Astronomy

"Historical Overview of the Discovery of Galaxies There are two privileged lines in the sky. One is the familiar ecliptic, the path traveled by the Sun. It is the image of our orbit around the central star, as seen from the Earth, against the stars. The Moon travels not far from this path and it even occasionally eclipses the Sun, and sometimes the Earth's shadow falls on the Moon. Likewise, the main planets stay fairly close to this line in the sky. We see that the orbits of the planets tend to lie in the same plane and this plane is close to that of Earth's orbit. The other privileged line is the one centered on the Milky Way. While the name of the first line (""ecliptic"") suggests a function (the line where eclipses occur), the name ""Milky Way"" suggests nothing useful other than general milky appearance (the name is derived from a Greek fairy tale involving a goddess and her milk). After telescopes became commonplace, following the efforts of Galileo, it was soon realized that the Milky Way is an unusually dense collection of stars, all lined up. Where it is dark, it was found, dust clouds block the light of stars behind. In essence, we are looking at the main plane of an enormous aggregation of stars, and we are right in the middle of it. Galaxy Cluster CL0024. Each of the bright ""nebulae"" are individual galaxies like our own Milky Way. (Source: STSCI, Space Telescope Science Institute) The realization that we live inside a galaxy and that there are other galaxies besides ours came in stages. It all started with the observation of fuzzy spiral objects which were grouped with other such ""nebulae"" and with star clusters as interesting curiosities in the sky, not to be confused with really interesting things such as comets. In western tradition, toward the end of the 18th century, the German philosopher Immanuel Kant (1724-1804), who had earlier proposed that the solar system arose from a disk of rotating dust and gas, suggested that the Milky Way is one of many ""island universes"", that is, that some of the ""nebulae"" seen in telescopes may be galaxies themselves. "