For 10 points, name these large systems of stars, examples of which include Andromeda and the Milky Way.
In astronomy, the eras of nuclei and atoms were followed by the era of these objects. The central dominant type of these objects becomes large through cannibalism. The formation of these objects is explained by the bottom-up theory, and one of them has thick dust lanes, which is partly why it resembles a sombrero. Rubin discovered that these objects have mostly flat rotation curves, and Hubble devised a classification system for them that resembles a tuning fork. There are around fifty of these objects in the Local Group, including elliptical and spiral types. For 10 points, name these large systems of stars, examples of which include Andromeda and the Milky Way.
In astronomy, the eras of nuclei and atoms were followed by the era of these objects.
galaxies [or galaxy; accept more specific types]