What mechanisms or physical processes are responsible for the emission of electromagnetic radiation in a galaxy in the different wavelength bands?
closed as too broad by Kyle Kanos, Emilio Pisanty, sammy gerbil, Chris♦, Mitchell Jan 31 '18 at 6:08
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
Most of the non-darkmatter mass of the galaxy is in the form of plasma or neutron density neutron stars and black holes. Almost all of the gamma ray photons originate from the nuclei of the various elements in the plasma as they fuse together to form new elements, matter-antimatter annihilation, gamma ray burst events, or emissions from nucleons relaxing back to ground quantum states of nuclear isomers.
X-rays can also originate from relaxing nucleons, but for lower quantum energy drops than gamma rays. A good portion of the x-ray spectrum comes from plasma and electrons being accelerated into black holes or neutron stars but colliding with charged plasma on the way. In fact, these x-ray emissions from Cygnus x-1 led to the first evidence of black holes.
Although photons of almost every frequency are emitted by supernovae explosions, a huge proportion of the spectrum emitted is in the x-ray range. This due again to the very great acceleration or deceleration (Bremsstrahlung) of plasma nuclei and electrons through magnetic and electrical fields in plasma resulting from the supernova explosions. The study of distant supernovae was one of the main reasons for launching the Chandra x-ray space telescope.
photons leaving stellar surfaces from being emitted as electrons rejoin the nuclei of cooled plasma
cosmic ray and magnetic field interactions
black body radiations (brown dwarfs, dead stars, non-nuclear reaction bodies)
left over microwave cosmic background radiation from big bang
radio waves from pulsars