The universe is bathed in diffuse, electromagnetic radiation left over from earlier epochs, and from astronomical sources such as quasars and primeval galaxies. There is X-ray, extreme ultraviolet and visible radiation, now shifted by cosmic expansion to the microwave region of the spectrum. Acting together, radiative transfer and atomic physics control the microscopic behavior of the baryonic gas. When it comes to simulating the birth and evolution of individual galaxies and stars, all the above physics must be modeled separately at suitably small scales according to various prescriptions . These prescriptions take account of the specifics of galaxy formation and starbirth , as well as stellar evolution, death, and the recycling of matter and energy.
Star birth occurs on galactic scales when galaxies first form.
Large amounts of energy are released when billions of stars within a given galaxy begin burning their nuclear fuel. This energy in turn has a substantial feedback effect on the gas out of which the galaxy condensed in the first place. Cosmologists believes this caused powerful galactic winds and intergalactic shock waves, both of which must also be incorporated into the most accurate simulations.