OFFICIAL42 What is the main purpose of the lecture?

[00:00.00]NARRATOR: Listen to part of a lecture in an astronomy class.
[00:04.03]MALE PROFESSOR: Before we continue talking about the properties of individual galaxies, it's worth talking about the distribution of galaxies in space. [00:12.57]Efforts at mapping, or surveying the universe, uh, making a sort of atlas of galaxies, have been going on for more than fifty years—[00:21.45]and, um, the creators of the first major map of the universe were the astronomers Harlow Shapley and Adelaide Ames.
[00:32.18]In 1932, Shapley and Ames catalogued the positions of 1,250 galaxies by photographing what they saw through their telescopes.
[00:43.86]And they made an important discovery. [00:46.37]Their survey was the first to indicate that galaxies were not distributed uniformly in space. [00:53.58]Some areas had a lot of galaxies, and other areas had just a few. [00:59.43]Uh, another way of putting this is to say that galaxies are clustered. [01:04.60]They're not spread evenly throughout the universe. [01:07.63]So we have stars grouped together in galaxies, and galaxies grouped together in clusters. OK?
[01:16.52]Now, uh, after their survey, other astronomers completed surveys that added to the number of clusters catalogued.
[01:24.61]One of the most important was done by the astronomer George Abell. [01:30.45]Abell completed his survey in 1958. [01:34.15]It added considerably to the map made by Shapley and Ames. [01:37.96]In fact, his map had over 2,700 clusters of galaxies, [01:43.47]that's 2,700 clusters of galaxies, [01:47.60]not just galaxies.
[01:50.17]But there's another aspect of Abell's work that makes this map so valuable to astronomers. [01:56.55]He introduced a classification scheme for the galaxy clusters. [02:02.15]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. [02:14.00]In fact, the Abell catalogue is used as a starting point for astronomers who study these objects.
[02:21.07]One of the reasons his scheme has been so widely accepted is because of his sample size. [02:27.85]With all the clusters in his sample, he could determine the different characteristics of clusters. [02:33.36]And these characteristics formed the basis of his classification scheme. [02:38.30]Now, two of the characteristics crucial to his classification were richness and symmetry. [02:46.22]So, ah, what did he mean by richness?
[02:50.27]Well, basically it refers to the number of galaxies there are within a cluster.
[02:57.22]FEMALE STUDENT: Is that the same as density?
[02:59.30]MALE PROFESSOR: That’s right. [03:00.25]Both, uh, “richness” and “density” refer to the number per area. [03:05.18]Rich clusters, or dense clusters, uh, contain a relatively high number of galaxies.
[03:11.37]FEMALE STUDENT: And symmetry just refers to its shape?
[03:14.12]MALE PROFESSOR: Mm, roughly speaking, yes. [03:16.70]Uh, whether the shape of the cluster was the same on the left side as on the right side.
[03:22.02]So Abell used categories like that to classify clusters on a scale, from regular to irregular.
[03:30.31]A “regular” cluster is sphere-shaped, symmetrical, and most dense in the middle, uh, [03:36.28]with the greatest number of galaxies concentrated in the middle of the cluster.
[03:41.15]An “irregular” cluster might appear to be lopsided, asymmetrical, with a low concentration of galaxies in the center.
[03:49.53]FEMALE STUDENT: You're talking about the shape of the cluster though, not the shape of the galaxies within the cluster.
[03:54.70]MALE PROFESSOR: Right. Uh, for example, let's consider the Coma cluster. [03:59.61]It's a symmetrical cluster, basically spherical in shape. But the individual galaxies within it are elliptical. [04:07.56]They're not spherical or spiral shaped. But the cluster itself shows spherical symmetry.
[04:14.07]Um, the Virgo cluster, on the other hand, is considered irregular; [04:19.92]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.
[04:30.50]FEMALE STUDENT: Another question—[04:31.71]you were saying how some clusters have more galaxies than others—[04:35.80]how many galaxies does a cluster have to have in order to even be a cluster?
[04:40.72]MALE PROFESSOR: Good question.
[04:42.12]Abell’s definition of a cluster is this.
[04:45.41]First, there have to be more than fifty galaxies within a specific amount of space. [04:51.27]He said basically that clusters have a radius of roughly 2 megaparsecs. [04:57.37]And it was just an assumption, that all clusters would be about the same size. [05:02.85]It's remarkable that it proved to be correct. [05:06.40]And this “standard” cluster radius is known today as the Abell radius.
[05:12.27]And second, those fifty-plus galaxies have to be a certain brightness. [05:17.99]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.