The Discovery of the Structure of Our Milky Way Galaxy

Abstract

Between the about 5000 stars that we can see with the naked eye on a dark moon-less night, stretches the faint-glowing band of the Milky Way (Fig. 4.1), which was recognized by man since time immemorial. Its name originates from ancient Greek mythology. Persian scholar Abu Rachman Biruni (973–1048)—inventor of spherical trigonometry and author of over 100 books in all fields of science, from mathematics, physics and astronomy, to geography and ethnography—was the first to propose that the Milky Way is a collection of uncountable numbers of faint stars. This suggestion was confirmed in the fall of 1609 when Galilei pointed his telescope—invented in the Netherlands 1 year earlier—at the Milky Way. The fact that the Milky Way stars are so faint implies that—on average—they are far more distant than the individual stars that we can see with the naked eye. Around 1750, English instrument builder and amateur astronomer Thomas Wright (1711–1786) proposed in his book An Original Theory or New Hypothesis of the Universe that the sun and the nearby stars are part of an almost flat disk, “resembling a millstone”, of millions of stars, which in the plane of the disk extends much farther than in the direction perpendicular to the disk, and which surrounds us on all sides. In those days this was a purely theoretical thought, as one was not yet able to determine the distances of the stars. The real mapping of the Milky Way system started near the end of the eighteenth century and took almost two centuries. Step by step it proceeded as follows.