We are used to characters in science fiction traveling around within the galaxy although inter-galactic travel is rare. It is worthwhile to reflect on facts about galaxies as summarized in Poul Anderson's Is There Life On Other Worlds? (New York, 1963, pp. 24-28). (Some blog readers might be able to update this information in the light of data gathered in the last fifty years.)
1963 Information
There might be a million million galaxies, each containing billions of stars, within range of the Mount Palomar telescope which cannot see to the end of the universe.
Clusters of galaxies are millions of light years apart.
The Milky Way and the Andromeda galaxy are the largest of the nineteen galaxies in the Local Group.
Our galaxy is 100,000 light years across. Its nucleus is 20,000 light years across with an axis of 6,500 light years. The galactic halo of thinly spread stars and gas has a volume fifty times that of the galaxy.
Several spiral arms with empty spaces between them radiate from the nucleus. Our sun, near the inner edge of one spiral arm, 100 light years from the galactic equator and 30,000 light years from the centre, travelling at 130 miles per second, revolves once around the galactic centre every 195 million years.
If possible, sf writers need to keep reminding us of the shape and size of the galaxy every time they describe their characters travelling around within it. Anderson's Terran Empire is only four hundred light years across but, later, mankind spreads through several spiral arms.
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