Galactic & Extragalactic Astronomy
Galactic astronomy is the study of our own Galaxy (The Milky Way) while extragalactic astronomy is the study of other galaxies. Our Sun is merely one of hundreds of billions of stars in the Galaxy, and our Galaxy is but one of hundreds of billions of galaxies in the observable universe. The study of galaxies aims to understand how these systems of stars, gas, and dark matter form and evolve in time. Our Galaxy is a disk (spiral) galaxy, as can be seen clearly in images of the entire sky.
The solar neighborhood is the name for the immediate vicinity of the Galaxy. Despite the long history of work in this area, there is ample evidence that there are still undiscovered stars and brown dwarfs within 25 parsecs (82 lightyears.) The solar neighborhood is part of the Galactic disk, located about 8 kiloparsecs (26,000 lightyears) from the Galactic Center, but a small fraction of nearby stars are members of the Thick Disk or Galactic Halo whose orbits have temporarily brought them into the disk. Studies of the composition, kinematics, and age of nearby stars can reveal the history of the Galaxy.
One of the most powerful tools to study galaxies is the 21cm emission from neutral hydrogen atoms. Radio telescopes can reveal this other invisible gas between the stars. Hydrogen gas is often found far front the centers of galaxies where no stars are visible. Through the doppler effect, the velocity of the gas can be measured, resulting in strong evidence that most of the mass in galaxies is Dark Matter.