Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of physical cosmology is theoretical astrophysics at its largest scale; conversely, since the energies involved in cosmology, especially the Big Bang, are the largest known, the observations of the cosmos also serve as the laboratory for physics at its smallest scales as well. See Astronomical_unit.