A. To explain the difficulty of classifying distant objects in the universe
B. To introduce a classification system for galaxy clusters
C. To present some recent discoveries about the shapes of galaxies
D. To describe some differences between galaxies and clusters
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NARRATOR:Listen to part of a lecture in an astronomy class.
MALE PROFESSOR: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, uh, making a sort of atlas of galaxies, have been going on for more than fifty years... and, um, the creators of the first major map of the universe were the astronomers Harlow Shapley and Adelaide Ames.
In 1932, Shapley and Ames catalogued the positions of 1,250 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.
Uh, 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.OK? Now, uh, 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 Abell.
Abell completed his survey in 1958.It added considerably to the map made by Shapley and Ames.In fact, his map had over 2,700 clusters of galaxies, that's 2,700 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 clusters.Now, uh, surveys completed since Abell's have catalogued additional galaxies and surveyed more of 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 formed the basis of his classification scheme.
Now, two of the characteristics crucial to his classification were richness and symmetry.So, ah, what did he mean by richness?Well, basically it refers to the number of galaxies there are within a cluster.
FEMALE STUDENT:Is that the same as density?
MALE PROFESSOR:That's right. Both, uh, "richness" and "density" refer to the number per area.Rich clusters, or dense clusters, uh, contain a relatively high number of galaxies.
FEMALE STUDENT:And symmetry just refers to its shape?
MALE PROFESSOR:Mm, roughly speaking, yes.Uh, whether the shape of the cluster was the same on the left side as on the right side.So Abell used 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, uh, with the greatest number of galaxies concentrated in the middle of the cluster.An "irregular" cluster might appear to be lopsided, asymmetrical, with a low concentration of galaxies in the center.
FEMALE STUDENT:You're talking about the shape of the cluster though, not the shape of the galaxies within the cluster.
MALE PROFESSOR:Right. Uh, 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.
Um, 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.
FEMALE 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?
MALE PROFESSOR:Good question.Abell's definition of a cluster is this: first, there have to be more than fifty 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 fifty-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.
旁白：请听天文学课上的部分内容。
教授：在我们继续讲每个星系的属性之前，这些星系在太空中的分布也值得一讲。绘制宇宙的地图或勘测宇宙、制作出星系的地图集的努力，已经进行了超过50年了。第一批重要的宇宙地图绘制者是天文学家Harlow Shapley和Adelaide Ames。
1932年，Shapley和Ames编录了1250个星系的位置，通过拍摄下他们在望远镜中看到的内容。而且他们做出了一项重大发现。他们的调查首次指出星系在太空中不是均衡分布的。一些区域有很多星系，而其他区域只有一点儿。
另一种表达方式就是说星系是成群分布的。它们在宇宙中不是均匀分布的。所以我们有聚集在星系中的星星和聚集成星团的星系。继他们的调查之后，其他天文学家完成了一些调查，增加了编录的星团数量。最重要的调查之一是由天文学家George Abell完成的。
Abell在1958年完成了他的调查。它对Shapley和Ames制作的地图进行了大量补充。事实上，他的地图上有超过2700个星团，那可是2700个星团啊！不只是星系。但是Abell的成果还有另一个方面使得这个地图对天文学家来说如此宝贵。他提出了星团的一个分类体制。自Abell之后完成的研究编录了额外的星系，并且探测了更外层的太空，但是没人能对Abell的分类体制加以改进。事实上，Abell的编录被研究这些物体的天文学家用作了一个起始点。
他的体制被如此广泛接受的原因之一是因为他的取样规模。借助样品中所有的星团，他能确定这些星团之间不同的特征。而这些特征形成了他的分类体制的基础。
对他的分类至关重要的两个特征是丰富程度和对称性。那么丰富程度是什么意思呢？它主要是指一个星团内的星系数量。
学生：是不是和密度一样？
教授：没错。丰富程度和密度指的都是每单位区域内的数量。丰富的星团或密度大的星团包含了相对较多的星系。
学生：而对称性指的只是它的形状？
教授：粗略来说的话，是的。指的是星团左边的形状和右边的是否一样。所以Abell用了像这样的类别对某个规模上的星团进行了分类：从常规的到非常规的。常规的星团是球形的、对称的、而且在中间最密集，也就是说星团中间聚集的星系数量最大。而非常规的星团可能看起来不太平衡、不对称、中间没有什么星系聚集。
学生：所以你讲的是星团的形状，不是星团里星系的形状。
教授：没错。比如我们来想想后发星系团。它是一个对称的星团，形状基本上是个球形。但是它里面的单独星系是椭圆的。它们不是球形或螺旋形的。但是星团本身展现出了球形对称。
而处女座星系团则被认为是非常规的；它的整体形状不对称，而且中心也没有星系聚集。但它碰巧既有椭圆的星系也有螺旋状星系。
学生：还有一个问题。你一直在说一些星团拥有比其他星团更多的星系。。。要成为一个星团的话，它需要有多少星系？
教授：好问题！Abell对星团的定义是：首先，在特定太空区域内必须有50个以上的星系。他说基本上星团的半径大约有2个百万秒差距。而且认为所有的星团规模都一样只是一个假设。值得一提的是，这被证明了是正确的。现在标准的星团半径被命名为“Abell半径”。第二，那50多个星系必须有一定亮度。当然这只是一个粗略的估算，但是看星系的亮度是区分附近的和位置较远的星团的一个好办法。