Galaxy Clusters

Week of November 1, 1999

Five weeks ago, these Snacks started small. We started with associations, then to constellations, from there to open clusters and then to globular clusters. The last step encompassed them all with galaxies, and how we discovered that the Milky Way was not the entire Universe.

What then, of the Universe? If the Milky Way isn't all of it, what is?

Galaxies, like stars in the sky, do not fall randomly strewn across the Universe. They aggregate in clusters much like the stars that comprise them. And just like those star clusters, there are different kinds of galaxy clusters. There are groups, which may contain from a handful to a few dozen galaxies. The Milky Way is part of the Local Group, which has two large spirals, perhaps three or four smaller ones, and dozens of tiny galaxies which are either elliptical (like the ones that orbit the Andromeda Galaxy) or irregular, like the two Magellanic Clouds, smallish galaxies that orbit our own Milky Way. These groups are gravitationally bound, which means all the galaxies move in complex orbits around each other. Sometimes the group members can collide. The Milky Way is colliding with a small irregular galaxy right now!

Galaxies also appear in larger clusters. The nearest such cluster lies in the direction of the constellation Virgo, and has hundreds and even thousands of galaxies in it. The largest is a monster elliptical that may have as many as a trillion stars in it, making it twice the size of our own Galaxy. This cluster of galaxies is so enormous in extent that even though it lies 60 million light years away, it occupies a large area of sky as seen from the Earth. You can barely cover it up with your outreached hand!

But even the Virgo Cluster is small potatoes. There are vast clusters of clusters, called superclusters, which contains hundreds of thousands or even millions of galaxies. Imagine! Our own Galaxy has 400 billion stars in it, yet there are clusters out there with millions of galaxies like our own. If galaxies are the universal small towns, then superclusters are the countries, collecting thousands of clusters and forming them into a colossal union.

On this scale, even the Universe itself has structure. Superclusters have galaxies strung out on vast sheets, as if the Universe were a sponge with holes in it; these holes have relatively few galaxies in them, with the majority falling along the bubbles' boundaries. No one is quite sure just why these structures formed, but the answer to it surely lies in the way the Universe itself was formed.

And as unimaginable as all this is, there is still one more stop left on our journey to bigger structures. This one is pure theory, but has implications philosophical in proportion. If we ever do find the answers to next week's questions, we may indeed have found out much more about ourselves, and our place in the cosmos.