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Name: Danbi K.
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
I have a question about the distribution of galaxies. I understand that they are distributed in a clustered way by gravitational forces (through collisions) however, pictures of galaxies look very randomly distributed. Why do scientists say that they are not when they look like they are?


Replies:
Yours is a good question because it raises an important point. Randomness or clustering depends upon what scale the observations are made. An example that illustrates the importance of scale of observation is a cloud. If you are flying through a large cloud in an airplane, the cloud looks uniformly gray (random), but the water droplets making up the cloud, under a microscope might look "lumpy". If you are standing on the ground, the cloud may look lumpy (clustered), or smooth (random) depending upon the type of cloud. If the cloud is very far away you may not be able to distinguish the cloud structure and it might look smooth (random) again.

What astronomers see is that galaxies on a large scale are not quite uniform but tend to form clusters. However if you are within a galaxy and only look at stars within that galaxy, the distribution may appear random.

Vince Calder


Dear Danbi,

This is a very good question and it opens up a fascinating look at the history of astronomy. First, galaxies are distributed in clusters, and I have seen that many many times during the observing that I do, However, in the 1920s, Edwin Hubble proposed that the clusters of galaxies were distributed evenly -- not the galaxies themselves, but the clusters of galaxies. Then Clyde Tombaugh (discoverer of Pluto) discovered that clusters were NOT distributed evenly through the space he examined during his planet search. He discovered superclusters, and voids without any galaxies. His work was built upon by George Abell in the 1950s, who confirmed that the clusters of galaxies are not evenly distributed. However, we now think that at the supercluster level, where hundreds of clusters gather into a supercluster, the superclusters are evenly distributed.

When you look at a picture of galaxies, it is probably of a single cluster.

Sincerely

David Levy



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