Galaxies and Clusters of Galaxies
The Milky Way Galaxy and Large Scale Structure of the Universe
Milky Way Galaxy
On a clear dark moonless night escape the city lights and look up at the sky. If you see a faint fuzzy white band across the sky that almost looks like thin clouds, you are seeing the Milky Way, our home galaxy. The Milky Way is a collection of a few hundred billion stars that is over a hundred thousand light years in diameter.
The Milky Way is a spiral galaxy. The stars are concentrated in a flat disk shape that is a few thousand light years thick but still over 100,000 light years in diameter. This disk has a spiral arm structure. Two arms come out from the nucleus, or center, of the galaxy and wind around several times to produce the spiral arm structure. The disk and spiral arms contain mostly young (called Population I) stars like our Sun. They also contain various types of nebulae and galactic or open clusters.
The Milky Way also has a spherical halo containing globular clusters and other old (called Population II) stars. This old spherical halo - young disk structure tells us that the galaxy was originally spherical but has flattened out in the 12 or so billion years since it formed. The galaxy spins as stars orbit its center to cause this flattening, just as pizza dough will flatten when a pizza maker spins it. The Milky Way is so vast that it takes our solar system about 250 million years to complete a single orbit.
Our current best evidence indicates that the central nucleus of the Milky Way contains a supermassive black hole. There is also evidence, but not proof that the Milky Way is a barred rather than a normal spiral galaxy.
Other Galaxies
The Milky Way is only one of billions of galaxies in the observable universe. Edwin Hubble classified the types of galaxies we observe into normal spiral, barred spiral, elliptical, and irregular galaxies.
Clusters and Superclusters of Galaxies
Galaxies group together into clusters of galaxies, just as stars group into galactic clusters and globular clusters. Do not confuse the terms "cluster of galaxies" and "galactic cluster"; they are two completely different scales. A cluster of galaxies is a group of galaxies, and a galactic cluster, also called open cluster, is a group of stars within a single galaxy.
Our Milky Way is part of a cluster of galaxies called the Local Group. The Local group consists of two spiral galaxies besides the Milky Way: the Andromeda Galaxy and M33. It also contains a number of dwarf elliptical and irregular galaxies. The Local group is a few million light years across.
On very large scales, there are superclusters, which are clusters of clusters of galaxies. The Local group is part of the Local Supercluster. Superclusters are typically a few hundred million light years in diameter. They are separated by similar sized empty regions called voids.
In astronomy clustering occurs on many scales ranging from clusters of stars, to superclusters of galaxies.
Further reading
Morrison, D., Wolff, S. and Fraknoi, A., Exploration of the Universe, Saunders, 1995.
Zeilik, M., Astronomy: The Evolving Universe, Cambridge, 2002.
