It is possible to see about 6,000 stars with the naked eye, and using a small telescope as many as 600,000 become visible. More powerful telescopes reveal, in addition to individual stars, many thousands of galaxies. Each of these galaxies contains as many as one trillion stars. The most powerful telescopes can probably detect 1 billion galaxies. Yet despite the immensity of these galaxies, they appear as the tiniest blotches of light surrounded by vast reaches of empty space. The universe, which by definition includes everything in existence, is unimaginably large. To study it, it must be broken down into its constituent parts.
The basic structure of the universe
An analysis of stars shows that many are part of clusters of as few as 10 stars to as many as 1 million stars. All revolve as a group about the center of the galaxy of which they are a part. This hierarchy continues to build up, with many galaxies being members of galactic clusters, bound together by their mutual gravitational interaction. The local group of about 20 galaxies, including our own, is one example of such a cluster. These clusters are typically tens of millions of parsecs across and contain several hundred galaxies. There is some evidence to suggest that clusters of clusters exist, or “super clusters” of galaxies.
The expansion of the universe
In the 1920’s the universe as a whole was discovered to be in a state of expansion. This finding was revealed in the recession of the galaxies: whereby the farther a galaxy is from the earth, the faster it appears to race away.
The density of the universe
The density of matter in the universe can be calculated once the number of galaxies per cubic mega parsec and the mass of each one is known. Counts of the numbers of galaxies exceeding a certain luminosity show that there is roughly 1 galaxy per 50 cubic mega parsecs of space. Taking an average galaxy mass as 10’1 solar masses, the density of space averages only 1 atom per 10 cubic yards (7.6 cubic meters).
Read More The stellar universe
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