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第一段
1 .Listen to part of a lecture in an astronomy class.
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第二段
1 .Before we continue talking about the properties of individual galaxies, it's worth talking about the distribution of galaxies in space.
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2 .Efforts at mapping or surveying the universe, making a sort of atlas of galaxies, have been going on for more than 50 years.
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3 .And the creators of the first major map of the universe were the astronomers Harlow Shapley* 1 and Adelaide Ames.
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第三段
1 .In 1932, Shapley and Ames3 catalogued the positions of 1250 galaxies by photographing what they saw through their telescopes.
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第四段
1 .And they made an important discovery.
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2 .Their survey was the first to indicate that galaxies were not distributed uniformly in space.
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3 .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.
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4 .They're not spread evenly throughout the universe.
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5 .So we have stars grouped together in galaxies and galaxies grouped together in clusters. Okay?
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第五段
1 .Now, after their survey, other astronomers completed surveys that added to the number of clusters catalogued.
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第六段
1 .One of the most important was done by the astronomer George Abell4
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2 .Abell completed his survey in 1958.
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3 .It added considerably to the map made by Shapley and Ames.
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4 .In fact, his map had over 2700 clusters of galaxies.
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5 .That is 2700 clusters of galaxies! Not just galaxies.
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第七段
1 .But there's another aspect of Abell's work that makes this map so valuable to astronomers.
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2 .He introduced a classification scheme for the galaxy clusters5
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3 .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.
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4 .In fact, the Abell catalogue is used as a starting point for astronomers who study these objects.
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第八段
1 .One of the reasons his scheme has been so widely accepted is because of his sample size.
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2 .With all the clusters in his sample, he could determine the different characteristics of clusters.
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3 .And these characteristics form the basis of his classification scheme.
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第九段
1 .Now, two of the characteristics crucial to his classification were richness and symmetry.
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2 .So what did he mean by "richness"?
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第十段
1 .Well, basically it refers to the number of galaxies there are within a cluster.
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第十一段
1 .Is that the same as density?
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第十二段
1 .That's right. Both richness and density refer to the number per area.
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2 .Rich clusters, or dense clusters, contain a relatively high number of galaxies.
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第十三段
1 .And symmetry just refers to its shape?
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第十四段
1 .Roughly speaking, yes.
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2 .Whether the shape of the cluster was the same on the left side as on the right side.
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第十五段
1 .So Abell use categories like that to classify clusters on a scale: from regular to irregular.
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第十六段
1 .A regular cluster is sphere shaped, symmetrical, and most dense in the middle.
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2 .The greatest number of galaxies concentrated in the middle of the cluster.
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第十七段
1 .An irregular cluster might appear to be lopsided, asymmetrical, with a little concentration of galaxies in the center.
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第十八段
1 .You are talking about the shape of the cluster though, not the shape of the galaxies within the cluster.
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第一十九段
1 .Right.
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2 .For example, let's consider the Coma Cluster.
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3 .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.
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第二十段
1 .The Virgo Cluster, on the other hand, is considered irregular.
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2 .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.
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第二十一段
1 .Another question. You were saying how some clusters have more galaxies than others.
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2 . How many galaxies does a cluster have to have in order to even be a cluster?
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第二十二段
1 .Good question!
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第二十三段
1 .Abell's definition of a cluster is this:
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第二十四段
1 .First, there have to be more than 50 galaxies within a specific amount of space.
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2 .He said basically that clusters have a radius of roughly 2 megaparsecs.
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3 .And it was just an assumption that all clusters would be about the same size.
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4 .It's remarkable that it proved to be correct.
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5 .And this standard cluster radius is known today as "The Abell Radius".
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第二十五段
1 .And second, those 50 plus galaxies have to be a certain brightness.
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2 .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.
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