Galaxies, and their accumulated gas, stars, and dust, are the main visible manifestations of structure in the Universe. Understanding galaxy formation and evolution is one of the key problems in astrophysics, combining the fields of stellar populations, gravitational dynamics, gas physics, active galactic nuclei, computer simulations, and cosmology. Studies can range in scale, from trying to understand the Milky Way star-by-star to measuring bulk properties of millions of distant galaxies.
