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The Hidden Lives of Galaxies - The Clustering of Galaxies

D. The Clustering of Galaxies

Like stars, galaxies often appear together in groups and clusters. Groups may consist of a few galaxies and are often a part of larger galaxy clusters. Galaxies also often have small companion galaxies. Our Milky Way Galaxy is accompanied by the Large and Small Magellanic Clouds, which are both irregular galaxies visible from the southern hemisphere. The Andromeda Galaxy has two small companion elliptical galaxies, M32 and M110.

Our nearest neighbor galaxies form the Local Group, which consists of about two dozen galaxies of various types - spiral, elliptical, and irregular. The nearest large cluster of galaxies is the Virgo Cluster. It covers a region in the sky about six degrees across in the constellation Virgo. It consists of over one hundred galaxies of many types, including spiral, elliptical, and irregular galaxies. The center of the Virgo Cluster is twenty million parsecs from Earth. Other clusters are farther, and some have a symmetric distribution of galaxies.

Some clusters are members of superclusters. The Local Group and Virgo Cluster are part of a supercluster that contains one hundred other clusters and is one hundred megaparsecs across. The study of these superclusters leads to understanding the very structure and evolution of the Universe.

II. The Hidden Lives of Galaxies

A. Hidden Objects

Observation of galaxies at wavelengths other than optical light reveals other objects and components. Some are also seen in optical wavelengths, but are brighter in other parts of the spectrum.

For example, at radio wavelengths astronomers can detect much of the hydrogen that lies between the stars. These hydrogen atoms emit radio waves having a frequency of 1420 MHz (= 1420 x 106 Hertz), or as it is more commonly referred to, a wavelength of 21 cm. Astronomers use the detection of this gas to map out the location of hydrogen in our Galaxy. Astronomers can also determine the velocity at which the gas is moving, and whether it is moving toward or away from us. In this manner, the general motion of gas, and presumably the stars formed from the gas, can be determined.

In X-ray wavelengths, we see individual stars, supernova remnants, binary star systems, and globular clusters. All of these occur in our own Galaxy, and we can see other galaxies which also contain these objects.


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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