尊敬的用户,为提升网站的使用体验,GRE考满分已全面改版升级,优化了所有的栏目页面,建议您体验新版。 进入新版

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 1.4M⊙. 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.
According to information provided by the passage, which of the following stars would astronomers most likely describe as a planetary nebula?
  • AA 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
  • BA 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
  • CA 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
  • DA 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
  • EA 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
显示答案

网友解答

当前版本由luofubin更新于2015-05-18 14:26:04

感谢由luofubin等1位用户对此题目的解答所做出的贡献。

题目问的是most likely describe as a planetary nebula,因此应该对应文中的full-fledged planetary nebula,而不是protoplanetary nebulas。c选项中surrounded by a circumstellar cloud that obscures its core from view,根据最后一段的描述,不是full-fledged planetary nebula的特征,故排除C.
当前版本感谢由luofubin等的贡献

题目讨论

如果对题目有任何的疑惑,欢迎在这里提出来,大家会帮你解答的

大家都在做

  • [填空]TC-8f416k32%人做对

    Even those who disagreed with Carmen’s views rarely faulted her for expressing them, ...

  • [等价]SE-b67tkh52%人做对

    Despite their initial fears, most environmentalists now concede that the artificial ree...

  • [填空]TC-41417k40%人做对

    Until the current warming trend exceeds the range of normal climatic fluctuations, ther...

  • [填空]TC-d1418k39%人做对

    While she initially suffered the fate of many pioneers—the incomprehension of her col...

  • [填空]TC-7e419k29%人做对

    Having no sense of moral obligation, Shipler was as little subject to the(i) o...

关于GRE®
  • 考试介绍
  • 考试题型
  • 评分标准
  • 如何备考
App下载
关注我们
联系我们
  • 考满分是专注于出国考试在线科学备考的留学生口碑
  • 品牌。已经服务了超过100万学生 ,累积学员备考记
  • 录超过10亿条,是出国留学考试领域新锐培训机构。
反馈意见

题目收藏成功!

你还可以通过添加标签来管理收藏的题目

标签:(多个标签用英文逗号隔开)
常用标签:

请描述一下这个错误: