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Good question! Abell’s definition of a cluster is this:
好問題！Abell對星團的定義是：
First, there have to be more than 50 galaxies within a specific amount of space.
首先，在特定太空區域內必須有50個以上的星系。
He said basically that clusters have radius of roughly 2 megaparsecs.
他說基本上星團的半徑大約有2個百萬秒差距。
And it was just an assumption that all clusters would be about the same size.
而且認為所有的星團規模都一樣只是一個假設。
It’s remarkable that it proved to be correct. And this standard cluster radius is known today as “the Abell Radius”.
值得一提的是，這被證明了是正確的。現在標準的星團半徑被命名為“Abell半徑”。
And second, those 50 plus galaxies have to be a certain brightness.
第二，那50多個星系必須有一定亮度。
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.
當然這只是一個粗略的估算，但是看星系的亮度是區分附近的和位置較遠的星團的一個好辦法。
