Short answers, pulled from the story.
Galileo Galilei first observed that the Milky Way consists of individual stars in 1610. He used a telescope to reveal a vast multitude of stars instead of the cloud of gas philosophers had claimed for centuries. This observation shattered the ancient belief that the Milky Way was a single atmospheric phenomenon.
Edwin Hubble resolved the Great Debate about whether the Milky Way is the entire universe in 1923. He used the 100-inch Mount Wilson telescope to identify Cepheid variables in the Andromeda Nebula. This discovery proved that Andromeda was far too distant to be part of the Milky Way.
The Hubble classification system for galaxies introduced in 1926 categorizes galaxies into elliptical, spiral, and irregular types. Elliptical galaxies range from nearly spherical E0 to highly elongated E7 and are dominated by older stars. Spiral galaxies feature rotating disks with bright arms where new stars are born.
Vera Rubin discovered the existence of dark matter in galaxies in the 1970s. She found that stars at the edges of spiral galaxies moved as fast as those near the center. This observation defied the laws of gravity as understood at the time and required the presence of invisible dark matter.
The current era of star formation is expected to continue for up to one hundred billion years. After this period, the smallest and longest-lived stars known as red dwarfs will begin to fade. This marks the end of the stellar age when galaxies will be composed of compact objects like white dwarfs and black holes.