Cosmology refers to the universe as a whole - someone who studies the origin and evolution of the universe. It's not a degree - you could work in cosmology with a degree in astronomy, astrophysics, or physics. Astrophysics has a bit less astronomy and a bit more physics, astronomy has a bit less physics, but there's no clear line between them. Astronomers and astrophysicists study stars, planets, galaxies, etc.
