GRE阅读题目解析:超巨星身世之谜

　　P2

　　1

　　Until recently astronomers have been puzzled by the fate of red giant and supergiant stars.

　　直到最近，天文学家才解开了红巨星超巨星的身世之谜。

　　2

　　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.

　　一旦巨星的质量超过了当下太阳质量(M⊙)的1.4倍，且耗尽了核聚变燃料，就不能支撑自身的重量，进而坍缩成一颗小中子星。

　　(一些其实没用的背景知识：

　　红巨星 [red giant]

　　A star of great size and brightness that has a relatively low surface temperature.

　　红巨星：体积巨大、十分明亮、表面温度相对较低的星

　　(美国传统词典AHD双解)

　　巨星 [giant star]

　　同质量和温度相比，半径较大的恒星;由于这类恒星的辐射表面相应也较大，亮度也就较大。巨星的一些次型是：具有更大半径和亮度的超巨星;温度虽低但亮度很大的红巨星;半径和亮度略小的亚巨星。有些巨星的光度为太阳的几十万倍，巨星和超巨星的质量有时是太阳质量的10～30倍，但体积常比太阳体积大几百万倍。因此，巨星和超巨星都是一些低密度的“弥漫”恒星。

　　超巨星 [supergiant star]

　　体积和真实光度都极大的恒星，明显的比巨星亮几个星等，并大好几倍。如同其他星系，超巨星在实践中通常通过鉴定其光谱中的某些特定谱线而鉴别它们之间的差别(参阅光谱学[spectroscopy])。超巨星的直径一般比太阳大好几百倍，光度大近百万倍。其寿命也许只有几百万年，为极为短暂的恒星。

　　中子星 [neutron star]

　　一种密度极高的恒星，认为主要由中子构成，外面有一薄层主要为铁原子和电子以及质子组成的大气。典型中子星的直径约20千米，但它们的质量大约是太阳的两倍，可见其密度之高(大约是水的密度的百万亿倍)。中子星有强度极高的磁场，与黑洞不同的是中子星有固态的表面。在这个表面下面，由于压力太高，不可能存在单个原子;质子与电子被紧压在一起成为中子。20世纪30年代初预言存在中子星，大多数研究人员认为中子星是超新星爆炸后形成的。1967年发现了脉冲星，首次证明了中子星的存在。参阅白矮星(white dwarf star)。

　　(大英袖珍百科))

　　3

　　The gravitational energy released during this implosion of the core blows off the remainder of the star in a gigantic explosion, or a supernova.

　　恒星的核心内爆的过程中，释放的引力势能把核心以外的其他物质以大爆炸的形式向外抛出，或者会形成超新星爆发。

　　(超新星 [supernova]

　　一类猛烈爆发的恒星，其光度在爆发后比平常猛增数百万倍至数十亿倍。与新星相类似，超新星的光度急遽上升并维持数周之久，随后缓慢变暗;光谱上有蓝移的发射线，表明有炽热气体往外喷发。超新星爆发是恒星的一场灾难，导致恒星瓦解成中子星或黑洞。爆发时相当于几个太阳的物质被突然抛入太空，爆炸所释放的能量能将其所在的整个星系全部照亮。17世纪以前历史上只记载过7颗超新星，最著名的一颗出现在1054年，此次爆发的残留物就是现在我们看到的蟹状星云。超新星爆发不但释放出大量的射电和X射线，还释放出宇宙线和组成太阳系各天体成分的重元素。

　　(大英袖珍百科))

　　4

　　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.

　　据信约有五成恒星初始质量都大于1.4M⊙，两颗恒星里怎么也该有一颗死于超新星爆发吧。

　　5

　　But in fact, only one star in thirty dies such a violent death.

　　事实并非如此，只有三十分之一的恒星如此暴亡。

　　6

　　The rest expire much more peacefully as planetary nebulas.

　　其他恒星的终结都相当平和，成了行星状星云。

　　(行星状星云 [planetary nebula]

　　用小望远镜观看颇似行星的亮星云，实际上都是一些垂死星体周围的发光气体的扩展的外壳。行星状星云是由一颗红巨星散发的外部包层，这种巨星还不够大到成为超新星。相反地，星体高度炽热的中心暴露出来(参阅白矮星[white dwarf star])，并使气体的周围外壳离子化，该外壳以每秒16千米的速度扩张。)

　　7

　　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.

　　很明显，多数大个儿恒星在耗尽核聚变能源以前，就能够丢掉足够多的物质，把体重降到关键的 1.4M⊙ 以下。

　　8

　　Evidence supporting this view comes from observations of IRC+10216, a pulsating giant star located 700 light-years away from Earth.

　　距离地球 700 光年，有一颗仍活动的红巨星 IRC+10216，对它的观测结果可以支持这种观点。

　　(pulsating 有脉搏的，有节奏扩张和收缩的)

　　9

　　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.

　　在 IRC+10216 周围的星云中用红外线手段探测到氨分子，据此可推断出其质量流失相当快(每一万年1M⊙).

　　10

　　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.

　　最近通过微波观测一氧化碳分子的结果，说明了相似的质量流失速度，也显示出逃逸的物质从恒星向外绵延，至少一光年的距离。

　　11

　　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.

　　我们知道 IRC+10216 周围星云的规模，并能够通过观测 NH3 或 CO 来测定物质外溢的速度，于是就可以计算周围星云的形成年代。

　　12

　　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.

　　显然，IRC+10216 在过去的一万年，以分子和星辰颗粒的形式向外抛出了的质量，相当于一个太阳。

　　13

　　This implies that some stars can shed huge amounts of matter very quickly and thus may never expire as supernovas.

　　这意味着，有些恒星可以非常快地脱去大量物质，因而永远不会经历超新星爆发。

　　14

　　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⊙.

　　关于超新星和行星状星云的理论模型与统计数据表明，初始质量 6M⊙ 左右的恒星会脱去足够多的物质，把质量降到 1.4M⊙ 以下。

　　15

　　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.

　　例如 IRC+10216，从最初形成仅五万年就开始了这个过程，对恒星来说只是一瞬。

　　(should 表示推测，因为计算结果不是观测结果，意为刚出生就开始了)

　　16

　　But what place does IRC+10216 have in stellar evolution?

　　但 IRC+10216 在恒星演化过程中处于什么阶段?

　　17

　　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.

　　天文学家们认为像 IRC+10216 这样的恒星，实际是 “原行星星云” —— 一些年老的巨星，其致密的核心快要、但还没完全脱离周围松散的气态膜。

　　(不翻译了，我也看不懂，大概看看。

　　A protoplanetary nebula or preplanetary nebula (Sahai, Sánchez Contreras & Morris 2005) (PPN) is anastronomical object which is at the short-lived episode during a star's rapid evolution between the lateasymptotic giant branch (LAGB)[a] phase and the subsequent planetary nebula (PN) phase. A PPN emits strongly in infrared radiation, and is a kind of reflection nebula. It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars (1–8 M☉). (Kastner 2005)

　　(https://en.wikipedia.org/wiki/Protoplanetary_nebula))

　　18

　　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.

　　一旦完全脱离，暴露的星核会成为行星状星云的中心恒星，加热残余的气态膜并使之离子化，最终逸散至太空。

　　19

　　This configuration is a full-fledged planetary nebula, long familiar to optical astronomers.

　　这样的结构才是成熟的行星状星云，也是天文观察者们长久以来最熟悉的。

　　(fledge 羽翼丰满;full-fledged 成熟的，充分发展的，有完全资格的，羽毛长全的)

　　2. The primary purpose of the passage is to

　　(A) offer a method of calculating the age of circum-stellar 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

　　选 D

　　解释了为什么死于超新星爆发的恒星比我们预想的要少。

　　3. The view mentioned in the middle of the second paragraph(句 4-9) 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

　　选 A

　　调和两种看似矛盾的事实。

　　本来两个里有一个就该爆，但事实上只有三十分之一爆掉了，因为其中多数恒星擅于减肥，在烧完前已经成功瘦身，就不用爆了。

　　4. 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.

　　选 B

　　A 未知信息，文章只说观测到两种分子，没说有没有其他成分。

　　B 正确项。

　　C 句 15 告诉我们中间差五万年，虽然对恒星来说很短，但也不能说 equal 。

　　D 文中信息不足以判断，只说天文学家算出来了，没说是不是匀速扩张。

　　E 未知信息，扩张速度与质量是否有关系我们不知道。

　　做完全陌生的学科的文章，有个好处就是你不大会发散思维，总是很老实地承认不知道。

　　5. 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

　　选 A

　　A 正确项，符合句 17-18 的描述。

　　B 6M⊙ - 4M⊙ = 2M⊙ > 1.4M⊙，所以，这颗是要爆的那种，三十分之一那种。

　　C 错在 obscure，应该是暴露出来的。

　　D 这个是正在爆的一颗。

　　E has yet to 有待于，还没枯竭，正在枯竭，信息不足以判断，起码我们不知道它剩余多少质量，所以完全不能判断会不会爆，还是成为行星状星云。

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