below the critical value of 1.4 M⊙. IRC+10216, for example, should do this in a mere 50,000 years from its birth, only an instant in the life of a star.
But what place does IRC+10216 have in stellar evolution? Astronomers suggest that stars like IRC+10216 are actually “protoplanetary nebulas”—old giant stars whose dense cores have almost but not quite rid themselves of the fluffy envelopes of gas around them. Once the star has lost the entire envelope, its exposed core becomes the central star of the planetary nebula and heats and ionizes the last vestiges of the envelope as it flows away into space. This configuration is a full-fledged planetary nebula, long familiar to optical astronomers.
GRE阅读全攻略同步练习题目：
21. The primary purpose of the passage is to
(A) offer a method of calculating the age of circumstellar clouds
(B) describe the conditions that result in a star’s expiring as a supernova
(C) discuss new evidence concerning the composition of planetary nebulas
(D) explain why fewer stars than predicted expire as supernovas
(E) survey conflicting theories concerning the composition of circumstellar clouds
22. The passage implies that at the beginning of the life of IRC+10216, its mass was approximately
(A) 7.0 M⊙
(B) 6.0 M⊙
(C) 5.0 M⊙
(D) 1.4 M⊙
(E) 1.0 M⊙
23. The view to which line 18 refers serves to
(A) reconcile seemingly contradictory facts
(B) undermine a previously held theory
(C) take into account data previously held to be insignificant
(D) resolve a controversy
(E) question new methods of gathering data
24. It can be inferred from the passage that the author assumes which of the following in the discussion of the rate at which IRC+10216 loses mass?
(A) The circumstellar cloud surrounding IRC+10216 consists only of CO and NH3 molecules.
(B) The circumstellar cloud surrounding IRC+10216 consists of material expelled from that star.
(C) The age of a star is equal to that of its circumstellar cloud.
(D) The rate at which IRC+10216 loses mass varies significantly from year to year.
(E) Stars with a mass greater than 6 M⊙ lose mass at a rate faster than stars with a mass less than 6 M⊙ do.
25. According to information provided by the passage, which of the following stars would astronomers most likely describe as a planetary nebula?
(A) A star that began its life with a mass of 5.5 M⊙, has exhausted its nuclear fuel, and has a core that is visible to astronomers
(B) A star that began its life with a mass of 6 M⊙, lost mass at a rate of 1 M⊙ per 10,000 years, and exhausted its nuclear fuel in 40,000 years
(C) A star that has exhausted its nuclear fuel, has a mass of 1.2 M⊙, and is surrounded by a circumstellar cloud that obscures its core from view
(D) A star that began its life with a mass greater than 6 M⊙, has just recently exhausted its nuclear fuel, and is in the process of releasing massive amounts of gravitational energy
(E) A star that began its life with a mass of 5.5 M⊙, has yet to exhaust its nuclear fuel, and exhibits a rate of mass loss similar to that of IRC+10216
26. Which of the following statements would be most likely to follow the last sentence of the passage?
(A) Supernovas are not necessarily the most spectacular events that astronomers have occasion to observe.
(B) Apparently, stars that have a mass of greater than 6 M⊙ are somewhat rare.
(C) Recent studies of CO and NH3 in the circumstellar clouds of stars similar to IRC+10216 have led astronomers to believe that the formation of planetary nebulas precedes the development of supernovas.
(D) It appears, then, that IRC+10216 actually represents an intermediate step in the evolution of a giant star into a planetary nebula.
(E) Astronomers have yet to develop a consistently accurate method for measuring the rate at which a star exhausts its nuclear fuel.
27. Which of the following titles best summarizes the content of the passage?
(A) New Methods of Calculating the Age of Circumstellar Clouds
(B) New Evidence Concerning the Composition of Planetary Nebulas
(C) Protoplanetary Nebula: A Rarely Observed Phenomenon
(D) Planetary Nebulas: An Enigma to Astronomers
(E) The Diminution of a Star’s Mass: A Crucial Factor in Stellar Evolution
GRE阅读全攻略同步练习真题答案：
D B A B A D E
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