Name: Danbi K.
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?
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.
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.
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Update: June 2012