关闭图标
log图标

toefl.viplgw.cn

  • 使用手机注册
  • 使用邮箱注册
  • 手机号不能为空!

    验证码不能为空!

    用户名不能为空!

    密码不能为空!

  • 邮箱不能为空!

    验证码不能为空!

    用户名不能为空!

    密码不能为空!

已有账号? 登录到雷哥托福
关闭图标
log图标
  • 使用手机找回密码
  • 使用邮箱找回密码
  • 手机号不能为空!

    验证码不能为空!

    密码不能为空!

  • 邮箱不能为空!

    验证码不能为空!

    密码不能为空!

又想起来了
加入生词本

listen

英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

已添加
×

我要举报草莓小菇凉评论

用户头像
草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

请选择举报类型:

举报电话:400 1816 180    举报QQ:2095453331
×
logo图标
分享到雷哥托福

分享成功图标分享成功

邀请名师点评成功,管理员正在安排老师进行点评。

继续做题 返回首页
支付雷豆失败图标 雷豆余额不足 购买雷豆 返回
报告题目错误
请选择错误类型:
请描述一下这个错误:

取消

文章结构

TPO 42-L2 .distribution of galaxies 点击收藏

标准

  • 高音
  • 静音
  • 句子
    精听
  • 全文
    精听
  • 听写
    模式
  • 点击查看本句

    听写本句 再播一遍 下一句
  • 点击查看全文

    再播一遍本文

    Listen to part of a lecture in an astronomy class.

    Before we continue talking about the properties of individual galaxies, it's worth talking about the distribution of galaxies in space.

    Efforts at mapping or surveying the universe, making a sort of atlas of galaxies, have been going on for more than 50 years.

    And the creators of the first major map of the universe were the astronomers Harlow Shapley* 1 and Adelaide Ames.

    In 1932, Shapley and Ames3 catalogued the positions of 1250 galaxies by photographing what they saw through their telescopes.

    And they made an important discovery.

    Their survey was the first to indicate that galaxies were not distributed uniformly in space.

    Some areas had a lot of galaxies, and other areas had just a few. Another way of putting this is to say that galaxies are clustered.

    They're not spread evenly throughout the universe.

    So we have stars grouped together in galaxies and galaxies grouped together in clusters. Okay?

    Now, after their survey, other astronomers completed surveys that added to the number of clusters catalogued.

    One of the most important was done by the astronomer George Abell4

    Abell completed his survey in 1958.

    It added considerably to the map made by Shapley and Ames.

    In fact, his map had over 2700 clusters of galaxies.

    That is 2700 clusters of galaxies! Not just galaxies.

    But there's another aspect of Abell's work that makes this map so valuable to astronomers.

    He introduced a classification scheme for the galaxy clusters5

    Now, surveys completed since Abell's have catalogued additional galaxies and surveyed more outer space, but no one has improved upon Abell's classification scheme.

    In fact, the Abell catalogue is used as a starting point for astronomers who study these objects.

    One of the reasons his scheme has been so widely accepted is because of his sample size.

    With all the clusters in his sample, he could determine the different characteristics of clusters.

    And these characteristics form the basis of his classification scheme.

    Now, two of the characteristics crucial to his classification were richness and symmetry.

    So what did he mean by "richness"?

    Well, basically it refers to the number of galaxies there are within a cluster.

    Is that the same as density?

    That's right. Both richness and density refer to the number per area.

    Rich clusters, or dense clusters, contain a relatively high number of galaxies.

    And symmetry just refers to its shape?

    Roughly speaking, yes.

    Whether the shape of the cluster was the same on the left side as on the right side.

    So Abell use categories like that to classify clusters on a scale: from regular to irregular.

    A regular cluster is sphere shaped, symmetrical, and most dense in the middle.

    The greatest number of galaxies concentrated in the middle of the cluster.

    An irregular cluster might appear to be lopsided, asymmetrical, with a little concentration of galaxies in the center.

    You are talking about the shape of the cluster though, not the shape of the galaxies within the cluster.

    Right.

    For example, let's consider the Coma Cluster.

    It's a symmetrical cluster basically spherical in shape, but the individual galaxies within it are elliptical. They're not spherical or spiral shaped, but the cluster itself shows spherical symmetry.

    The Virgo Cluster, on the other hand, is considered irregular.

    There's no symmetry to its overall shape, no central concentration of galaxies, but it happens to have both elliptical and spiral galaxies within it.

    Another question. You were saying how some clusters have more galaxies than others.

    How many galaxies does a cluster have to have in order to even be a cluster?

    Good question!

    Abell's definition of a cluster is this:

    First, there have to be more than 50 galaxies within a specific amount of space.

    He said basically that clusters have a radius of roughly 2 megaparsecs.

    And it was just an assumption that all clusters would be about the same size.

    It's remarkable that it proved to be correct.

    And this standard cluster radius is known today as "The Abell Radius".

    And second, those 50 plus galaxies have to be a certain brightness.

    Of course it was a rough estimate, but looking at galaxies' brightness was a good way to distinguish between clusters that were nearby and those that were more distant.

    • special
    ☞查看答案 再播一遍

联系雷哥托福

全国免费咨询热线:400 1816 180

  • 上海(总部)
  • 北京
  • 成都
  • 杭州
  • 武汉
  • 南京
  • 广州
  • 西安
  • 重庆
  • 深圳
  • 上海(总部)服务中心

    电话:400 1816 180

    地址:上海市徐汇区文定路218号德必徐家汇WE艺术湾B座205

    公交路线:76路、93路、138路宜山路蒲汇塘路站;205路、732路、754路南丹路宜山路站;205路、754路、830路南丹路文定路站等

    地铁路线:地铁3号线、4号线宜山路站出站470m;地铁1号线、4号线上海体育馆站出站600m。

  • 北京服务中心

    电话:400 1816 180

    地址:北京市朝阳区雅宝路7号 E园EPARK大厦508

    公交路线:乘坐公交44路外环;58路;139路;特12路外环;特2路;夜20路外环到雅宝路公交站 下车,走140米即可达到。

    地铁路线:乘坐地铁1号线、2号线到建国门站(A出口)步行1000米即可到达;乘坐地铁2号线、6号线到朝阳门站(A出口)步行1200米即可到达雅宝路7号 E园EPARK大厦。

  • 成都服务中心

    电话:400 1816 180

    地址:成都市锦江区总府路31号Z31六楼(皇冠假日酒店旁)

    公交路线:乘坐58路、98路、锦城观光3号线、熊猫快线5号线在春熙路步行街北站下 步行约281米。

    地铁路线:乘坐地铁2号线或者3号线到春熙路C口下,步行约982米

  • 杭州服务中心

    电话:400 1816 180

    地址:杭州市江干区江锦路159号 平安金融中心B座8楼

    公交路线:901路、105路环线、106路环线、9M路:富春路民心路口站 176路:市民中心北大门站,市民中心西大门站

    地铁路线:号线钱江路站;4号线钱江路,江锦路

  • 武汉服务中心

    电话:400 1816 180

    地址:武汉市珞瑜路光谷世界城广场写字楼1栋1单元14层11421室

    公交路线:乘坐18路,25路,340路,510路,518路,521路等公交车到珞瑜路湖北省中医院站下车

    地铁路线:乘坐地铁2号线至珞雄路A口出,往湖北省中医院光谷院区方向步行50米至世界城广场写字楼

  • 南京服务中心

    电话:400 1816 180

    地址: 南京市秦淮区中山东路532号金蝶科技园 H1幢308号(林客社 内)

    公交路线:5路 34路 34路区间 36路 55路 59路 y5路夜间 17路 65路 115路 118路 y17路夜间

    地铁路线:地铁2号线明故宫站1号口出,向东200米

  • 广州服务中心

    电话:400 1816 180

    地址:广州市天河区体育西路103号维多利广场B座26楼K15

    公交路线:11,18,20,230,278,293,45,583,669,886冼村路中站下。 194,40,777,778,90,观光2路花城广场路站下。

    地铁路线:乘坐地铁APM线至花城大道站下B口出,步行约500米即可到达保利威座大厦北塔。

  • 西安服务中心

    电话:400 1816 180

    地址:西安市莲湖区北大街西华门十字西南角78号嘉会广场C座4层B021号

    公交路线:4路、12路、26路、36路、37路、229路、游7钟楼北站;15路、16路、32路、102路、182路、236路、286路西华门站。

    地铁路线:地铁二号线钟楼站150米;地铁一号线/二号线北大街站700米。

  • 重庆服务中心

    电话:400 1816 180

    地址:重庆市渝中区邹容路68号大都会广场16楼1603-1604室

    公交路线:401、135、114、111、401、151、862、181、262、105、866、153、466

    地铁路线:2号线临江门站500米、1号线小什字站500米、6号线小什字站500米。

  • 深圳服务中心

    电话:400 1816 180

    地址:深圳市罗湖区书城路都市名园B栋5楼B区(只接受预约拜访)

    公交路线:101路 10路 12路 203路 214路 215路 223路 29路 3路 85路 M112路 M191路 M192路 N3路 P100路(定制公交请预约) 高快巴士200号线 高快巴士20路 高快巴士863号线到地王大厦下,步行431米即可到达。

    地铁路线:乘坐1号线/罗宝线/2号线/蛇口线到大剧院(E口)出,步行498米即可到达都市名园B栋

预约托福考试规划师