『新东方在线托福听力』为托福考生准备了ETS托福TPO/Official 42 Lec 2听力练习：Why does the professor emphasize the number of clusters mapped by Abell?原文题目答案解析，TPO/Official 42 Lec 2听力原文范文音频翻译。

托福TPO/Official 42 Lec 2听力原文Why does the professor emphasize the number of clusters mapped by Abell?【新东方在线】

旁白：听一段天文学课程节选。

Narrator: Listen to part of a lecture in an astronomy class.

教授：在我们继续讨论单体星系的特点之前，值得聊一聊星系在太空当中的分布。

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

绘制或者测量，以及制作某种星系地图册的努力已经持续了五十多年。

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

并且第一份重要的宇宙地图的创造者是天文学家Harlow Shapley和Adelaid Ames。

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

在1932年，Shapley和Ames通过他们利用望远镜所拍下的照片，给1250个星系的位置进行了分类。

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

而且他们有了一个重大的发现。

他们的测量，第一次表明了星系在太空中不是统一分布的。。

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

他们的测量首次表明星系在太空中不是均匀分布的。

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.

它们并不在整个宇宙中均衡地分布。

They're not spread evenly throughout the universe.

所以，恒星组成星系，星系组成星团

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

现在，经过了他们的调查之后，其他天文学家完成了增加了对星团数量分类的调查。

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

最重要的调查当中，有一个是由天文学家George Abell完成的。

One of the most important was done by the astronomer George Abell.

这极大地扩充了Shapley和Ames制作的地图的内容。

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

事实上，他的地图上有超过2700个星系群。

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

但Abell的工作在另一方面使得这张地图对天文学家来说非常宝贵。

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

他介绍了对星系群的分类方案。

He introduced a classification scheme for the galaxy clusters.

自阿贝尔以来完成的调查已经记录了更多的星系，并调查了更多的外太空，但没有人对阿贝尔的分类方案进行改进。

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.

事实上，Abellde的分类已经被当做天文学家研究这些事物的起点了。

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

他的方案被广泛接受的原因之一是因为他的样本大小。

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

在他样本中的所有星团里，他可以确定不同星团的不同特征。

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

这些特征构成了他分类方案的基础。

And these characteristics form the basis of his classification scheme.

目前，两个对他的分类来说至关重要的特点是富有性和对称性。

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

那么他所说的“富有性”是什么意思？

基本上它指在一个星团里星系的数量。

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

学生：是不是和密度是一样的？

富有性和密度都是指在每个区域当中的数量。

Both richness and density refer to the number per area.

富有的星团，或密集的星团，都包含了相对较多的星系。

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

学生：那对称只是指它的形状吗？

Student: And symmetry just refers to its shape?

指的是星团右侧的形状和左侧的形状是不是一样。

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

所以Abell使用像那样的类别成规模地对星团进行分类：从规则的到不规则的。

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

一个规则的星团是球状的，对称的，并且中间密度最高。

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

星系的最大数量集中在星团的中间。

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

一个不规则的星团可能出现不平衡，不对称，只有少量星系集中在中间的情况。

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

学生：您是在说星团的形状，而不是星团中星系的形状。

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

教授：对。比如，让我们考虑下Coma星系团。

Professor: Right. For example, let's consider the Coma Cluster.

它是一个对称的星团，基本上是球形的，但是当中的个体星系是椭圆形的。

It's a symmetrical cluster basically spherical in shape, but the individual galaxies within it are elliptical.

它们不是球形的或者螺旋形的，但是星团本身呈现球形对称。

They're not spherical or spiral shaped, but the cluster itself shows spherical symmetry.

另一方面，处女座星团被认为是不规则的。

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

它的整体形状并不对称，中心没有星系集中，但是它恰好拥有椭圆的星系和螺旋形的星系。

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.

您刚才说一些星团如何拥有比其他星团更多的星系。

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

一个星团中拥有多少星系才能成为一个星团呢？

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

Abell给出的星团定义是这样的：

首先，在一个特定数量的空间里，拥有超过50个星系。

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

他说基本上星团有差不多两百万秒误差的半径。

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

这仅仅是个假设，设定所有的星团都是一样大小。

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

值得注意的是，这点被证明是对的。

It's remarkable that it proved to be correct.

这个经典的星团半径到今天被称为“Abell半径”。

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

第二，那50多个星系必须有一定的亮度。

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

当然了这也是个粗略的估计，但是观察星系的亮度是区分附近的星团和那些较远的星团的一个很好的方法。

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.

What did Shapley and Ames discover about the universe?

Why does the professor emphasize the number of clusters mapped by Abell?

What aspects of clusters did Abell use to classify them? Click on two answers.

Why does the professor discuss the Coma and Virgo clusters?

What is the professor's opinion of Abell's assumption that all clusters are about the same size?

Listen to part of a lecture in an astronomy class
Professor 
Before we continue talking about the properties of individual galaxies, it’s worth talking about the distribution of galaxies in space. Maping or surveying the universe making sort of, that was galaxies, have been going on for more than fifty years. And the creators of the first major map in the universe were the astronomers Harlow Shapley and Adelaide Ames. In 1932, Shapley and Ames catalog the positions of 1250 galaxies by photographing what they saw through the telescopes. And they made an important discovery. Their survey was the first to indicate that galaxies were not distributed uniformly in space. Some areas have a lot of galaxies, and other areas have just a few. Another way of putting this is to say that galaxies are clustered. They are not spread evenly throughout the universe. So we have stars grouped together in galaxies, and galaxies grouped together in clusters.
Ok, now, after their survey, other astronomers completed the surveys that added to the number of the clusters catalog. One of the most important was done by the astronomer George O. Abell.
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’s 2700 clusters of galaxies, not just galaxies. But there is another aspect of Abell’s work, that makes the map so valuable to astronomers. He introduced classification scheme for the galaxy clusters. Now surveys completed since Abell catalog additional galaxies, and survey more about the space. But no one has improved Abell’s classification scheme. In fact, the Abell’s catalogue is used as starting point for astronomers to study this objects. One of the reasons his scheme has been so widely accepted is because his sample size. With all these clusters in his sample, he could determine the different characteristics of clusters. And these characteristics formed 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 the cluster.
Student 
Is it the same as density?
Professor 
That’s right. Both richness and density refer to the number per area. Rich clusters or dense clusters contain relatively high number of galaxies.
Student 
And symmetry just refers to its shape?
Professor 
Um, roughly speaking, yes. Whether the shape of a cluster was the same on the left side is on the right side. So Abell use categories like that to classify clusters on the scale from regular to irregular. A regular cluster is sphere shaped, symmetrical and most 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 low concentration of galaxies in the center.
Student 
You are talking about the shape of the cluster, though, not the shape of the galaxies within the cluster.
Professor 
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 are not spherical or spiral shaped. But the cluster itself shows spherical symmetry. The Virgo cluster, on the other hand, is considered irregular. There is no symmetry to its overall shape. No center concentration of galaxies. But it happened to have both elliptical and spiral galaxies within it.
Student 
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?
Professor 
Good question. Abell’s definition of cluster is this. First, there has to be more than 50 galaxies within the specific amount of space. He said basically that clusters have radius of roughly two megaparsecs. And it was just an assumption that all clusters should 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 fifty plus galaxies have to be a certain brightness. Of course, it was a rough estimate, but looking at galaxy’s brightness was a good way to distinguish between clusters that were nearby and those that were more distant.<img src="/upload/image/1516176447946.png" alt="" width="510" height="382" /><img src="/upload/image/1516176463756.png" alt="" width="512" height="385" /><img src="/upload/image/1516176483049.png" alt="" width="513" height="382" /><img src="/upload/image/1516176495087.png" alt="" width="507" height="386" /><img src="/upload/image/1516176506087.png" alt="" width="512" height="387" /><img src="/upload/image/1516176518522.png" alt="" width="508" height="385" />

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