passage 53 蓝离散星

★★★

批量审题：

1. The reference to a “larger target” serves primarily to suggest why a

信息目的题，一般在定位当句可做

2. Information presented in the passage suggests which of the following about blue stragglers? 间接细节，遇到也不一定好做

3. The passage cites which of the following as evidence undermining the theory presented in the second sentence? 直接细节+削弱

做题优先级要出来，3 1 2

文章分析：

①Hotter and more massive than the Sun, stars called “stragglers” are puzzling to astronomers because such rapidly burning stars would not be expected to persist in ancient star clusters. ②Some researchers believe that the typical blue stragglers formed when two ancient, lower-mass stars collide and merge form more massive, hotter star. ③Peter Leonard theorizes alternatively that in low density globular clusters, where mergers between single stars occur too infrequently to account for the observed quantity of blue stragglers, these stragglers are created instead by a group of stars. ④He suggests that a pair of stars already orbiting each other presents a larger target for a third star or another pair. ⑤Once this new grouping forms, close encounters between the stars could prompt any two to merge as a blue straggler. ⑥Leonard’s model predicts that each blue straggler has a distant orbiting companion—as appears true of many blue stragglers in the M67 cluster of the Milky Way galaxy.

① 观点，天文学家们觉得这个现象很奇怪；

② 顺承观点，v1=两个小的撞成一个大的；

③ PL的转折观点，v2=不是单个撞的，那样几率太小了，应该是一群撞的；

④+⑤+⑥ 顺承观点，∈v2

①

↓

②  ↔  ③

            ↓
           ④→⑤→⑥

题型解析：

1. The reference to a “larger target” serves primarily to suggest why a

信息目的题，句内局部信息先服务于句子主干，主干则服务于逻辑上层

A.blue straggler would be more likely to collide and merge with another star than would be a lower-mass star 没有证据/相反，不是bs跟其他的撞

B.pair of stars would be more likely to encounter other stars than would the typical blue straggler 没有证据/相反，bs不是撞击对象

C. pair of stars would be more likely to interact with other stars than would a single star 

D.blue straggler would be more likely to interact with a pair of stars than it would with a third star 同B

E.third star would be more likely to encounter a pair than it would to encountera blue straggler 

其实四个错误选项中都有一个相同问题：文中blue straggler都是撞击结果，不是撞击对象。

2. Information presented in the passage suggests which of the following about blue stragglers? 间接细节

A.They originate from stars that are hotter and more massive than the Sun. 没有证据

B. They are burning more rapidly than other types of stars observed in ancient star clusters.看起来好像没有证据，但是属于第①句的直接推断，考场上一定记得结合排除法解题，利用好错误标签，单选选最优。

C.They are older than most other types of stars within the same starcluster. 没有证据，甚至相反

D.They are less numerous in low-density globular clusters than are pairs of stars. 没有证据

E.They generally originate from the oldest stars among those found in ancient star clusters. 没有证据

3. The passage cites which of the following as evidence undermining the theory presented in the second sentence? 直接细节，对应③

A. A discrepancy between the number of mergers between single stars in certain low-density globular clusters and that in other low-density globular clusters 没有证据

B. A discrepancy between the heat and mass of blue stragglers formed by one type of process and the heat and mass of blue stragglers formed by another type of process 相反/没有证据

C. A discrepancy between the frequency of star mergers in low-density globularclusters and those in high-density globular clusters 没有证据

D. A discrepancy between the amount of heat and mass of ancient single stars and that of blue stragglers 没有证据

E. A discrepancy between the number of mergers between single stars in certain star clusters and the number of blue stragglers in those clusters

背景拓展：

A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point （主序折向点） for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3.

蓝离散星是疏散星团或球状星团中与其他主序成员成表面温度较高、光度更高的蓝色恒星。因此，在星团的赫罗图中，它们的位置有别于星团的其他成员。按理说，光度这样高的恒星在球状星团和年老的疏散星团中应早已离开主序，进入演化。蓝离散星看似违背了现行恒星演化的标准理论，因为在同时诞生的恒星在赫罗图上应该很明确的位在同一条曲在线，而由此依位置可以测量出它们原始的质量。因为蓝离散星偏离了这条曲线，显示它们在恒星演化中有不同的经历。

以下知识不知道什么时候会有用，但多了解总不会吃亏，假如下次考相关了呢，不用深究，有个印象就行~

关于蓝离散星的解释有很多种，主要分为单星论和多星相互作用论。现在看来，单星的“返老还童”不太可能，只有多星相互作用更有可能。主要有两种解释模型：恒星碰撞和物质传输。

1 恒星碰撞：

两颗恒星的合并只会创造出一颗质量更大的恒星，这颗恒星的表面温度会比同年龄的恒星更热和更亮。如果这种理论是正确的，则蓝离散星就不会成为恒星演化理论上的问题，合并后的恒星在核心会有更多的氢，使它的行为像较年轻的恒星。有证据支持这种看法，在星团内蓝离散星的数量明显的与恒星的密集度有关，越密集处的术量越多，特别是在球状星团的核心区域。因为在单位体积内的恒星数目越多，碰撞和密接的机会越高，而星团内确实比其他区域更容易发生。

检验这种假说的一种方法是研究蓝离散星中的脉动变星。在星震学中，合并的恒星在脉动上的特性会与一般的恒星有所不同，或许可以测量出其间的差异。尽管在拥挤的区域中经常能找到小光度振幅的蓝离散星，然而，在缺乏明显的蓝离散脉动星下，脉动的测量是非常困难的。

2 物质传输：

由一颗大质量星和一颗小质量星组成的双星系统，大质量星先演化成为超巨星并充满它的洛希半径（双星的允许半径），这样就成了一对半相接双星，红巨星给没有演化的小星传送物质，小型的质量变大，同时温度增高（变蓝），光度增高，成为看似“掉队”的蓝离散星。

最近的研究显示蓝离散星的碳和氧比附近区域的其他恒星要少，这意味著一颗恒星的变热和变蓝是从轨道上的另一颗恒星攫取物质所导致的；而质量被窃取的恒星会使得原本在深层含有较多碳和氧等重元素的区域被暴露在表面。

蓝离散星的自转迅速，通常都在太阳的75倍以上，质量也是星团中其他成员的2-3倍。