Galaxies are composed of stars in various stages of evolution, planets, glowing nebulae, dust, gas, and empty space. The galaxy with which we are most familiar is our own Milky Way. Early Greeks looked up into the starry sky and saw a glowing band of light. It looked like a river of milk; hence they gave it the name "Milky Way." That eventually became the name of our galaxy.

The invention of the telescope and its use by Galileo revealed that this band of light was composed of countless stars. The idea that our sun was just a single star in a system of many others was first suggested by Thomas Wright in 1750. In 1755, philosopher Immanuel Kant (1724-1804) published a book in which he made the visionary suggestion that the Milky Way galaxy was composed of a collection of stars in the shape of a lens, and that...