Notes:
A Note on Galaxies
In the centuries that followed Messier’s first catalogue of deep space nebulae, telescopes became big enough to differentiate between the nebulae. It was seen that some of these nebulae could actually be resolved into individually stars, confirming old ideas that we live in an island universe which is but one of many. These island universes were named galaxies, and it was decided that the Milky Way - the band of faint stars that stretches across the sky must represent our galaxy. These newly defined galaxies were then classified into different types as astronomers always do. There are the spiral galaxies, with large spiral arms and dark dust lanes, and the ellipitical galaxies, apparently rather boring large elliptical collections of stars. The spiral galaxies were further sub-divided into those with and those without bars across their nucleus.