When galaxies were discovered telescopically, and cataloged by Messier two centuries ago, they were known as little more than comet-like fuzzballs, or
sometimes spiral like "nebulae." It wasn't until early in this century that
very powerful telescopes resolved for the first time individual stars within
the "Great Nebula" in Andromeda. Edwin Hubble established the size and
distance scales of galaxies by observing variable stars with known luminosities
in external galaxies. Knowing the intrinsic luminosity of a particular class of star, and knowing how bright the star
Hubble went on to produce his famous "Hubble Types" which cataloged galaxies into a sequence leading from elliptical galaxies to spiral galaxies, with roughly 20 different categories. In the 1950's the greatest contributions to our understanding of galaxies were observationally spearheaded by Allan Sandage, whose crowning work was the "Hubble Atlas of the Galaxies," which gorgeously illustrated the different Hubble types. In the seventies, astronomers first nailed down the concept of "dark matter" within galaxies, which gravitationally binds the orbits of the stars within them, and increases the masses of galaxies by a factor of ten. The exact character of this extra matter remains an enigma today, but is so compelling observationally that astronomers take its existence as textbook knowledge.
Galaxies populate the Universe in a complicated three-dimensional web of
filaments, sheets and clusters, whose dimensions we're just getting a handle
on. As they interact gravitationally, they move at velocities of up to 1,000
kilometers per
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